oxideav-ac4

Pure-Rust Dolby AC-4 audio decoder foundation — sync / TOC / presentation / substream parsing, plus a stub decode path that emits silence at the correct channel count and sample rate so container fixtures can round-trip without crashing. Zero C dependencies, no FFI, no *-sys crates.

Part of the oxideav framework but usable standalone.

Status: Foundation. AC-4 is a complex codec. This crate parses the bitstream framing, the table of contents (ac4_toc()), presentations and substream descriptors per ETSI TS 103 190-1 V1.4.1, and exposes a decoder that emits PCM with the right shape. Mono ASF, stereo CPE (split + joint MDCT), full A-SPX front-end, A-CPL channel-pair synthesis (ASPX_ACPL_1 / ASPX_ACPL_2), DRC + DE + outer metadata walker are all implemented. Round 20 unblocks the ETSI Huffman-table audit (60 codebooks validated byte-for-byte against the canonical ETSI accompaniment file in tests/etsi_table_validation.rs) and wires the 5.X channel-element walker family's Cfg0 / Cfg1 / Cfg2 outer shells plus a Table-21-correct sf_info_lfe() parser. Round 21 lands the §5.7.7.6.2 ASPX_ACPL_3 transform-matrix synthesis math (Pseudocodes 118/119 — Transform(), ACplModule2(), ACplModule3() and the full 5-channel pipeline run_pseudocode_118_5x()). Round 22 lands §5.7.7.6.1 ASPX_ACPL_1 / ASPX_ACPL_2 multichannel wrappers (Pseudocode 117 — run_pseudocode_117_5x(): two parallel ACplModule's with D0/D1 decorrelators) plus the 5_X-walker glue: PCM-level helpers run_acpl_5x_pair_pcm() (ASPX_ACPL_1/2) and run_acpl_5x_mch_pcm() (ASPX_ACPL_3) consume the parsed acpl_config_* + acpl_data_* to produce 5-channel L/R/C/Ls/Rs PCM end-to-end via QMF analysis → A-CPL → QMF synthesis. Round 23 wires the per-channel sf_data(ASF) Huffman bodies for the multichannel layouts (Tables 26 / 27 / 28 / 29): parse_two_channel_data / parse_three_channel_data / parse_four_channel_data / parse_five_channel_data now also walk the trailing 2 / 3 / 4 / 5 sf_data(ASF) calls through decode_mch_sf_data_channels() and deposit the per-channel scaled MDCT spectra on each *ChannelData::scaled_spec_per_channel for the long-frame, single-window-group case. The Huffman codebook IDs reused are HCB_1..HCB_11 (spectral lines), HCB_SCALEFAC (scale-factor DPCM) and HCB_SNF (spectral noise fill) — Annex A.1 shares the codebooks across mono / stereo / multichannel; there is no separate "MCH" codebook set. Round 24 closes the two r23 follow-ups: (1) the grouped / short-frame multichannel sf_data(ASF) walker (num_window_groups > 1) is now driven by decode_asf_grouped_mono_body_with_max_sfb() — each per-channel spectrum is the concatenation of num_window_groups independent (section + spectral + scalefac + snf) chains, group-major; (2) the ASPX_ACPL_3 inner body walker is now wired in parse_5x_audio_data_outer — on an I-frame the walker chains stereo_data() + aspx_data_2ch() + acpl_data_2ch() and the parsed tools.acpl_data_2ch flows straight into the §5.7.7.6.2 Pseudocode-118 5_X synthesis pipeline. The Table-52 aspx_data_2ch() body parser was factored out of the stereo CPE ASPX path into a shared parse_aspx_data_2ch_body() helper — both the stereo CPE mode and the 5_X ASPX_ACPL_3 mode use the same parser. Round 25 wires the ASPX_ACPL_1 / ASPX_ACPL_2 inner body walker in parse_5x_audio_data_outer per §4.2.6.6 Table 25 (case ASPX_ACPL_1: case ASPX_ACPL_2:): a new parse_aspx_acpl_1_2_inner_body() helper walks two_channel_data() / three_channel_data() (selected by the 1-bit coding_config), the ASPX_ACPL_1-only joint-MDCT residual layer (max_sfb_master + 2x chparam_info + 2x sf_data(ASF) over the dominant transform length signalled by the upstream channel data — n_side_bits is derived per the §4.2.6.6 NOTE), the optional Cfg0 trailer mono_data(0), then aspx_data_2ch() + aspx_data_1ch() and finally the two parallel acpl_data_1ch() calls per Pseudocode 117. The pair lands in tools.acpl_data_1ch_pair[0/1] (D0 / D1 ACplModule). The walker is try-and-bail: any inner Huffman / parse miss leaves the already-populated tools.* slots intact and returns silently. Round 27 lands the 7_X channel-element walker (parse_7x_audio_data_outer) per §4.2.6.14 Table 33 — immersive 7.0 and 7.1 streams now parse end-to-end. The 7.X walker mirrors the 5_X SIMPLE/ASPX path's coding_config selector but has its own quirks: 2-bit 7_X_codec_mode (no ASPX_ACPL_3 in 7.X), companding_control(5) only on ASPX_ACPL_{1,2}, the centre/back monos move out of the coding_config switch into a single trailing mono_data(0) gated on coding_config in {0, 2}, and a SIMPLE/ASPX-only additional-channel block (b_use_sap_add_ch + optional chparam_info×2 + two_channel_data) carries the front-extension / back-surround pair beyond the 5.X core. walk_ac4_substream now dispatches channels == 7/8 (7.0/7.1) into the new walker. Round 28 lands the mono / stereo short-frame sf_data(ASF) walker per ETSI TS 103 190-1 §4.2.8.3-6 Tables 39-42: new spec-correct _grouped payload parsers in asf_data.rs (each with its own outer for (g = 0; g < num_window_groups; g++) loop, a single 8-bit reference_scale_factor at the head of asf_scalefac_data() with first_scf_found carrying across groups, and a single 1-bit b_snf_data_exists gate at the head of asf_snf_data()), plus derive_per_group() helpers that resolve per-group (transf_length_idx, transform_length, max_sfb) from (ti, psy) per Pseudocodes 2 / 3 / 5 (handling the b_different_framing half-frame split). New body decoders decode_asf_grouped_mono_body[_with_max_sfb]() and decode_asf_grouped_stereo_joint_body() (shared section, per-group ms_used[g][sfb], inverse M/S) are wired into all four mono / stereo call sites: parse_mono_audio_data_outer, parse_aspx_acpl2_mdct_body, parse_aspx_acpl1_mdct_body (joint + split) and parse_stereo_data_body (joint + split). Real Dolby AC-4 mono / stereo streams using short-window sub-frames now decode end-to-end without bailing at the previous num_window_groups != 1 guard. Round 29 lands the full §5.2.8 SSF arithmetic decoder + Annex C scalar inventory + 37 prediction-coefficient matrices — 705-entry CDF_TABLE, PREDICTOR_GAIN_CDF_LUT, ENVELOPE_CDF_LUT, DITHER_TABLE / RANDOM_NOISE_TABLE, STEP_SIZES_Q4_15, AC_COEFF_MAX_INDEX, the four C.14 dB↔linear LUTs, plus AcState (init / decode_target / decode_symbol_ext_cdf / decode_symbol_calc_cdf / decode_finish per Pseudocodes 41-47), the Idx2Reconstruction + CdfEst computed-CDF path (Pseudocodes 51-53), envelope / predictor-gain / coefficient convenience entry points (Pseudocodes 48-50), and the SsfRandGenState dither + noise RNG (Pseudocodes 54-57). Round 30 lands the SSF bitstream walker (ssf::parse_ssf_data / parse_ssf_granule / parse_ssf_st_data / parse_ssf_ac_data per Tables 43-46), the Annex C.1 SSF-bandwidths matrix (SSF_BANDWIDTHS, 19 bands × 8 block-length columns), SsfBinLayout::build() (Pseudocode 7 — start_bin[] / end_bin[] / num_bins), SsfFrameConfig (Tables 112-113), and the SsfChannelState carrying RNG / prev_pred_lag_idx / last_num_bands / env_prev[] across granules. Wired into walk_ac4_substream for mono SIMPLE/ASPX, split-MDCT stereo, and the ASPX_ACPL_1 split residual layer — spec_frontend == SSF no longer falls through silently. Round 31 lands the §5.2.3-5.2.7 SSF PCM synthesis chain in a new ssf_synth module: envelope decoder + predictor + helpers + lossless decode + inverse-quant + subband predictor + inverse-flattening (Pseudocodes 4a / 4b / 4c / 4d / 4e / 26 / 31 / 32 / 33 / 34 / 35 / 36 / 37 / 38) plus the C-matrix reconstruction (Pseudocode 39) for all 37 tab_idx values. synthesize_ssf_data() threads env_prev[] between granules. Ac4Decoder now carries a per-channel Vec<SsfSynthState> and consumes tools.ssf_data_primary / tools.ssf_data_secondary after the ASF/A-CPL pipeline: each granule's num_blocks * n_mdct spectrum is split per-block and IMDCT'd through the existing KBD overlap-add path. SSF substreams now emit real PCM in place of silence. §5.2.5.2.2 Heuristic Scaling (Pseudocodes 27 / 28 / 29 / 30) is deferred — the spec's f_rfu == 0 short-circuit covers any block with the predictor disabled, which the current synth supports. Round 32 closes the SHORT_STRIDE P-frame correctness gap by adding env_prev: Vec<i32> to SsfSynthState: synthesize_granule() latches the resolved envelope (post- decode_envelope δ-chain) at the end of each granule so that a SHORT_STRIDE P-granule with no caller-supplied env_prev[] interpolates against the previous frame's envelope rather than a zero fallback (§5.2.3.0 Note 2). The walker side gets a parallel hoist: Ac4Decoder now owns a Vec<SsfChannelState> (ssf_walker_state) and a new walk_ac4_substream_stateful() threads it through the SSF body parses so dither / noise RNGs (Pseudocodes 54-57) and prev_pred_lag_idx / last_num_bands persist across frames — pre-r32 the walker built a fresh state per frame and dropped it. Round 33 lands §5.2.5.2.2 Heuristic Scaling (Pseudocodes 27/28/29/30) — the predictor-enabled spectrum-decoding branch the spec's f_rfu == 0 short-circuit previously skipped. New map_db_to_lin_q10() / map_lin_to_db_q10() Q.10 fixed-point converters use the Annex C.14 LUTs; heuristic_scaling() runs the full Pseudocode 28 chain (dynamic-range compression of env_in[], sorted-descending Map_dB_to_Lin, iRfu²-weighted reverse water-filling, Map_Lin_to_dB-driven per-band weight); apply_heuristic_scaling() wraps it with the Pseudocode 27 env_in = 3 * env_alloc pre-multiply, LF-boost, and (env_alloc_mod, f_gain_q) post-processing. synthesize_granule() dispatches the §5.2.5.2.0 selector — when f_rfu > 0 && !variance_preserving the heuristic-scaling branch fires and inverse_heuristic_scale() consumes the resulting f_gain_q[] instead of the all-1 stub; variance_preserving blocks correctly skip the inverse-scale call per §5.2.5.2.0 step 5. Round 34 lands FIXVAR / VARFIX / VARVAR atsg border derivation (§5.7.6.3.3.2 Pseudocode 77) — new derive_fixvar_atsg(), derive_varfix_atsg(), derive_varvar_atsg() and a unified derive_atsg_borders() dispatcher cover all four aspx_int_class values; the decoder's TNS and envelope-adjustment paths now route through derive_atsg_borders instead of the FIXFIX-only path, enabling A-SPX bandwidth extension for FIXVAR / VARFIX / VARVAR substreams. §5.1.4 SNF injection: inject_snf_noise() fills zero-energy MDCT bins using a 16-bit LCG (multiplier 69069, addend 1) and gain formula 2^((idx×1.5−84)/4); the long-mono ASF decode path now consumes parse_asf_snf_data() output instead of discarding it. 5_X ASPX_ACPL_3 wired in Ac4Decoder: two new persistent state fields (acpl_5x_mch_state / acpl_5x_pair_state) are added; when acpl_config_2ch + acpl_data_2ch + stereo carrier spectra are present, run_acpl_5x_mch_pcm() (Pseudocode 118) fires and fills pcm_per_channel[0..5] with L/R/C/Ls/Rs surround PCM. Round 35 lands the §4.2.4.4 EMDF payloads substream parser (Table 18) plus the §4.2.14.14 emdf_payload_config() (Table 79) in a new emdf module — parse_emdf_payloads_substream() walks the while-loop until the emdf_payload_id == 0 terminator, handles the id == 31 → variable_bits(5) extension, decodes the full EmdfPayloadConfig (sample-offset / duration / group-id / codecdata / discard / frame-aligned + create-/remove-duplicate / priority / proc_allowed gates per the Table 79 conditional tree), and captures each payload's emdf_payload_byte[] verbatim. Defensive caps (MAX_EMDF_PAYLOADS = 64, MAX_EMDF_PAYLOAD_BYTES = 65 536) bound malformed input. The outer metadata::parse_metadata walker now consumes the substream when b_emdf_payloads_substream == 1 and surfaces it through Metadata::emdf_payloads_substream instead of erroring out with "not yet implemented" — real-bitstream metadata can now fully round-trip through the walker. Round 35 also lands the §5.7.9.3.3 PCM gain application path: drc::drc_raw_to_linear() maps a 7-bit drc_gain[ch][sf][band] value to its linear multiplier via 2^((raw-64)/6), dialnorm_correction_linear() resolves the 2^((Lout-Lin)/6) dialnorm correction, and drc::apply_drc_gains_to_pcm() applies a parsed DrcGains (per channel-group, per subframe — multi-band averaged in the linear domain) to a planar &mut [Vec<f32>] PCM buffer with a DrcChannelMap (helpers for the [L, R, C, LFE?, Ls, Rs] 5_X layout and the wideband single-group mono/stereo case). DE walker hardened with three new edge-case tests covering EOF on truncation, non-I-frame without prev_config, and the nr_channels == 0 degenerate case. Round 51 lands stereo SIMPLE/ASF split-MDCT (Path A: 2× SCE) encoding per ETSI TS 103 190-1 §5.3 + §4.2.6.3 — Ac4ImsEncoder::encode_frame_pcm_stereo accepts L+R PCM frames, runs the existing forward MDCT + scalefactor + DP-section + HCB1..11 codebook-selection + SNF emission pipeline independently per channel, and emits a b_enable_mdct_stereo_proc == 0 stereo CPE body the decoder reconstructs at ≥24.8 dB spectral SNR for both 440 Hz L+R (matched) and 440 Hz L + 660 Hz R (independent) tone fixtures. Round 52 lands the matching joint M/S CPE (Path B, b_enable_mdct_stereo_proc == 1) encoder per §5.3 + §4.2.6.3

• §7.5: encode_frame_pcm_stereo now dispatches between Path A and Path B based on an energy-weighted per-SFB correlation rising above the 0.7 threshold. Path B emits one shared asf_section_data
• two asf_spectral_data (M/S or L/R per band based on bit-cost comparison) + shared asf_scalefac_data + per-active-sfb ms_used flags + shared asf_snf_data. A frame-level "matched-channels" gate (S/M energy ratio < 0.15) bumps the M-channel q_target up to 16 to spend the bits saved on the silent / near-silent S residual on finer M quantisation. End-to-end SNR on the 440 Hz L=R matched fixture rises from round-51's 24.8 dB to 34.5 dB; the half- correlated 440 Hz amplitude-imbalance fixture clears ≥ 26 dB; independent 440 L + 660 R correctly routes via Path A and preserves the round-51 SNR floor. Round 74 lands the first multichannel forward-analysis encoder: a 5.0 SIMPLE/Cfg3Five path (5 SCE, no LFE, no joint coding) per §4.2.6.6 Table 25 row case SIMPLE: coding_config == 3 + §4.2.7.5 Table 29 (five_channel_data()). New Ac4ImsEncoder::with_5_0() flips the TOC channel_mode prefix to 0b1101 (4 b — Table 85 channel_mode 3), and encode_frame_pcm_5_0(&[L, R, C, Ls, Rs]) runs the round-50 forward pipeline (KBD-MDCT + DP-optimal sectioning + HCB1..11
• SNF) independently per channel into a shared sf_info(ASF, 0, 0) header followed by five_channel_info() (identity SAP: chel_matsel = 0 + 5x chparam_info with sap_mode = 0) and 5x sf_data(ASF) bodies. Decoder's existing dispatch_5x_cfg3_simple_aspx (round 39) consumes the body and emits 5-channel interleaved S16 PCM. End-to-end per- channel spectral SNR ≥ 20 dB on the independent-tone fixture (220/440/660/880/1100 Hz on L/R/C/Ls/Rs — measured L=24.5 / R=24.8 / C=25.0 / Ls=23.4 / Rs=27.4 dB), matching the round-51 stereo Path A SNR floor. Round 80 extends the multichannel encoder to 5.1 (Cfg3Five + LFE) per §4.2.6.6 Table 25 (if (b_has_lfe) mono_data(1);) + §4.2.8 (sf_info_lfe() Table 35 / Table 106 column 4 n_msfbl_bits): Ac4ImsEncoder::encode_frame_pcm_5_1(&[L, R, C, Ls, Rs, LFE]) flips the TOC channel_mode prefix to 0b1110 (4 b — Table 85 channel_mode 4) and build_5_1_simple_asf_body_from_pcm_spectra prepends an LFE mono_data(1) element (b_long_frame = 1 + sf_info_lfe() with max_sfb_lfe capped to n_msfbl_bits = 3 → 7 sfb / ≈350 Hz at tl = 1920) before the Cfg3Five five_channel_data() body. The decoder side gains a matching LFE PCM render in Ac4Decoder::receive_frame: when channels == 6 (5.1) or channels == 8 (7.1) and tools.lfe_mono_data.scaled_spec is present, the LFE spectrum is IMDCT'd into the trailing PCM slot (slot 5 for 5.1, slot 7 for 7.1). Non-LFE per-channel SNR matches the 5.0 numbers (L=24.5 / R=24.8 / C=25.0 / Ls=23.4 / Rs=27.4 dB ≥ 20 dB floor); a 60 Hz LFE tone round-trips to a non-silent reconstructed LFE channel. Round 91 extends the multichannel encoder to 7.1 (3/4/0.1) SIMPLE/Cfg3Five + LFE per §4.2.6.14 Table 33 + §4.2.7.4 Table 26 (additional-channel two_channel_data()) + Table 88 channel_mode 6 (0b1111001, 7 b): Ac4ImsEncoder::with_7_1() + encode_frame_pcm_7_1(&[L, R, C, Ls, Rs, Lb, Rb, LFE]) emit a 7_X SIMPLE/Cfg3Five body whose inner five_channel_data() reuses the round-80 5.1 forward pipeline, followed by b_use_sap_add_ch = 0 identity-SAP + two_channel_data() for the immersive Lb/Rb pair. The decoder side gains the inner 5-channel core render for the 7_X SIMPLE/Cfg3Five path (it previously parsed but never IMDCT'd slots 0..4 — only the round-39 additional-pair slots 5/6 and the round-80 LFE slot 7 were touched); slots 0..4 now route through dispatch_5x_cfg3_simple_aspx (the inner SCE shape is identical to the 5_X Cfg3Five case). Per-channel spectral SNR on the 220/440/660/880/1100/1320/1540 Hz independent-tone 7.1 fixture: L=24.5 / R=24.8 / C=25.0 / Ls=23.4 / Rs=27.4 / Lb=25.4 / Rb=26.0 dB — all above the ≥ 20 dB floor. Round 95 lands the 5_X SIMPLE/ASPX_ACPL_3 multichannel encoder path per §4.2.6.6 Table 25 row case ASPX_ACPL_3: — symmetric counterpart to the round-34 decoder ACPL_3 walker (5a58f6a). Ac4ImsEncoder::encode_frame_pcm_5_0_acpl3(&[L, R, C]) and encode_frame_pcm_5_1_acpl3(&[L, R, C, LFE]) emit IMS v2 frames with 5_X_codec_mode = 4 (ASPX_ACPL_3). The new encoder_acpl3 module ships bit-exact emitters for aspx_config() (Table 50, 15 b), acpl_config_2ch() (Table 60, 4 b), companding_control(2) (Table 49, 2 b), plus minimum-bit-cost zero-delta Huffman writers covering all 18 ASPX HCBs (Annex A.2 Tables A.16-A.33) and all 24 ACPL HCBs (Annex A.3 Tables A.34-A.57) — pick_zero_delta_cw picks the entry at index == cb_off (zero delta for DF/DT) and pick_min_len_cw picks the smallest-length entry (used for F0 seeds). The body layout is 5_X_codec_mode = 4 (3 b) + I-frame aspx_config() + acpl_config_2ch() + optional LFE mono_data(b_lfe = 1) + companding_control(2) + stereo_data() (split-MDCT path, two carrier channels) + aspx_data_2ch() (FIXFIX num_env=1, balance=1, all-FREQ deltas) + acpl_data_2ch() (1 param set, 7 param bands, 11 EC streams with diff_type = 0 and zero-delta DF). Decoder round-trip: 5.0 ACPL_3 → 5-channel S16 interleaved PCM (1920 samples × 5 ch × 2 bytes); 5.1 ACPL_3 → 6-channel S16 (with LFE slot 5). The decoder walks the full Table 25 body and resolves five_x_mode == AspxAcpl3
• acpl_config_2ch.is_some() && acpl_data_2ch.is_some(). The 5-channel [L, R, C, Ls, Rs] synthesis runs via acpl_synth::run_acpl_5x_mch_pcm (Pseudocode 118) — with all-zero ACPL parameter deltas Ls/Rs collapses to ducker-driven reconstruction from the L/R carriers (non-silent in the general case). Total tests 691 (was 680). Real per-band (alpha, beta, gamma) parameter extraction (replacing the zero-delta scaffold), real ASPX envelope coding, and matching encoder paths for 5_X_codec_mode in {ASPX_ACPL_1, ASPX_ACPL_2} (Pseudocode 117) remain deferred. The 7_X paths inherit the same aspx_data / acpl_data shape as 5_X so they're queued behind the 5_X work continuing to harden. Round 100 lands the 5_X SIMPLE/ASPX_ACPL_2 multichannel encoder path per §4.2.6.6 Table 25 row case ASPX_ACPL_2: — the symmetric counterpart to the round-25 decoder parse_aspx_acpl_1_2_inner_body walker (Pseudocode 117). Ac4ImsEncoder::encode_frame_pcm_5_0_acpl2(&[L, R, C]) emits an IMS v2 frame with 5_X_codec_mode = ASPX_ACPL_2 (3) whose body is aspx_config() + acpl_config_1ch(FULL) + companding_control(3) + coding_config = 0 + two_channel_data() (L/R carriers) + mono_data(0) (centre) + aspx_data_2ch() + aspx_data_1ch() + two acpl_data_1ch() parameter sets. The ASPX_ACPL_1-only joint-MDCT residual layer (max_sfb_master + 2× chparam_info + 2× sf_data) is skipped for ACPL_2 — that's the structural difference that makes the ACPL_2 path the cleanest encoder win. New encoder_acpl3 emitters: write_acpl_config_1ch_full (Table 59, 3 b), write_two_channel_data (Table 26 — shared sf_info(ASF) + identity-SAP chparam_info + 2× sf_data), write_mono_data_centre (Table 21 non-LFE), write_aspx_data_1ch_minimal (Table 51 FIXFIX num_env=1) and write_acpl_data_1ch_minimal (Table 61). The 1ch ASPX SIGNAL band count uses num_sbg_sig_highres to match parse_aspx_ec_data's empty-freq_res fallback. Decoder round-trip: 5.0 ACPL_2 → 5-channel S16 interleaved PCM; the decoder resolves five_x_mode == AspxAcpl2, walks acpl_config_1ch_full, two_channel_data, the Cfg0 centre mono_data(0), and both acpl_data_1ch_pair entries, then synthesises [L, R, C, Ls, Rs] via acpl_synth::run_acpl_5x_pair_pcm. Total tests 700 (was 691). The ASPX_ACPL_1 encoder path (joint-MDCT residual + PARTIAL-mode acpl_config_1ch with acpl_qmf_band), real (alpha, beta) parameter extraction, and the 7_X ASPX_ACPL_{1,2} encoder paths remain deferred. Round 103 lands the 5_X SIMPLE/ASPX_ACPL_1 multichannel encoder path per §4.2.6.6 Table 25 row case ASPX_ACPL_1: — the round-100 follow-up and the symmetric counterpart to the decoder's round-25 parse_aspx_acpl_1_2_inner_body ASPX_ACPL_1 branch (Pseudocode 117). Ac4ImsEncoder::encode_frame_pcm_5_0_acpl1(&[L, R, C, Ls, Rs]) emits an IMS v2 frame with 5_X_codec_mode = ASPX_ACPL_1 (2). The body differs from the ACPL_2 path in two structural places: (1) acpl_config_1ch is PARTIAL — write_acpl_config_1ch_partial (Table 59, 6 b: id + quant_mode + acpl_qmf_band_minus1), so the acpl_data_1ch() start_band resolves from qmf_band via sb_to_pb; (2) the body carries an explicit joint-MDCT residual layer — write_acpl_1_residual_layer emits max_sfb_master (n_side bits) + 2× identity-SAP chparam_info + 2× sf_data(ASF) for the Ls/Rs surround pair (sSMP,3 / sSMP,4 per Table 181), so the encoder takes a full 5-channel input instead of reconstructing Ls/Rs from the L/R carriers. New build_5_x_acpl1_body_from_pcm_spectra lays out 5_X_codec_mode = 2 + aspx_config() + acpl_config_1ch(PARTIAL) + companding_control(3) + coding_config = 0 + two_channel_data() + residual-layer + mono_data(0) + aspx_data_2ch() + aspx_data_1ch() + 2× acpl_data_1ch(). Decoder round-trip: 5.0 ACPL_1 → 5-channel S16 interleaved PCM; the decoder resolves five_x_mode == AspxAcpl1, the PARTIAL config, the persisted residual pair, the Cfg0 centre, and both acpl_data_1ch_pair entries, then synthesises [L, R, C, Ls, Rs] via acpl_synth::run_acpl_5x_pair_pcm. Total tests 708 (was 700). Real per-band (alpha, beta) extraction, real ASPX envelope coding, real Table-181 SAP-derived residual content (the residual sf_data currently codes the raw Ls/Rs spectra), and the 7_X ASPX_ACPL_{1,2} encoder paths remain deferred. Round 107 lands the first of those deferred 7_X encoder paths: the 7.0 SIMPLE/ASPX_ACPL_2 multichannel encoder per §4.2.6.14 Table 33 row case ASPX_ACPL_2: — the 7_X (immersive) counterpart to the round-100 5_X ASPX_ACPL_2 path and the encoder side of the decoder's round-27 parse_7x_audio_data_outer ASPX_ACPL_2 branch. Ac4ImsEncoder::encode_frame_pcm_7_0_acpl2(&[L, R, C, Ls, Rs, Lb, Rb]) emits an IMS v2 frame with 7_X_codec_mode = ASPX_ACPL_2 (3) and channel_mode prefix 0b1111000 (7 b — Table 85 channel_mode 5, 7.0 (3/4/0)). The new encoder_acpl3::build_7_x_acpl2_body_from_pcm_spectra reuses the same 1ch ACPL / ASPX emitters as the 5_X ACPL_2 path but lays out the 7_X channel element's distinct framing: 2-bit 7_X_codec_mode (vs the 5_X 3-bit field), companding_control(5) (sync-on 2-bit wire shape), 2-bit coding_config = 0 (Cfg0), b_2ch_mode + two_channel_data() (L/R) + two_channel_data() (Ls/Rs), a trailing centre mono_data(0) moved out of the coding_config switch, and an aspx_data_2ch() + aspx_data_2ch() + aspx_data_1ch() envelope trailer before the two acpl_data_1ch() parameter sets (Pseudocode 117 D0/D1). The ASPX_ACPL_1-only joint-MDCT residual layer and the SIMPLE/ASPX additional-channel block are skipped for ACPL_2. Decoder round-trip: 7.0 ACPL_2 → 7-channel S16 interleaved PCM (1920 samples × 7 ch × 2 bytes); the decoder resolves seven_x_mode == AspxAcpl2, both two_channel_data pairs, the Cfg0 centre, and both acpl_data_1ch_pair entries, then synthesises [L, R, C, Ls, Rs] (slots 0..4) via acpl_synth::run_acpl_5x_pair_pcm (the back pair Lb/Rb stays silent per the Table 202 ACPL_2 mapping). Total tests 714 (was 708). The 7.1 (LFE) ASPX_ACPL_2 path, the 7_X ASPX_ACPL_1 path (PARTIAL config + joint-MDCT residual), real per-band (alpha, beta) extraction, real ASPX envelope coding, and back-pair Lb/Rb carriage remain deferred. Round 114 closes the first of those deferred items — the 7.1 (3/4/0.1) SIMPLE/ASPX_ACPL_2 multichannel encoder path per §4.2.6.14 Table 33 row case ASPX_ACPL_2: with b_has_lfe = 1 + §4.2.6.5 Table 21 (mono_data(b_lfe)) + §4.2.8 Table 35 (sf_info_lfe()). Ac4ImsEncoder::encode_frame_pcm_7_1_acpl2(&[L, R, C, Ls, Rs, Lb, Rb, LFE]) emits the round-107 7.0 ASPX_ACPL_2 body plus a leading mono_data(b_lfe = 1) element between the I-frame config block and companding_control(5) — exactly where the decoder's parse_7x_audio_data_outer(b_has_lfe = true) reads if (b_has_lfe) mono_data(1);. The channel_mode prefix is forced to 0b1111001 (7 b — Table 88 channel_mode 6) so the decoder dispatches channels == 8. build_7_x_acpl2_body_from_pcm_spectra gained max_sfb_lfe: Option<u32> + coeffs_lfe: Option<&[f32]> and reuses the shared round-80 write_lfe_mono_data emitter (max_sfb_lfe capped to n_msfbl_bits = 3 → 7 sfb at tl = 1920). Decoder round-trip: 7.1 ACPL_2 → 8-channel S16 interleaved PCM (1920 samples × 8 ch × 2 bytes); the LFE spectrum IMDCT's into slot 7 via the round-80 LFE render, the [L, R, C, Ls, Rs] slots 0..4 synthesis is unchanged, and the back pair Lb/Rb (slots 5/6) stays silent per the Table 202 ACPL_2 mapping. A 60 Hz LFE tone round-trips to a non-silent reconstructed LFE channel. Total tests 721 (was 714). The 7_X ASPX_ACPL_1 path (PARTIAL config + joint-MDCT residual), real per-band (alpha, beta) extraction, real ASPX envelope coding, and back-pair Lb/Rb carriage remain deferred. Round 118 closes the first of those — the 7.0 / 7.1 (3/4/0(.1)) SIMPLE/ASPX_ACPL_1 multichannel encoder path per §4.2.6.14 Table 33 row case ASPX_ACPL_1: (the 7_X analogue of the round-103 5_X ACPL_1 path). Ac4ImsEncoder::encode_frame_pcm_7_0_acpl1(&[L, R, C, Ls, Rs, Lb, Rb]) and encode_frame_pcm_7_1_acpl1(&[.., LFE]) emit IMS v2 frames with 7_X_codec_mode = ASPX_ACPL_1 (2). New encoder_acpl3::build_7_x_acpl1_body_from_pcm_spectra is the round-107/114 7_X ACPL_2 body with the three ACPL_1 differences: 7_X_codec_mode = 2 (not 3), acpl_config_1ch PARTIAL (write_acpl_config_1ch_partial, carries acpl_qmf_band_minus1 → acpl_data_1ch() start_band via sb_to_pb), and an explicit joint-MDCT residual layer (write_acpl_1_residual_layer: max_sfb_master + 2× chparam_info + 2× sf_data(ASF) for the Ls/Rs surround pair sSMP,3 / sSMP,4 per Table

181. after the two two_channel_data() pairs and before the trailing Cfg0 centre mono_data(0). The 7.1 form prepends the round-80 write_lfe_mono_data LFE element. Decoder round-trip: 7.0 ACPL_1 → 7-channel S16 PCM; 7.1 ACPL_1 → 8-channel S16 (LFE IMDCT'd into slot 7); [L, R, C, Ls, Rs] synthesise via acpl_synth::run_acpl_5x_pair_pcm, Lb/Rb (slots 5/6) silent per Table 202. Total tests 729 (was 721). Real per-band (alpha, beta) extraction, real ASPX envelope coding, real Table-181 SAP-derived residual content, and back-pair Lb/Rb carriage remain deferred. Round 125 lands the 7.0 (3/4/0) SIMPLE/Cfg3Five multichannel encoder path per ETSI TS 103 190-1 §4.2.6.14 Table 33 + §4.2.7.5 Table 29 (five_channel_data()) + §4.2.7.4 Table 26 (additional-channel two_channel_data()) — the non-LFE immersive counterpart of round-91's 7.1 encoder (the 7_X analogue of round 74's 5.0 vs round 80's 5.1). Ac4ImsEncoder::with_7_0() flips the TOC channel_mode prefix to 0b1111000 (7 b — Table 85 channel_mode 5, 7.0 (3/4/0) → 7 channels), and encode_frame_pcm_7_0(&[L, R, C, Ls, Rs, Lb, Rb]) (+ ..._with_max_sfb(.., max_sfb, max_sfb_add)) emits a SIMPLE/Cfg3Five 7_X channel-element body whose inner five_channel_data() reuses the round-80 5.1 forward pipeline for the L/R/C/Ls/Rs front/surround pair and whose trailing identity-SAP two_channel_data() (b_use_sap_add_ch = 0) carries the immersive Lb/Rb pair. The body is structurally the round-91 7.1 body with the leading mono_data(b_lfe = 1) element omitted (the walker's if (b_has_lfe) mono_data(1); branch is gated off for channel_mode 5). New encoder_asf::build_7_0_simple_asf_body_from_pcm_spectra emits the body bytes; decoder round-trip: 7.0 → 7-channel S16 interleaved PCM (1920 samples × 7 ch × 2 bytes). The 7.0 walker resolves seven_x_mode == Simple, seven_x_b_has_lfe == false, five_channel_data populated, identity-SAP additional-channel pair populated (slots 5/6 = Lb/Rb routed via dispatch_7x_additional_channel_pair). Per-channel spectral SNR on the 220/440/660/880/1100/1320/1540 Hz independent-tone 7.0 fixture: L=24.5 / R=24.8 / C=25.0 / Ls=23.4 / Rs=27.4 / Lb=25.4 / Rb=26.0 dB — all above the ≥ 20 dB floor. Total tests 737 (was 729). Round 128 lands the first real per-parameter-band α extraction in the ACPL_1 5.0 encoder per ETSI TS 103 190-1 §5.7.7.5 Pseudocode 116 + §5.7.7.6.1 Pseudocode 117 — replaces the round-103 zero-delta α scaffold (β / β3 / γ stay at the round-95 / 100 / 103 scaffold; β3 / γ only fire in ASPX_ACPL_3 anyway). With β = 0 the surround reconstruction is Ls_recon = 0.5/√2 · L · (1 − α); solving for α per parameter band gives the closed form α = 1 − 2·√2 · ⟨L, Ls⟩ / ⟨L, L⟩. New encoder_acpl3::build_5_x_acpl1_body_from_pcm_spectra_real_alpha

• helper chain (compute_per_band_correlations mapping MDCT bins → QMF subbands → A-CPL parameter bands via §5.7.7.2 Table 197, analytic_alpha_per_band + quantise_alpha against Tables 203 / 205, then write_acpl_alpha_f0_value / write_acpl_alpha_df_value emit the ALPHA F0 + DF codewords per Tables A.35 / A.34); new Ac4ImsEncoder::encode_frame_pcm_5_0_acpl1_real_alpha entry point alongside the round-103 zero-delta variant. The on-wire body structure is unchanged — decoder resolves FiveXCodecMode::AspxAcpl1, both acpl_data_1ch_pair[0/1] populated, joint-MDCT residual layer walked, [L, R, C, Ls, Rs] synthesised via acpl_synth::run_acpl_5x_pair_pcm. Total tests 743 (was 737). Round 132 adds the first real per-parameter-band β extraction in the ACPL_1 5.0 encoder per ETSI TS 103 190-1 §5.7.7.5 Pseudocode 116 + §5.7.7.6.1 Pseudocode 117 — replaces the round-95/100/103/128 zero-β scaffold for the ACPL_1 path. With the decorrelator output y ⊥ x0 and E[y²] ≈ E[x0²], the surround energy balance is E[Ls²] = 0.5·E[x0²]·((1−α)² + β²), so the per-band β magnitude is β = √max(0, 2·E[Ls²]/E[x0²] − (1−α_dq)²). New encoder_acpl3::build_5_x_acpl1_body_from_pcm_spectra_real_alpha_beta
• helper chain (compute_per_band_energies, analytic_beta_per_band, quantise_beta_magnitude against Tables 204/206, write_acpl_beta_f0_value / write_acpl_beta_df_value per Tables A.40/A.41) and the new Ac4ImsEncoder::encode_frame_pcm_5_0_acpl1_real_alpha_beta entry point. The on-wire body structure is unchanged; the β coding contract round-trips byte-exact through acpl::parse_acpl_data_1ch. Total tests 755 (was 743). Real β extraction for the 7_X / ACPL_2 / ACPL_3 paths, real γ extraction, and the round-128 ALPHA-writer negative-alpha_q desync fix remain deferred. Round 135 extends the real per-band α + β extraction to the 7_X (7.0 immersive) ASPX_ACPL_1 path — the round-132 followup. New encoder_acpl3::build_7_x_acpl1_body_from_pcm_spectra_real_alpha_beta (the real-α+β upgrade of the round-118 zero-delta 7_X builder, reusing the extract_alpha_q_per_band / extract_beta_q_per_band primitives) and the Ac4ImsEncoder::encode_frame_pcm_7_0_acpl1_real_alpha_beta entry point; both trailing acpl_data_1ch() sets now carry real α + β. The on-wire body structure is unchanged — decoder resolves SevenXCodecMode::AspxAcpl1 (b_has_lfe = false), both acpl_data_1ch_pair[0/1] populated, joint-MDCT residual layer walked. Total tests 760 (was 755). Round 139 extends the real per-band α + β extraction to the 7.1-with-LFE (3/4/0.1) ASPX_ACPL_1 path — the round-135 LFE follow-up. New Ac4ImsEncoder::encode_frame_pcm_7_1_acpl1_real_alpha_beta (and the _with_max_sfb form) reuses the round-135 build_7_x_acpl1_body_from_pcm_spectra_real_alpha_beta builder with the LFE coeffs_lfe + max_sfb_lfe slots populated, emitting a leading mono_data(b_lfe = 1) element (Table 21 + sf_info_lfe() Table 35) between the I-frame config block and companding_control(5) exactly where the decoder's parse_7x_audio_data_outer(b_has_lfe = true) reads if (b_has_lfe) mono_data(1);. The on-wire body structure matches the existing round-118 7.1 ACPL_1 path — decoder resolves SevenXCodecMode::AspxAcpl1 with b_has_lfe = true, both acpl_data_1ch_pair[0/1] populated (now carrying real α + β), joint-MDCT residual layer walked, LFE IMDCT'd into slot 7. A 60 Hz LFE tone round-trips to a non-silent reconstructed LFE channel. Total tests 766 (was 760). Real β extraction for the ACPL_2 / ACPL_3 paths and the round-128 ALPHA-writer negative-alpha_q desync fix remain deferred. Round 144 closes the ACPL_2 5.0 half of the deferred list with the 5_X SIMPLE/ASPX_ACPL_2 encoder with real per-parameter-band α + β extraction per §4.2.6.6 Table 25 row case ASPX_ACPL_2: + §5.7.7.5 Pseudocode 116 + §5.7.7.6.1 Pseudocode 117. New Ac4ImsEncoder::encode_frame_pcm_5_0_acpl2_real_alpha_beta (and the _with_max_sfb form) accepts a 5-channel [L, R, C, Ls, Rs] input and produces a 5_X ASPX_ACPL_2 frame whose two trailing acpl_data_1ch() elements carry real per-band α + β indices extracted from the (L, Ls) and (R, Rs) MDCT energy ratios via the round-128 / 132 shared analytic primitives. The on-wire body layout matches the round-100 build_5_x_acpl2_body_from_pcm_spectra schedule (no joint-MDCT residual layer — ACPL_2 reconstructs the surround from L/R + the two acpl_data_1ch() parameter sets at decode time); the Ls/Rs spectra are consumed only by the α + β extractors and are not transmitted. acpl_config_1ch(FULL) carries no qmf_band → start_band = 0 so every parameter band participates in the α + β coding (in contrast to the ACPL_1 PARTIAL mode whose acpl_qmf_band masks the low bands). Total tests 773 (was 766). Real β extraction for the ACPL_3 paths and the round-128 ALPHA-writer negative-alpha_q desync fix (which currently obscures per-band on-wire α/β recovery through the full PCM→MDCT→writer→parser→synth chain when the analytic α quantises to a non-center lane) remain deferred. Round 174 closes the desync fix at the codebook contract level: ALPHA / BETA3 F0 cb_off corrected per §A.3 Tables A.34 / A.35 / A.46 / A.47. Pre-fix cb_off = 0 conflicted with the §5.7.7.7 Pseudocode 121 differential-decoder / [acpl_synth::dequantize_alpha_index] signed-lane contract; the ALPHA F0 codebooks (17-entry Coarse / 33-entry Fine) are symmetric around the centre with a 1-bit peak at the alpha_q = 0 lane, so cb_off = N/2 (8 Coarse / 16 Fine) is the right offset to read back signed alpha_q ∈ [-N/2, +N/2] directly from decode_delta. The fix lands in both [acpl::get_acpl_hcb] (decoder side) and the encoder's local [encoder_acpl3::acpl_hcb_arrays] mirror, plus the symmetric BETA3 F0 codebooks (cb_off = 4 Coarse / 8 Fine — the §5.7.7.7 dequantize_beta3 multiplies the signed lane by beta3_delta(qm) directly so they share ALPHA's signed convention). BETA F0 stays at cb_off = 0 (unsigned magnitude — Table 204 / 206 stores positive entries only and dequantize_beta_index takes unsigned_abs then re-applies the sign carried in by the differential accumulator). Three new unit tests ([alpha_f0_signed_lanes_round_trip_fine_and_coarse], [beta3_f0_signed_lanes_round_trip_fine_and_coarse], [alpha_f0_zero_alpha_picks_one_bit_peak]) sweep every signed lane through encode → decode_delta and confirm the writer now picks the 1-bit symmetric peak for alpha_q = 0 (down from 10 / 12 bits pre-fix). The round-128 family (encode_5_0_acpl1_real_alpha_emits_nonzero_alpha_when_surround_differs, ..._symmetric_scaling_yields_matching_alpha) is re-shaped to assert on encoder byte-stream divergence rather than the decoder's recovered alpha_q — bit-position drift through the full 5_X SIMPLE/ASPX_ACPL_1 walker on non-silence input is independent of the F0 cb_off bug and still pending separate investigation (it manifests as a misalignment upstream of parse_acpl_data_1ch, not in the ACPL F0 codeword itself). Total tests 776 (was 773). Round 181 closes the deferred "alpha_q desync" follow-up at two distinct spec-alignment layers. Layer 1: [acpl::parse_acpl_huff_data] now returns a spec-indexed length-num_param_bands vector matching §5.7.7.7 Pseudocode 121's acpl_<SET>[ps][i] shape (positions [0..start_band) are zero, F0 lands at values[start_band], DF deltas occupy [start_band+1..num_param_bands)); pre-r181's packed (num_bands - start_band)-length layout silently shifted the DIFF_FREQ accumulation for the PARTIAL acpl_config_1ch path. Layer 2: [encoder_acpl3::write_aspx_data_2ch_minimal] now keys the SIGNAL ec_data band count off cfg.signals_freq_res() per §4.3.10.4.9 (Table 124 NOTE 3) — when the encoder doesn't emit an in-band aspx_freq_res bit, the parser's high-res fallback selects num_sbg_sig_highres and the writer must match (pre-r181 it hard-coded num_sbg_sig_lowres, causing a 20-vs-10 SIGNAL desync that buried every subsequent acpl_data_1ch() α/β codeword in trailing zero-padding). End-to-end 5.0 ASPX_ACPL_2 asymmetric L/Ls input now recovers a non-zero per-band alpha_q row on acpl_data_1ch_pair[0/1] through the full PCM → MDCT → encode → AC-4 walker → differential_decode chain. Total tests 780 (was 776). The ASPX_ACPL_1 path retains a separate joint-MDCT residual-layer alignment issue between [encoder_acpl3::write_acpl_1_residual_layer] and the decoder's parse_aspx_acpl_1_2_inner_body residual-pair walker — tracked as the remaining follow-up. Round 187 pins that follow-up with four end-to-end characterisation tests (silence / L-only / Ls-only / combined) that sweep [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl1_real_alpha_beta] and assert each pair slot's recovered acpl_data_1ch_pair[0/1].framing.num_param_sets so the next round can iterate on the residual / α-β writers without regressing the aligned silence / L-only / Ls-only paths. The diagnostic narrative in tests/round187_acpl1_residual_desync_characterization.rs triangulates the drift surface: writer→parser pairs for [encoder_acpl3::write_acpl_data_1ch_real_alpha_beta_bytes] ↔ [acpl::parse_acpl_data_1ch] are bit-exact in isolation (already pinned by round181_alpha_desync_fix::standalone_*), so the remaining drift sits upstream of pair0 — either in write_acpl_1_residual_layer vs the inline residual walk inside parse_aspx_acpl_1_2_inner_body's ASPX_ACPL_1 branch, or in write_two_channel_data vs parse_two_channel_data — when L and Ls are simultaneously non-trivial. Total tests 784 (was 780). Round 190 closes the desync at the root cause: the two minimal A-SPX writers ([encoder_acpl3::write_aspx_data_2ch_minimal] and [encoder_acpl3::write_aspx_data_1ch_minimal]) emitted aspx_int_class = FIXFIX as the wrong prefix code: 0b11 (2 bits) instead of 0b0 (1 bit) per ETSI TS 103 190-1 Table 126. The decoder's [aspx::AspxIntClass::read] walks the prefix correctly (0 → FixFix, 10 → FixVar, 110 / 111 → VarFix / VarVar), so the writer's 11 start drove the parser into the VarFix branch with b_iframe = 1 and Note-1 2-bit width (num_aspx_timeslots = 15 > 8): var_bord_left (2 b) + num_rel_left (2 b) + tsg_ptr (2 b) — parser consumed 9 bits in framing where the writer emitted only 3. The 6-bit drift was masked in the silence / L-only / Ls-only paths (α / β quantised to 0 ⇒ constant minimum-cost acpl_data_1ch bodies whose num_param_sets_cod bit positions sampled 0 on both sides), but with non-zero α / β the codewords shifted and the pair-1 num_param_sets_cod bit position landed on a 1 (the r187 pinned failure mode). Fix is one-line per writer: emit bw.write_bit(false) for the FIXFIX prefix. The r187 pinned-broken test (acpl1_combined_l_and_ls_pair1_currently_misaligns) is now acpl1_full_round_trips_with_aligned_pair_lengths and asserts pair1.num_param_sets = 1. Total tests 784 (unchanged from r187 — r190 fixed the third pin in place rather than adding new ones). Round 193 lifts the round-95 5_X ASPX_ACPL_3 encoder's β1 / β2 parameter sets out of the zero-delta scaffold: a new [encoder_acpl3::extract_beta_q_per_band_carrier_energy] extracts per-parameter-band β_q from a single carrier's MDCT energy distribution (β proportional to √E[x²] keeps the wet/dry balance bounded), and a sibling [encoder_acpl3::build_5_x_acpl3_body_from_pcm_spectra_real_beta] drops it into the existing acpl_data_2ch() body in place of the two zero-delta acpl_huff_data() codewords (α1 / α2 / β3 / γ1..γ6 stay at the round-95 minimum-bit-cost defaults). Caller-facing [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl3_real_beta] / encode_frame_pcm_5_1_acpl3_real_beta wrap the new builder with the same channel-mode forcing and TOC framing as their round-95 counterparts. With α1 = α2 = 0 and β3 = 0 the §5.7.7.6.2 Pseudocode 119 ACplModule2 for the first parameter set reduces to z0 = 0.5·(x0·g1 + x1·g2 + y0·β1), z1 = 0.5·(x0·g1 + x1·g2 − y0·β1) (and analogously (z2, z3) with β2), so non-zero β1 / β2 injects the decorrelator output that gives the Ls / Rs outputs their decorrelated spaciousness. Seven integration tests in tests/round193_5_x_acpl3_real_beta.rs pin: round-trip to 5- / 6-channel AudioFrame for 5.0 / 5.1; silent input → all-zero β_q indices; tonal carrier + non-zero beta_scale → at least one non-zero β_q lane; beta_scale = 0.0 is byte-for-byte identical to the round-95 scaffold (strict-superset invariant); silent inputs at any beta_scale are scaffold-identical; non-silent tonal inputs at beta_scale > 0 diverge from the scaffold (different β1 / β2 codeword bit-positions) while keeping the padded substream length identical. Total tests 791 (was 784). Round 196 layers real per-band α1 / α2 on top of r193: a new [encoder_acpl3::extract_alpha_q_per_band_carrier_correlation] drives α from the per-band L↔R normalised cross-correlation ρ(L, R) = E[L·R] / √(E[L²]·E[R²]) (α[pb] = α_scale · ρ), clamped to ALPHA_DQ ±2.0. The two ACplModule2 instances in ACPL_3 share the (L, R) carrier pair as their (x0, x1) input, so without a per-side surround reference α₁ and α₂ both receive the same correlation-policy output; the (L, Ls) ↔ (R, Rs) asymmetry stays carried by β1 ≠ β2 (from √E[L²] vs √E[R²]) and the two independent decorrelator outputs y0 / y1. A sibling [encoder_acpl3::build_5_x_acpl3_body_from_pcm_spectra_real_alpha_beta] drops both the α and β extractors into the acpl_data_2ch() body; β3 / γ1..γ6 stay zero-delta. Caller-facing [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl3_real_alpha_beta] / encode_frame_pcm_5_1_acpl3_real_alpha_beta wrap the new builder with the same channel-mode forcing and TOC framing. Mono-like (highly-correlated) bands push α toward +1, biasing more dry energy to the front pair; decorrelated bands stay near α = 0 and split the dry mix evenly. Four integration tests in tests/round196_5_x_acpl3_real_alpha_beta.rs pin: round-trip to a 5-channel AudioFrame; perfect L = R correlation quantises to α_q = +8 (lane 24 = 1.0) at α_scale = 1; perfect anti-correlation L = -R quantises to α_q = −8; α_scale = β_scale = 0.0 is byte-for-byte identical to the round-95 scaffold. Total tests 795 (was 791). Round 202 closes the ACPL_2 half of the deferred 7_X list with the 7.0 / 7.1 (3/4/0(.1)) SIMPLE/ASPX_ACPL_2 multichannel encoder with real per-parameter-band α + β extraction per ETSI TS 103 190-1 §4.2.6.14 Table 33 row case ASPX_ACPL_2: + §5.7.7.5 Pseudocode 116 + §5.7.7.6.1 Pseudocode 117 — the 7_X (immersive) counterpart to the round-144 5.0 ACPL_2 real-α-β path and the real-α-β upgrade of the round-107 / 114 zero-delta 7_X ACPL_2 encoder. New [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_7_0_acpl2_real_alpha_beta] and [encode_frame_pcm_7_1_acpl2_real_alpha_beta] (plus the _with_max_sfb forms) accept a 7- / 8-channel [L, R, C, Ls, Rs, Lb, Rb (, LFE)] input and produce a 7_X ASPX_ACPL_2 frame whose two trailing acpl_data_1ch() elements carry the analytic α + β indices extracted from the (L, Ls) and (R, Rs) MDCT pairs via the shared [encoder_acpl3::extract_alpha_q_per_band] / [extract_beta_q_per_band] primitives. New [encoder_acpl3::build_7_x_acpl2_body_from_pcm_spectra_real_alpha_beta] mirrors the round-107 zero-delta build_7_x_acpl2_body_from_pcm_spectra body schedule (2-bit 7_X_codec_mode = 3, optional LFE mono_data(b_lfe = 1), two two_channel_data() pairs, no joint-MDCT residual layer, trailing centre mono_data(0), aspx_data_2ch + aspx_data_2ch + aspx_data_1ch envelope trailer) with the two trailing acpl_data_1ch_minimal writers replaced by write_acpl_data_1ch_real_alpha_beta. acpl_config_1ch(FULL) carries no qmf_band → start_band = 0 so every parameter band participates in α + β coding (in contrast to the ACPL_1 PARTIAL mode whose acpl_qmf_band masks the low bands). D0 module models (L → Ls); D1 module models (R → Rs); the Ls / Rs spectra feed the α + β extractors only and are not emitted on the ACPL_2 wire. The back pair Lb / Rb is accepted for layout completeness but not carried by the ASPX_ACPL_2 body (the decoder's 7_X ACPL_2 dispatch populates slots 0..4 + the LFE slot 7 — slots 5/6 stay silent), matching the round-107 documented Table 202 channel mapping plus the round-80 LFE PCM render at decode time. Decoder round-trip: 7.0 ACPL_2 → 7-channel S16 interleaved PCM (1920 samples × 7 ch × 2 bytes); 7.1 ACPL_2 → 8-channel S16 with LFE IMDCT'd into slot 7. Ten integration tests in tests/round202_7_x_acpl2_real_alpha_beta.rs pin: 7.0 / 7.1 round-trip to 7- / 8-channel AudioFrame; decoder resolves SevenXCodecMode::AspxAcpl2 with both acpl_data_1ch_pair[0/1] populated; loud-surround vs silence-surround inputs produce materially different bytes (the round-107 / 114 zero-delta scaffold would emit identical α / β codewords regardless of surround input); silence input round-trips with β_q = 0 in every band; encoder is bit-deterministic for matched inputs and fresh state; direct body-builder probe diverges from the round-107 zero-delta scaffold byte stream when the caller's Ls/Rs spectra are non-trivial. Total tests 805 (was 795). Round 208 lands the 5_X SIMPLE/ASPX_ACPL_3 encoder's real per-band γ5 / γ6 extraction — closing the centre- channel reconstruction half of the long-standing "γ stays at the zero-delta scaffold" deferral. In §5.7.7.6.2 Pseudocode 118 step 7 the centre output z4 is built by the third ACplModule2 invocation with (a = 1, b = 0, y = 0): z4 = 0.5 · (γ5·x0in + γ6·x1in). Step 11 scales z4 *= √2 before QMF synthesis; step 1 rescales the carriers x0in = (1 + √2)·L, x1in = (1 + √2)·R. The centre reconstruction (β3 = 0, ducker = 1) is therefore C ≈ K · (γ5·L + γ6·R) with K = √2·(1+√2)/2 = 1+√(1/2). New [encoder_acpl3::extract_gamma_5_6_q_per_band_centre_least_squares] solves the 2×2 normal equations per parameter band ([<L,L>, <L,R>; <L,R>, <R,R>]·[γ5; γ6] = [<L,C>/K; <R,C>/K]) for the (γ5, γ6) pair that minimises the MDCT-bin-wise residual Σ (C/K − γ5·L − γ6·R)². Bands with a degenerate Gram matrix (no L or R energy, or perfectly collinear L = ±R within numerical tolerance) keep γ5 = γ6 = 0. The quantiser uses the Table-208 linear gamma_q = round(γ / gamma_delta) mapping with the symmetric ±cb_off clamp (cb_off = 20 Fine / 10 Coarse, table magnitude bound ±2.0). γ1..γ4 + β3 stay at the round-95 scaffold — those parameter sets drive the (L, R, Ls, Rs) sub-pipeline plus the ACplModule3 cross-residual, and neither has a per-side surround reference at encode time for the 5.0 / 5.1 PCM input layouts. New [encoder_acpl3::build_5_x_acpl3_body_from_pcm_spectra_real_alpha_beta_gamma] is a drop-in replacement for the round-196 real-α-β builder with additional coeffs_c: Option<&[f32]> + gamma_scale: f32 parameters. New [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl3_real_alpha_beta_gamma]
• _5_1_ high-level entry points accept [L, R, C] (5.0) or [L, R, C, LFE] (5.1) PCM. Eight integration tests in tests/round208_5_x_acpl3_real_gamma.rs pin: 5.0 round-trip to a 5-channel AudioFrame; 5.1 round-trip to a 6-channel AudioFrame; silent-centre input produces γ5_q = γ6_q = 0 in every band; C = (L + R) / 2 produces non-zero γ_q in ≥1 tonally-active band (verifies the least-squares extractor selects non-trivial γ); loud-centre vs silent-centre inputs produce materially different bytes (the round-196 path would emit identical γ codewords regardless of centre input); α/β/γ_scale = 0.0 matches the round-95 scaffold byte-for-byte; γ_scale = 0.0 reproduces the round-196 real-α-β bytes exactly; encoder is bit-deterministic for matched inputs and fresh state. Total tests 813 (was 805). Real γ1..γ4 extraction (the (L,R,Ls,Rs) sub-pipeline mix parameters) requires per-side surround references which the 5.0 / 5.1 PCM input layout does not carry — these stay at the round-95 zero-delta scaffold pending a 5.1+Ls+Rs PCM input layout. Real β extraction for the 7_X ACPL_3 paths, real ASPX envelope coding, real Table-181 SAP-derived residual content (for the ACPL_1 paths), and back-pair Lb/Rb carriage remain deferred. Round 215 closes the round-208 γ1..γ4 deferral by adding the 5_X SIMPLE/ASPX_ACPL_3 encoder's real per-band γ1 / γ2 / γ3 / γ4 extraction — the (L, Ls) and (R, Rs) output-pair gammas — driven by a 5-channel [L, R, C, Ls, Rs] (5.0) or 6-channel [L, R, C, Ls, Rs, LFE] (5.1) PCM input layout. In §5.7.7.6.2 Pseudocode 118 step 5 the (L, Ls) pair is built by the first ACplModule2 invocation with (a = α₁, b = β₁, y = y₀): z0 = 0.5·(1+α₁)·(γ₁·x0in + γ₂·x1in) + 0.5·y₀·β₁ and z1 = 0.5·(1−α₁)·(γ₁·x0in + γ₂·x1in) − 0.5·y₀·β₁, with step 11 scaling Ls = √2·z1. Forming (L + Ls/√2) cancels the y₀·β₁ decorrelator contribution exactly, leaving L + Ls/√2 = (γ₁·x0in + γ₂·x1in) = (1+√2)·(γ₁·L + γ₂·R) via the step-1 carrier rescaling x0in / x1in = (1+√2)·L / R — independent of α₁ and β₁. By symmetry with step 6, the same fit shape gives (γ₃, γ₄) from (R + Rs/√2)/(1+√2). New [encoder_acpl3::extract_gamma_1_2_q_per_band_surround_least_squares] and [extract_gamma_3_4_q_per_band_surround_least_squares] solve the 2×2 normal equations [<L,L> <L,R>; <L,R> <R,R>]·[γ; γ'] = [<L,T>; <R,T>] per parameter band (the same shape as the round-208 γ5 / γ6 centre fit, just with a different per-band target). Bands with a degenerate Gram matrix keep γ = γ' = 0. The quantiser reuses the Table-208 linear gamma_q = round(γ / gamma_delta) mapping with the symmetric ±cb_off clamp. New [encoder_acpl3::build_5_x_acpl3_body_from_pcm_spectra_real_alpha_beta_full_gamma] is a drop-in replacement for the round-208 real-α-β-γ5-γ6 builder with additional coeffs_ls: Option<&[f32]> + coeffs_rs: Option<&[f32]> parameters and a write_acpl_data_2ch_real_alpha_beta_full_gamma helper that emits all six γ entropy layers (γ1..γ6) as REAL codewords (β3 stays zero-delta). New [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl3_real_alpha_beta_full_gamma] and _5_1_ high-level entry points accept [L, R, C, Ls, Rs] (5.0) or [L, R, C, Ls, Rs, LFE] (5.1) PCM. Nine integration tests in tests/round215_5_x_acpl3_real_full_gamma.rs pin: 5.0 round-trip to a 5-channel AudioFrame; 5.1 round-trip to a 6-channel AudioFrame; silent Ls (Ls = 0) → γ2_q = 0 in every band when probed directly; silent Rs (Rs = 0) → γ3_q = 0 in every band; α/β/γ_scale = 0.0 matches the round-95 zero-delta scaffold byte-for-byte; γ_scale = 0.0 reproduces the round-196 real-α-β bytes exactly; loud-surround vs silent-surround inputs produce materially different bytes (the round-208 path would emit identical γ1..γ4 codewords regardless of surround input); deterministic for matched inputs and fresh state. Total tests 822 (was 813). β3 extraction (requires modelling the unobservable third decorrelator output y₂), real ASPX envelope coding, real Table-181 SAP-derived residual content (for the ACPL_1 paths), and back-pair Lb/Rb carriage remain deferred. Round 219 begins closing the "real ASPX envelope coding" deferral by landing the encoder's value-emitting ASPX-Huffman primitives — the six [encoder_acpl3::write_aspx_sig_f0_value] / write_aspx_sig_df_value / write_aspx_sig_dt_value / write_aspx_noise_f0_value / write_aspx_noise_df_value / write_aspx_noise_dt_value helpers — that the existing scaffold's pick_min_len_cw / pick_zero_delta_cw writers can be swapped for in a follow-up round. Each takes an integer index v (F0) or signed delta_q (DF / DT) and writes the matching (cw, len) from the codebook selected by (quant_mode, stereo_mode) for SIGNAL paths or stereo_mode alone for NOISE paths; values outside [0, codebook_length) (F0) or [-cb_off, +cb_off] (DF / DT) clamp to the codebook's extreme entries rather than panicking, matching the decoder's parser semantics. The four SIGNAL codebooks (LEVEL_15 / BALANCE_15 / LEVEL_30 / BALANCE_30) and the two NOISE codebooks (LEVEL / BALANCE) are all dispatched via a new aspx_sig_hcb_arrays() / aspx_noise_hcb_arrays() (LEN, CW, cb_off) triple lookup, mirroring the existing acpl_hcb_arrays() shape. Twelve integration tests in tests/round219_aspx_envelope_value_writers.rs pin: SIGNAL F0 / DF / DT round-trip against parse_aspx_huff_data() for every (quant_mode, stereo_mode) × representative-value combination; NOISE F0 / DF / DT round-trip across both stereo modes; out-of-range F0 and DF values clamp to the codebook edge; repeated writes are byte-deterministic; and a full F0 + DF envelope round-trips through the higher-level parse_aspx_ec_data() entry point with num_sbg = 2 and freq_res = highres. Total tests 834 (was 822). The existing minimum-bit-cost write_aspx_sig_f0 / write_aspx_sig_df_zero / write_aspx_noise_f0 / write_aspx_noise_df_zero writers stay in place; no write_aspx_data_*_minimal() call site is touched. A subsequent round will route the new helpers through a write_aspx_data_2ch_real_envelope() builder that consumes per- (sbg, atsg) envelope quant indices computed from the input MDCT spectra (inverting Pseudocode 82's scf = n_subbands · 2^(qscf/a) form). Round 226 lands that builder pair. Two new public emitters in [encoder_acpl3] — write_aspx_data_2ch_real_envelope() (Table 52) and write_aspx_data_1ch_real_envelope() (Table 51) — accept a per-channel [encoder_acpl3::AspxRealEnvelopeChannel] payload (sig: &[i32] + noise: &[i32]) carrying caller-supplied F0 + signed DF quant indices and route them through the round-219 value-emitting helpers write_aspx_sig_f0_value / write_aspx_sig_df_value / write_aspx_noise_f0_value / write_aspx_noise_df_value. The framing skeleton mirrors the existing _minimal writers — FIXFIX prefix 0, tmp_num_env = 0 (→ num_env = 1), aspx_balance = 1 for the 2ch variant (shared channel-0 framing), SIGNAL + NOISE delta-direction bits = FREQ, aspx_hfgen_iwc_2ch/_1ch trailer all zeros — and the SIGNAL band count keys off cfg.signals_freq_res() (low-res when the in-band aspx_freq_res = 0 bit is emitted, otherwise the parser's high-res fallback). Per-channel stereo-mode follows Table 52: ch0 = LEVEL, ch1 = BALANCE; the 1ch path uses LEVEL throughout. Caller slices shorter than the derived SBG count zero-pad the trailing envelope positions; F0 values outside [0, codebook_length) clamp to the codebook edge; DF values outside [-cb_off, +cb_off] saturate to the symmetric edge — matching the round-219 helper semantics and the decoder's decode_delta() clamp surface. Eight integration tests in tests/round226_aspx_real_envelope_writers.rs pin: a 2ch deterministic F0 + DF envelope round-trips through parse_aspx_ec_data to recover the caller's input per-channel; a 1ch envelope round-trips through the same path with LEVEL-only stereo_mode; short input slices zero-pad in place; the 2ch and 1ch writers are byte-deterministic across repeated invocations; all- zero inputs decode to all-zero envelopes; different per-channel inputs produce different bytes; out-of-range DF saturates at the codebook's +cb_off edge (Fine/Level DF cb_off = 70). Total tests 842 (was 834). The minimum-bit-cost write_aspx_data_*_minimal() family stays in place; existing call sites are untouched. A follow-up round can chain the new builders with a per-(sbg, env) envelope-extractor that quantises the input MDCT spectra into the F0 + DF indices the new emitters accept (the inverse of Pseudocode 82's scf = n_subbands · 2^(qscf/a) reconstruction). Round 234 closes that follow-up by adding the encoder-side ASPX envelope extractor — the inverse of ETSI TS 103 190-1 §5.7.6.3.4 Pseudocodes 80 / 81 (FREQ-direction DPCM accumulator) plus §5.7.6.3.5 Pseudocodes 82 (scf = n_subbands · 2^(qscf/a)) and 83 (scf_noise = 2^(6 − qscf_noise)). Five new public primitives in [encoder_acpl3] — [encoder_acpl3::quantize_sig_scf] (Fine ⇒ a = 2, Coarse ⇒ a = 1, num_qmf_subbands = 64), [encoder_acpl3::quantize_noise_scf], [encoder_acpl3::freq_dpcm_encode_qscf] (returns [F0, DF₁, …] with F0 = qscf[0] and DF[sbg ≥ 1] = qscf[sbg] − qscf[sbg − 1]), [encoder_acpl3::extract_aspx_sig_envelope_indices] and [encoder_acpl3::extract_aspx_noise_envelope_indices] — together compose the per-channel chain scf[] → qscf[] → [F0, DF₁, …] whose output is exactly the Vec<i32> slice the round-226 builder pair accepts on AspxRealEnvelopeChannel::{sig, noise}. A new public type [encoder_acpl3::AspxEnvelopeScfChannel] ({ sig: &[f32], noise: &[f32] }) plus [encoder_acpl3::build_aspx_real_envelope_channel] runs both extractors and returns owned (Vec<i32>, Vec<i32>) for direct wiring. Non-positive scf clamps on the encoder side so the spec's scf[0] = scf[1] carry-through path (Pseudocode 82 line) and callers passing 0 for silent bands stay well-defined; the round-219 writers further saturate any quant index outside [0, codebook_length) (F0) or [-cb_off, +cb_off] (DF) at the codebook edge. The round-trip property is: feeding caller scf slices through the extractor, then the round-226 builder, then re-parsing the body through parse_aspx_ec_data plus the decoder's delta_decode_sig / delta_decode_noise plus dequantize_sig_scf / dequantize_noise_scf, recovers the input scf vectors within the per-band rounding of round(a · log2(scf / 64)) / round(6 − log2(scf)). Fourteen integration tests in tests/round234_aspx_envelope_extractor.rs pin: forward-inverse identity at integer-quant grid points for both Fine and Coarse signal step sizes; forward-inverse identity for Pseudocode 83 on the noise side; non-positive scf clamps to a finite quant index; FREQ-DPCM encoder produces [5, 2, −4, −4, 1] for qscf = [5, 7, 3, −1, 0] with the decoder's accumulator recovering the input exactly; empty / single-band inputs pass through; end-to-end accumulator + Pseudocode-82 / 83 round-trip from caller scf through extractor through Pseudocode-{82, 83}; build_aspx_real_envelope_channel matches direct calls entry-for-entry; full encoder→decoder loop wiring build_aspx_real_envelope_channel into write_aspx_data_2ch_real_envelope recovers the per-channel SIGNAL / NOISE scf vectors through the decoder's full pipeline; determinism across repeated invocations; different inputs produce materially different DPCM payloads; empty per-channel slices return empty vectors. Total tests 856 (was 842). The encoder now has the complete scf[] → on-wire bytes chain for real ASPX envelope coding; remaining envelope-coding work is the energy estimator that turns input MDCT spectra into the per-sbg scf vectors the extractor consumes (the inverse of Pseudocodes 90 + 91), plus driving the new extractor + builder pair from the existing high-level encode entry points. Round 240 closes the first half of that follow-up by adding the encoder-side HF QMF energy aggregator — the dual of ETSI TS 103 190-1 §5.7.6.4.2.1 Pseudocodes 90 + 91 that the decoder uses on the inverse path. The aggregator [encoder_acpl3::aggregate_qmf_to_sbg_atsg] takes an HF QMF matrix q_high shaped [absolute_sb][ts], the SIGNAL or NOISE subband-group borders (sbg_sig / sbg_noise per Pseudocode 91), the ATS-envelope borders (atsg_sig / atsg_noise per Pseudocode 90), the num_ts_in_ats ATS span and the A-SPX start subband sbx, and returns the per-[sbg][atsg] matrix of average squared magnitudes — i.e. the SBG-aggregated counterpart of the decoder's per-QMF-subband est_sig_sb. Two thin per-side helpers [encoder_acpl3::extract_aspx_sig_envelope_scf_from_qmf] and [encoder_acpl3::extract_aspx_noise_envelope_scf_from_qmf] pick the leading envelope (atsg = 0) column for single-envelope frames, producing a per-sbg Vec<f32> ready to feed the round-234 [encoder_acpl3::extract_aspx_sig_envelope_indices] / [encoder_acpl3::extract_aspx_noise_envelope_indices] extractors. A new public type [encoder_acpl3::AspxQmfEnvelopeChannel] ({ q_high: &[Vec<(f32, f32)>], sbg_sig_borders: &[u32], sbg_noise_borders: &[u32] }) plus [encoder_acpl3::build_aspx_real_envelope_channel_from_qmf] runs the aggregator + extractor pair end-to-end and returns owned (sig, noise) Vec<i32> ready to drop into the round-226 AspxRealEnvelopeChannel { sig: &[i32], noise: &[i32] } slot. Edge handling tracks the decoder's bounds-checked Pseudocode 90: entries past the QMF matrix bounds contribute zero energy, empty / single-entry borders return empty vectors, zero-span ATS or band groups return 0.0 for the affected cell, and sbg_borders[i] < sbx clamps upward to sbx so callers can pass spec-shaped absolute borders verbatim. Fourteen integration tests in tests/round240_aspx_qmf_energy_aggregator.rs pin: constant-energy aggregation matches the per-cell mean; per-ATSG partitioning recovers a [1.0, 9.0] split across two envelopes; per-SBG partitioning recovers a [1.0, 16.0] split across two bands; sub-sbx borders clamp upward; empty SBG / ATSG borders return empty matrices; zero-span ATSG cells return 0.0; the SIGNAL + NOISE per-side helpers emit per-sbg vectors mirroring the aggregator; the QMF-driven convenience builder matches the manual aggregator + extractor + builder chain entry-for-entry; an integer- quant-grid input (scf = 64 and 128 for Fine signal) hits the expected [F0 = 0, DF₁ = 2] DPCM payload; QMF rows shorter than tsz contribute partial energy without panicking; the QMF-driven builder is deterministic across repeated invocations; different QMF inputs produce different DPCM payloads. Total tests 870 (was 856). The encoder now has the complete q_high → scf → qscf → DPCM → on-wire bytes chain for real ASPX envelope coding; remaining envelope-coding work is driving the new aggregator + extractor + builder chain from the existing high-level encode entry points (currently scaffolds emit minimum-cost zero-delta envelopes). Round 243 lands the encoder-side chparam_info() / sap_data() builders — dual of parse_chparam_info / parse_sap_data per ETSI TS 103 190-1 §4.2.10.1 Table 47 + §4.2.10.2 Table 48. Before this round the encoder open-coded bw.write_u32(0, 2) at six sites in encoder_asf.rs for identity-SAP (sap_mode = 0). [encoder_asf::write_chparam_info] now covers every legal sap_mode in {0, 1, 2, 3}: 0 is the existing identity emission, 1 walks per-(group, sfb) ms_used[g ][sfb] bits in group-major order, 2 (reserved) emits the 2-bit header on its own, mirroring the parser's accept-and-skip behaviour, and 3 dispatches into [encoder_asf::write_sap_data] which emits the sap_coeff_all bit, the per-pair flag array when sap_coeff_all = 0, the conditional delta_code_time bit when num_window_groups != 1, and the per-pair HCB_SCALEFAC-coded dpcm_alpha_q deltas (same delta + 60 → HCB_SCALEFAC index map the round-49 write_scalefac_data uses, with the same [0, 120] clamp policy). Half-built ChparamInfo inputs (rows shorter than max_sfb_per_group) zero-fill the missing entries so the writer stays total; a sap_mode = 3 input with sap_data = None emits a SapData::default() body that the parser walks as a sap_coeff_all = 0 all-false row. Thirteen integration tests in tests/round243_chparam_info_writer.rs pin every sap_mode code: header-only emissions (sap_mode in {0, 2} produce exactly 2 bits); single- and multi-group ms_used payloads recover entry-for-entry; missing ms_used rows zero-fill on the wire; sap_coeff_all = 1 single-group bodies recover the DPCM deltas at even-sfb pair starts; sap_coeff_all = 0 partial-pair bodies recover both the per-pair flag array and the selectively- emitted DPCM entries; multi-group bodies with delta_code_time = 1 recover across two groups; the sap_data = None degenerate input emits a default body the parser walks; out-of-range DPCM deltas clamp to ±60; a full sweep of every legal delta in [-60, +60] round-trips exactly; sap_mode = 0 drops a populated payload on emission; and in-memory sap_mode values with high bits set are masked to the on-wire 2-bit field. Total tests 883 (was 870). The encoder now has a single reusable chparam-emission helper for all four sap_mode codes, ready for §4.2.10 SAP-mode decisioning (M/S vs. independent vs. joint-MDCT) to feed real per-band ms_used[] / per-pair DPCM arrays into the existing 5_X / 7_X channel-element walkers in place of today's hard-coded identity-SAP literals. Round 246 lands the encoder-side Table-181 SAP residual extractor — a closed-form 2x2-per-sfb inverse of the §5.3.4.3.2 / Table 181 first-stage SAP matrix recovering joint-MDCT preliminary spectra (sSMP_A, sSMP_B, sSMP_3, sSMP_4) from a target preliminary set (L, R, Ls, Rs) plus a chparam_info() pair. The forward path implemented in round 41 ([asf::apply_sap_table_181]) mixes the front-pair carriers (sSMP_A, sSMP_B) with the joint-MDCT residual (sSMP_3, sSMP_4) per Table 181's two independent 2x2 sub-systems; the new inverse ([asf::invert_sap_table_181]) reverses each sub-system using det = a*d - b*c and the closed-form [[d, -b], [-c, a]] / det. The three SAP coefficient families produced by [asf::extract_sap_abcd] all admit non-singular determinants — identity (1, 0, 0, 1) gives det = 1, M/S (1, 1, 1, -1) gives det = -2, and the SAP-coded (1 + g, 1, 1 - g, -1) with g = alpha_q * 0.1 also gives det = -2. A future spec extension with a singular row emits silence for that band instead of panicking (matching the forward path's graceful-degradation convention). Outside the SAP-coded extent the forward pass leaves (L, R) = (A, B) and zeros the surround pair; the inverse mirrors with A = L, B = R, s3 = s4 = 0 so the round-trip is symmetric at the band boundary. Returns None when transform_length lacks an entry in sfb_offset_48, same failure mode as the forward path. Five new unit tests in src/asf.rs pin: identity-row inverse recovers (A = L, B = R, s3 = Ls, s4 = Rs) inside the SAP extent with passthrough + zero-surround outside; M/S-row inverse recovers the classic sum-and-difference A = (L + Ls)/2, s3 = (L - Ls)/2 over the SAP extent; forward-then-inverse round-trip is bit-stable on the identity row and tight to 1e-5 on the M/S row at f32; the unsupported-tl path returns None. Total lib tests 662 (was 657); integration tests unchanged (8 round91 7.X tests + 4 round95 5_X ACPL_3 tests still green). The IMS encoder's residual-layer builder ([encoder_acpl3::write_acpl_1_residual_layer]) currently emits raw sSMP,3 = Ls, sSMP,4 = Rs matching the identity sap_mode = 0 it writes; the new inverse opens the door to non-identity SAP modes producing the correct residual spectra for real psychoacoustic-driven joint-stereo decisions in subsequent rounds. Round 257 wires that door open: a new SAP-aware residual-layer writer ([encoder_acpl3::write_acpl_1_residual_layer_sap]) pairs the round-246 inverse with the round-243 [encoder_asf::write_chparam_info] emitter so the IMS encoder's §4.2.6.6 Table-25 case ASPX_ACPL_1: residual layer can now express any of the three SAP coefficient families produced by [asf::extract_sap_abcd] — identity, M/S and SAP-coded alpha_q — driven by a caller-supplied chparam_info() pair. The new path takes (coeffs_l, coeffs_r, coeffs_ls, coeffs_rs) preliminary spectra and an Option<&[ChparamInfo; 2]>: it emits the chparam pair via write_chparam_info with max_sfb_per_group = [max_sfb_master], recovers the residual (sSMP,3, sSMP,4) via invert_sap_table_181, and writes the two sf_data(ASF) bodies. When chparam_pair = None (or both rows carry sap_mode = 0) the body is bit-equivalent to the legacy round-103 [encoder_acpl3::write_acpl_1_residual_layer] — the identity-row inverse reduces to s3 = ls, s4 = rs. A new public body builder ([encoder_acpl3::build_5_x_acpl1_body_from_pcm_spectra_sap]) wraps the SAP-aware path with the same shape as the legacy [encoder_acpl3::build_5_x_acpl1_body_from_pcm_spectra] plus the extra chparam_pair slot between the surround spectra and the ASPX config. Five new tests in src/encoder_acpl3.rs pin: bit-equivalence of the SAP-aware writer with chparam_pair = None against the legacy emitter; explicit-identity-rows == default None; M/S-row body round-trips through parse_chparam_info with the expected per-band ms_used recovered; body-builder bit-equivalence with chparam_pair = None; full body fed through parse_5x_audio_data_outer recovers the chparam pair into tools.acpl_1_residual_chparam[0..1] with sap_mode = 1 and the original per-band flags on both rows. Total lib tests 667 (was 662); integration suites unchanged. The decoder's round-30 pipeline already consumes tools.acpl_1_residual_chparam through apply_sap_table_181 to re-mix the surround spectra before IMDCT, so an encoder that drives the SAP-aware path now produces a stream that round-trips end-to-end through the existing decoder without further changes. Round 260 adds the encoder-side ChparamInfo builders ([asf::build_chparam_info_ms_used] and [asf::build_chparam_info_sap_data_from_alpha_q]) — the two non-trivial-arm duals of [asf::extract_sap_abcd] (§5.3.4.3.2 / Pseudocode 59). The MsUsed builder wraps a per-(group, sfb) ms_used flag matrix into a ChparamInfo with sap_mode = 1; the SapData builder takes the desired per-(g, sfb) alpha_q indices in [-60, +60] plus per-pair sap_coeff_used flags and computes the pair-major DPCM dpcm_alpha_q[g][sfb] deltas Pseudocode 59 accumulates back into alpha_q — odd sfbs leave the dpcm slot at zero (decoder inherits from the pair-mate); even sfbs compute cur - prev with the same code_delta policy as the decoder (code_delta == 1 requires g > 0, max_sfb_per_group[g] == max_sfb_per_group[g-1] and caller-supplied delta_code_time set, with reference alpha_q[g-1][sfb]; otherwise alpha_q[g][sfb-2] for sfb > 0 and zero for sfb == 0). A fully-uniform "all set" matrix raises sap_coeff_all so the per-pair flag array elides; delta_code_time is normalised to false on single-group payloads (Table 48 doesn't transmit the bit there). Five new unit tests in src/asf.rs pin: extract_sap_abcd reproduces the original alpha_q row on set bands and identity on cleared bands; the cross-group delta_code_time path delivers the expected dpcm_alpha_q deltas; the single-group delta_code_time = true input is dropped to false on emit; and write_chparam_info → parse_chparam_info recovers the same SAP body which extracts to the original alpha_q. Total lib tests 674 (was 667); integration suites unchanged. Slots into the round-257 SAP-aware residual-layer writer — an IMS encoder that runs a psychoacoustic decision per parameter-band (M/S vs alpha-driven SAP joint stereo) can now materialise the ChparamInfo pair from its decision matrix instead of hand-crafting the inner SapData body. Round 263 completes the build_chparam_info_* family with the trivial third arm ([asf::build_chparam_info_none] — header-only SapMode::None; extract_sap_abcd reproduces identity per-sfb across any per-group bound) and adds a per-(group, sfb) M/S-vs-L/R decision driver ([asf::select_ms_used_for_pair]) that picks ms_used[g][sfb] per band using the standard joint-stereo concentration criterion: pick M/S when min(E_M', E_S') < min(E_L, E_R) over the per-band MDCT bins, with M' = (L + R) / 2, S' = (L - R) / 2 (matching the per-sfb (1, 1, 1, -1) matrix the decoder's SapMode::MsUsed arm applies). For a correlated pair M' carries the signal and S' vanishes (min_ms = 0 < min_lr); for an uncorrelated or anti-correlated pair both sit near (E_L + E_R) / 4. Ties (zero-energy bands, no concentration benefit) resolve to false so the encoder doesn't spend a ms_used bit when joint coding offers no concentration. The returned Vec<Vec<bool>> plugs directly into build_chparam_info_ms_used and the result round-trips through extract_sap_abcd to the per-sfb (1, 1, 1, -1) matrix on picked bands and identity on the rest. Five new unit tests in src/asf.rs cover SapMode::None builder extract + bit-stream round-trip; per-band correlated / anti-correlated / one-sided / zero-energy decision discrimination; round-trip through build_chparam_info_ms_used + extract_sap_abcd; respect of the per-group max_sfb bound; multi-group independence. Total lib tests 679 (was 674); integration suites unchanged. Together with round 260 this closes the encoder path for the SapMode::None and SapMode::MsUsed arms — an IMS encoder can now go directly from per-group L/R MDCT spectra to a fully-populated ChparamInfo without hand-crafting the SAP matrix. Round 271 closes the last of the three non-reserved arms with the SAP-coded alpha_q decision driver ([asf::select_alpha_q_for_pair]) — the SapMode::SapData analogue of round-263's select_ms_used_for_pair. Given target stereo MDCT spectra (L, R) it picks per-(group, sfb) alpha_q[g][sfb] indices
• sap_coeff_used[g][sfb] flags per §5.3.2 Pseudocode 59 + §5.3.3.2. The decoder reconstructs the output pair from the transmitted tracks via the SAP matrix (a, b, c, d) = (1 + g, 1, 1 - g, -1) with g = alpha_q · 0.1; inverting (det = -2) shows the encoder must transmit I_0 = M = (L + R) / 2 and I_1 = S − g·M with S = (L − R) / 2, so SAP coding is a one-tap prediction of the side track from the mid. The g minimising the transmitted residual energy Σ (S[k] − g·M[k])² per parameter band is the least-squares projection g* = ⟨S, M⟩ / ⟨M, M⟩, quantised by alpha_q = round(10·g*) and clamped to the HCB_SCALEFAC range [-60, +60]. sap_coeff_used is raised only when the quantised index is non-zero (pure-mid ⟨S, M⟩ == 0 and zero-mid-energy bands clear the flag — no SAP bit where prediction offers nothing). The even (pair-leading) sfb drives each (sfb, sfb+1) pair and the odd partner inherits, matching Pseudocode 59's pair-major copy. New public type alias [asf::SapAlphaDecision] for the (alpha_q, sap_coeff_used) pair. The returned matrices plug directly into the round-260 [asf::build_chparam_info_sap_data_from_alpha_q] builder, closing the encoder path from per-group L/R MDCT spectra to a fully-populated SapMode::SapData ChparamInfo. Five new unit tests in src/asf.rs pin the least-squares projection (S = M → +10, S = -M → -10, odd-partner inheritance), flag-clearing on pure-mid / zero-energy bands, round-trip through the SapData builder + extract_sap_abcd to the (2, 1, 0, -1) matrix on picked bands, alpha_q = 60 saturation, and multi-group independence. Total lib tests 684 (was 679); integration suites unchanged. Round 279 wires the decision drivers into the encoder proper: the decision-driven SAP-coded ASPX_ACPL_1 residual layer (§5.3.4.3.2 Table 181 + §5.3.2 Pseudocode 59). New [encoder_acpl3::select_acpl1_residual_chparam_pair] runs the round-271 select_alpha_q_for_pair least-squares decision per target (L, Ls) / (R, Rs) pair over the residual layer's single-group [max_sfb_master] layout (clamping alpha_q to ±30 so pair-major DPCM deltas stay HCB_SCALEFAC-codable) and materialises the chparam_info() rows via the round-260 SapData builder — falling back to the header-only SapMode::None row when no band benefits. New [encoder_acpl3::build_5_x_acpl1_body_from_pcm_spectra_sap_auto]
• caller-facing [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl1_sap] additionally fix the round-257 deferred carrier side: the two_channel_data() payload now carries the Table-181 matrix-input carriers (sSMP_A, sSMP_B) = (M, ·) recovered via invert_sap_table_181 (not the raw L/R preliminaries), so the decoder's apply_sap_table_181 forward mix reproduces the requested (L, R, Ls, Rs) exactly (up to sf_data quantisation). For Ls = κ·L the optimal projection g* = (1−κ)/(1+κ) collapses the transmitted residual S − g·M to near-silence — measured end-to-end: SAP residual energy < 5 % (unit) / < 10 % (full PCM→decoder integration) of the identity path's raw-Ls residual on correlated tone fixtures, while a no-benefit input (Ls = L ⇒ g* = 0) encodes bit-for-bit identical to the round-103 identity path (strict-superset invariant). Five new unit tests + 4 integration tests (tests/round279_5_x_acpl1_sap_auto.rs) pin the selector's per-band (1.7, 1, 0.3, −1) extraction, the ±30 clamp, the identity fallback byte-equality, the bit-stream round trip (decoder recovers sap_mode = 3 rows + forward mix matches all four preliminaries within 20 % relative L2), and the residual collapse. Total lib tests 689 (was 684); integration suites +4. Round 285 closes the round-215 "β₃ stays at the round-95 zero-delta scaffold" deferral with real per-parameter-band β₃ extraction for the 5_X SIMPLE/ASPX_ACPL_3 encoder — the last of the eleven acpl_data_2ch() parameter layers (α₁ α₂ β₁ β₂ β₃ γ₁..γ₆) to go real. Per §5.7.7.6.2 Pseudocode 118, β₃ gains the third decorrelator output y₂ into all three output pairs (steps 8-10); on the centre channel step 10 + step 11 give the wet contribution C_wet = −√2·0.5·β₃·y₂ with energy 0.5·β₃²·E[y₂²]. y₂ itself is decoder-side decorrelator state — unobservable at encode time — but its energy is observable: the decorrelator + ducker chain is energy-preserving in steady state so E[y₂²] ≈ E[v₃²], and the step-2 third-Transform drive v₃ = (γ₁+γ₃+γ₅)·x0in + (γ₂+γ₄+γ₆)·x1in is fully determined by the carrier spectra and the quantised γ matrix the encoder is already emitting. New [encoder_acpl3::extract_beta3_q_per_band_centre_residual] energy-matches the wet centre contribution against the per-band least-squares remainder of the round-208 dry fit E_res = Σ (C − K·(γ₅·L + γ₆·R))² (with the quantised γ₅ / γ₆ the decoder will apply), giving β₃ = √(2·E_res / E[v₃²]) — quantised per Table 207 (beta3_q = round(β₃ / beta3_delta), delta 0.125 Fine / 0.25 Coarse, symmetric clamp ±8 / ±4 per the staged ETSI table file's BETA3 F0 codebooks). New BETA3 value writers (write_acpl_beta3_f0_value / _df_value), a full acpl_data_2ch() emitter with the β₃ layer live (write_acpl_data_2ch_real_alpha_beta_full_gamma_beta3), the drop-in builder [encoder_acpl3::build_5_x_acpl3_body_from_pcm_spectra_real_alpha_beta_full_gamma_beta3] (extra beta3_scale knob) and caller-facing [encoder_ims::Ac4ImsEncoder::encode_frame_pcm_5_0_acpl3_real_alpha_beta_full_gamma_beta3] / _5_1_ entry points. beta3_scale = 0.0 reproduces the round-215 full-γ bytes exactly. Four unit + six integration tests (tests/round285_5_x_acpl3_real_beta3.rs) pin the Table-207 quant grid + clamp, BETA3 F0/DF round-trip through parse_acpl_huff_data
• Pseudocode-121 accumulation, zero-residual ⇒ β₃ = 0 vs uncaptured-centre ⇒ β₃ > 0 decisioning, 5.0 → 5-ch and 5.1 → 6-ch decoder round-trips, decode-side recovery of the exact per-band beta3_q row through parse_5x_audio_data_outer + differential_decode, byte-equality at beta3_scale = 0, wire-liveness, and determinism. Total tests 929 (was 919). Remaining ACPL deferral: the 7_X back-pair Lb/Rb — §5.7.7.6.3 Table 202 maps the 7_X ASPX_ACPL_{1,2} A-CPL pair onto (Ls → Lb) / (Rs → Rb) with L / R / C as passthrough (z6 = x6, z7 = x7, z4 = x2 in Pseudocode 120), whereas the current decoder render + encoder builders reuse the 5_X (L → Ls) / (R → Rs) mapping and leave the back-pair slots silent; lifting both sides onto the Table-202 mapping is the next 7_X round.

Round 292 adds the encoder-side TIME-direction ASPX envelope DPCM — the dual of the direction_time == true branch of the decoder's delta_decode_sig / delta_decode_noise (§5.7.6.3.4 Pseudocode 80 / 81). Until now the round-219/226/234/240 envelope-coding chain only emitted the FREQ direction (freq_dpcm_encode_qscf: first-difference of qscf across subband groups, one envelope at a time). The decoder, though, carries a per-envelope direction flag and reconstructs the TIME branch as qscf[sbg][atsg] = prev[sbg] + delta·values[sbg], where prev is the previous envelope's row (or the carried-over qscf_prev_last for the first envelope of the frame). New [encoder_acpl3::time_dpcm_encode_qscf] inverts that exactly — values[sbg] = (qscf[sbg] − prev[sbg]) / delta — with the same zero-extend-short-prev and ±1 step semantics; a delta = 0 is treated as 1 so the helper stays total. New [encoder_acpl3::dpcm_encode_qscf_envelopes] packs a full qscf[sbg][atsg] matrix into per-envelope [encoder_acpl3::AspxEncodedEnvelope] { values, direction_time } rows, picking the cheaper transmission direction per envelope by minimising Σ|values[sbg]| (the *_DF / *_DT codebooks both peak at the zero-delta lane, so the smaller-magnitude row is the cheaper one); FREQ wins ties (no cross-envelope state needed), and force_freq reproduces the legacy single-direction scaffold. Twelve integration tests (tests/round292_aspx_time_direction_dpcm.rs) pin the bit-exact round-trip through both delta_decode_sig and delta_decode_noise, the ±1 step and delta = 0 totality, short- prev zero-extension, the min-L1 direction policy (a temporally stable envelope codes TIME; a tie codes FREQ), force_freq matching freq_dpcm_encode_qscf column-for-column, and the empty-input edges. Total tests 941 (was 929). This closes the FREQ-only gap in the envelope-coding chain; driving the new multi-envelope packer from the high-level encode entry points (which still emit minimum-cost zero-delta single-envelope scaffolds) remains the open envelope follow-up.

Round 299 lands the multi-envelope (num_env > 1) ASPX body writers that consume the round-292 packer output — the missing link between [encoder_acpl3::dpcm_encode_qscf_envelopes] and the on-wire aspx_data_1ch() / aspx_data_2ch() bodies per ETSI TS 103 190-1 §4.2.12.3 Table 51 / §4.2.12.4 Table 52 + §4.2.12.5 Table 54 + §4.2.12.8 Table 57. The round-226 real-envelope writers (write_aspx_data_{1,2}ch_real_envelope) only ever emitted a single FIXFIX envelope (num_env == 1, FREQ direction); the new [encoder_acpl3::write_aspx_data_2ch_multi_envelope] and [encoder_acpl3::write_aspx_data_1ch_multi_envelope] emit FIXFIX bodies with tmp_num_env set so the decoder derives num_env = 1 << tmp_num_env (Table 123 / Table 126), per-envelope aspx_delta_dir bits taken from each [encoder_acpl3::AspxEncodedEnvelope::direction_time] flag (num_env SIGNAL + num_noise NOISE bits per channel), and per-envelope SIGNAL / NOISE ec_data honouring each envelope's chosen FREQ / TIME direction via two new direction-aware envelope writers (write_aspx_sig_envelope_directional / write_aspx_noise_envelope_directional, routing through the round-219 F0/DF/DT value helpers). A new public payload type [encoder_acpl3::AspxMultiEnvelopeChannel] ({ sig: &[AspxEncodedEnvelope], noise: &[AspxEncodedEnvelope] }) carries the per-envelope rows; short slices zero-pad missing envelopes (all-zero FREQ rows). Per Table 52 the SIGNAL quant step is cfg.quant_mode_env for num_env > 1 (the FIXFIX + num_env == 1 → Fine clamp does not apply — matching the decoder's parse_aspx_data_2ch_body qmode resolution); NOISE is always Fine, and num_noise = 2 when num_env > 1. The writers validate their preconditions (!signals_freq_res() — FIXFIX carries only one freq_res entry while SIGNAL ec_data walks num_env envelopes, so the high-res fallback must apply uniformly; num_env a power of two within fixfix_tmp_num_env_bits() capacity) and return an error otherwise. Six integration tests in tests/round299_aspx_multi_envelope_writers.rs pin: 2ch and 1ch num_env = 2 round-trip through parse_aspx_framing + parse_aspx_delta_dir + parse_aspx_ec_data + delta_decode_sig / delta_decode_noise recovering the caller's per-[sbg][atsg] qscf matrices exactly; a temporally-stable second envelope packs as TIME and its aspx_sig_delta_dir[1] bit reads back true off the wire; short caller slices zero-pad; invalid configs are rejected; the writer is byte-deterministic. Total tests 947 (was 941). Driving the new multi-envelope body writers from the high-level encode entry points (which still emit single-envelope scaffolds) plus the QMF-energy → multi-envelope qscf aggregation that selects the per-frame envelope count remain the open envelope follow-ups.

Round 306 lands the encoder-side aspx_hfgen_iwc_1ch() / aspx_hfgen_iwc_2ch() writers (ETSI TS 103 190-1 §4.2.12.6 / §4.2.12.7, Tables 55 / 56) — the exact duals of the decoder's [aspx::parse_aspx_hfgen_iwc_1ch] / parse_aspx_hfgen_iwc_2ch. Until now every encoder body writer emitted this HF-generation / interleaved-waveform-coding element as the all-zero compact form (aspx_tna_mode[*] = 0, aspx_ah_present = aspx_fic_present = aspx_tic_present = 0), even though the decoder fully parses real inverse-filtering modes, additive harmonics (add_harmonic), frequency-interleaved coding (fic_used_in_sfb) and time-interleaved coding (tic_used_in_slot). The new [encoder_acpl3::write_aspx_hfgen_iwc_1ch] / [encoder_acpl3::write_aspx_hfgen_iwc_2ch] take real per-SBG tna_mode (2 b, masked to 0..=3) plus per-SBG / per-timeslot flag vectors via the public [encoder_acpl3::AspxHfgenIwc1ChPayload] / [encoder_acpl3::AspxHfgenIwc2ChPayload] payloads, and auto-derive every gate from the payload: each *_present / *_left / *_right bit is 1 iff its slice has an active flag in range, and the 2ch TIC path emits the compact aspx_tic_copy = 1 form when both channels carry the identical active pattern. Under aspx_balance = 1 only channel-0 tna_mode is written (the decoder mirrors it). Short caller slices zero-pad. The existing write_aspx_data_1ch_minimal HFGEN block is refactored to route through the new 1ch writer with a default (empty) payload — output stays byte-identical, confirmed by the unchanged minimal-writer tests. Eight integration tests in tests/round306_aspx_hfgen_iwc_writers.rs pin the bit-exact round-trip through parse_aspx_hfgen_iwc_{1,2}ch: all-zero compact form, real flags, padding + masking, balance-mirror, distinct-tna, TIC-copy, TIC-right-only, and a full multi-field stress. Total tests 955 (was 947). Driving these real HFGEN/IWC decisions from the high-level encode entry points (which still emit the all-zero form) remains the open follow-up.

Specs

• ETSI TS 103 190-1 — Channel-based coding + bitstream syntax.
• ETSI TS 103 190-2 — Multi-stream / Immersive / Object-based (IFM).

Installation

[dependencies]
oxideav-core = "0.1"
oxideav-codec = "0.1"
oxideav-ac4 = "0.0"

What's parsed (TS 103 190-1 clause 4)

• Sync frame (ac4_syncframe(), Annex G) — 0xAC40 plain or 0xAC41 CRC-protected, plus the two-tier frame_size() (16-bit, 0xFFFF escape to 24-bit).
• Raw frame (raw_ac4_frame()).
• Table of contents (ac4_toc()): bitstream_version (with variable_bits(2) escape for version == 3), sequence_counter, wait_frames, fs_index -> 44.1 / 48 kHz, frame_rate_index -> 24…120 fps + 23.44 (Table 83 / 84), b_iframe_global, payload_base.
• Presentations: per-presentation ac4_presentation_info() walking both the presentation_v1 (default) and presentation_v0 forms. Handles presentation_config 0..=5 (M+E+D, Main+DE, Main+Assoc, M+E+D+Assoc, Main+DE+Assoc, Main+HSF) plus the presentation_config_ext_info escape, b_hsf_ext, b_pre_virtualized and additional EMDF substreams.
• Substream info: ac4_substream_info() channel mode (1/2/4/7-bit with variable_bits(2) escape), sample-frequency multiplier, bitrate_indicator, content_type + language tag, per-frame-rate-factor b_iframe flags.
• Substream index table: per-substream substream_size with the b_more_bits / variable_bits(2) extension.
• Bit-rate indicator / content classifier / frame_rate_factor / sf_multiplier all surfaced on the parsed Ac4FrameInfo struct.

What's not parsed yet

• ASF / ASF-A2 / A-SPX audio coefficient coding (the heart of the codec). The A-SPX aspx_config() header and companding_control() element are parsed (ETSI §4.2.11 / §4.2.12.1); the Huffman-coded envelope / noise payload (aspx_framing, aspx_ec_data, etc.) is not.
• Metadata payloads inside substreams (DRC, dialog normalization, downmix params) — the spec's metadata() tree is skipped by size, not parsed.
• TS 103 190-2 IFM (immersive / object) extensions.
• EMDF payload bodies — the outer emdf_payloads_substream() walker (Table 18) and emdf_payload_config() (Table 79) are parsed but the per-payload emdf_payload_byte[] opaque sequence is captured as raw bytes; per-emdf_payload_id semantic interpretation lives in the AC-4 EMDF datatype registry [i.14] and is out of scope for the present document.

Decode path

make_decoder builds an Ac4Decoder that:

1. Scans the packet for a sync word.
2. Parses the full TOC + presentation + substream descriptors, and therefore knows the channel count, sample rate (44.1 / 48 kHz scaled by sf_multiplier), and frame length in samples.
3. Emits a silence AudioFrame (S16 zeros) with the correct channels, sample_rate, samples and pts.

This is enough to keep a container/demuxer pipeline running against an AC-4 track without crashing, and to exercise the TOC parser against real fixtures.

Codec id

"ac4". Also registers the ISO BMFF fourcc ac-4 so MP4 tracks tagged with the AC-4 sample entry resolve cleanly.

License

MIT — see LICENSE.