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//! SBR envelope / noise-floor dequantization — ISO/IEC 14496-3
//! §4.6.18.3.5 "Dequantization and stereo decoding".
//!
//! Converts the reconstructed *quantized* scalefactors
//! ([`crate::sbr_reconstruct`]'s `E_Q(k,l)` / `Q(k,l)`) into the linear
//! energy values `EOrig(k,l)` / `QOrig(k,l)` the envelope adjuster
//! (§4.6.18.7) consumes:
//!
//! * Single channel (or an uncoupled pair, `bs_coupling == 0`):
//! `EOrig = 64 · 2^(E/a)` with `a = 2` for `bs_amp_res = 0` (1.5 dB
//! steps) and `a = 1` for `bs_amp_res = 1` (3.0 dB steps);
//! `QOrig = 2^(NOISE_FLOOR_OFFSET − Q)` with
//! `NOISE_FLOOR_OFFSET = 6` (§4.6.18.2.5).
//! * Coupled pair (`bs_coupling == 1`): channel 0 carries the
//! level average and channel 1 the pan ratio;
//! `panOffset = [24, 12]` (§4.6.18.2.6) recentres the ratio. The
//! left / right split divides the doubled average
//! `64·2^(E0/a + 1)` by `1 + 2^(±(panOffset − E1)/a)` (and the
//! noise analogue with `panOffset(1) = 12`), which preserves
//! `ELeft + ERight = 2 · (64·2^(E0/a))`.
//!
//! ## Provenance
//!
//! Every formula and constant is from the §4.6.18.3.5 text and the
//! §4.6.18.2.5 / §4.6.18.2.6 constant lists of the staged spec. No part
//! of this implementation is derived from any external decoder.
use crate::sbr_reconstruct::{EnvelopeScalefactors, NoiseScalefactors};
/// `NOISE_FLOOR_OFFSET = 6` (§4.6.18.2.5).
pub const NOISE_FLOOR_OFFSET: f64 = 6.0;
/// `panOffset = [24, 12]` indexed by `bs_amp_res` (§4.6.18.2.6).
#[inline]
#[must_use]
pub fn pan_offset(amp_res: bool) -> f64 {
if amp_res {
12.0
} else {
24.0
}
}
/// The §4.6.18.3.5 amplitude-resolution divisor `a`: `2` for
/// `bs_amp_res = 0` (1.5 dB), `1` for `bs_amp_res = 1` (3.0 dB).
#[inline]
#[must_use]
pub fn amp_divisor(amp_res: bool) -> f64 {
if amp_res {
1.0
} else {
2.0
}
}
/// Dequantized (linear-energy) envelope and noise-floor scalefactors
/// for one channel.
#[derive(Debug, Clone, PartialEq)]
pub struct DequantizedSbr {
/// `EOrig[l][k]` — linear envelope energies, one band vector per
/// envelope (band count follows the envelope's frequency
/// resolution).
pub e_orig: Vec<Vec<f64>>,
/// `QOrig[l][k]` — linear noise-floor energies, one `NQ`-band
/// vector per noise floor.
pub q_orig: Vec<Vec<f64>>,
}
/// §4.6.18.3.5 single-channel dequantization:
/// `EOrig = 64·2^(E/a)`, `QOrig = 2^(NOISE_FLOOR_OFFSET − Q)`.
#[must_use]
pub fn dequant_single(
env: &EnvelopeScalefactors,
noise: &NoiseScalefactors,
amp_res: bool,
) -> DequantizedSbr {
let a = amp_divisor(amp_res);
let e_orig = env
.eq
.iter()
.map(|l| {
l.iter()
.map(|&e| 64.0 * (f64::from(e) / a).exp2())
.collect()
})
.collect();
let q_orig = noise
.q
.iter()
.map(|l| {
l.iter()
.map(|&q| (NOISE_FLOOR_OFFSET - f64::from(q)).exp2())
.collect()
})
.collect();
DequantizedSbr { e_orig, q_orig }
}
/// §4.6.18.3.5 coupled-pair dequantization.
///
/// `ch0` carries the level average (`E0` / `Q0`), `ch1` the pan ratio
/// (`E1` / `Q1`). Returns the `(left, right)` linear energies.
#[must_use]
pub fn dequant_coupled(
env0: &EnvelopeScalefactors,
noise0: &NoiseScalefactors,
env1: &EnvelopeScalefactors,
noise1: &NoiseScalefactors,
amp_res: bool,
) -> (DequantizedSbr, DequantizedSbr) {
let a = amp_divisor(amp_res);
let pan = pan_offset(amp_res);
let mut left_e = Vec::with_capacity(env0.eq.len());
let mut right_e = Vec::with_capacity(env0.eq.len());
for (l0, l1) in env0.eq.iter().zip(env1.eq.iter()) {
let mut le = Vec::with_capacity(l0.len());
let mut re = Vec::with_capacity(l0.len());
for (&e0, &e1) in l0.iter().zip(l1.iter()) {
// 64·2^(E0/a + 1) split by the pan ratio.
let avg2 = 64.0 * (f64::from(e0) / a + 1.0).exp2();
let ratio = ((pan - f64::from(e1)) / a).exp2();
le.push(avg2 / (1.0 + ratio));
re.push(avg2 / (1.0 + 1.0 / ratio));
}
left_e.push(le);
right_e.push(re);
}
// Noise floors always use panOffset(1) = 12 (§4.6.18.3.5: the
// noise formulas are written with panOffset(1) regardless of
// bs_amp_res).
let noise_pan = pan_offset(true);
let mut left_q = Vec::with_capacity(noise0.q.len());
let mut right_q = Vec::with_capacity(noise0.q.len());
for (l0, l1) in noise0.q.iter().zip(noise1.q.iter()) {
let mut lq = Vec::with_capacity(l0.len());
let mut rq = Vec::with_capacity(l0.len());
for (&q0, &q1) in l0.iter().zip(l1.iter()) {
let avg2 = (NOISE_FLOOR_OFFSET - f64::from(q0) + 1.0).exp2();
let ratio = (noise_pan - f64::from(q1)).exp2();
lq.push(avg2 / (1.0 + ratio));
rq.push(avg2 / (1.0 + 1.0 / ratio));
}
left_q.push(lq);
right_q.push(rq);
}
(
DequantizedSbr {
e_orig: left_e,
q_orig: left_q,
},
DequantizedSbr {
e_orig: right_e,
q_orig: right_q,
},
)
}
#[cfg(test)]
mod tests {
use super::*;
fn env(eq: Vec<Vec<i32>>) -> EnvelopeScalefactors {
let n = eq.len();
EnvelopeScalefactors {
eq,
freq_res: vec![true; n],
}
}
fn noise(q: Vec<Vec<i32>>) -> NoiseScalefactors {
NoiseScalefactors { q }
}
/// `EOrig = 64·2^(E/a)`: exact powers for both amplitude
/// resolutions.
#[test]
fn single_channel_envelope_powers() {
let e = env(vec![vec![0, 2, 4]]);
let q = noise(vec![vec![6]]);
// bs_amp_res = 1 → a = 1: 64·2^E.
let d = dequant_single(&e, &q, true);
assert_eq!(d.e_orig[0], vec![64.0, 256.0, 1024.0]);
// bs_amp_res = 0 → a = 2: 64·2^(E/2).
let d = dequant_single(&e, &q, false);
assert_eq!(d.e_orig[0], vec![64.0, 128.0, 256.0]);
}
/// `QOrig = 2^(6 − Q)`: Q = 6 is unity, each +1 halves.
#[test]
fn single_channel_noise_powers() {
let e = env(vec![vec![0]]);
let q = noise(vec![vec![0, 6, 8]]);
let d = dequant_single(&e, &q, true);
assert_eq!(d.q_orig[0], vec![64.0, 1.0, 0.25]);
}
/// A balanced pan (`E1 == panOffset`) splits the energy equally:
/// both channels get exactly the mono dequantization.
#[test]
fn coupled_balanced_pan_is_symmetric() {
for amp_res in [false, true] {
let e0 = env(vec![vec![4, 8]]);
let q0 = noise(vec![vec![3]]);
let e1 = env(vec![vec![
pan_offset(amp_res) as i32,
pan_offset(amp_res) as i32,
]]);
let q1 = noise(vec![vec![12]]);
let (l, r) = dequant_coupled(&e0, &q0, &e1, &q1, amp_res);
let mono = dequant_single(&e0, &q0, amp_res);
for k in 0..2 {
assert!((l.e_orig[0][k] - mono.e_orig[0][k]).abs() < 1e-12);
assert!((r.e_orig[0][k] - mono.e_orig[0][k]).abs() < 1e-12);
}
assert!((l.q_orig[0][0] - mono.q_orig[0][0]).abs() < 1e-12);
assert!((r.q_orig[0][0] - mono.q_orig[0][0]).abs() < 1e-12);
}
}
/// The coupled split preserves the pair sum:
/// `ELeft + ERight = 2·(64·2^(E0/a))` for every pan value, and the
/// same for the noise floors.
#[test]
fn coupled_split_preserves_energy_sum() {
for amp_res in [false, true] {
for e1v in [0, 5, 11, 17, 24] {
let e0 = env(vec![vec![6]]);
let q0 = noise(vec![vec![4]]);
let e1 = env(vec![vec![e1v]]);
let q1 = noise(vec![vec![(e1v % 12) * 2]]);
let (l, r) = dequant_coupled(&e0, &q0, &e1, &q1, amp_res);
let mono = dequant_single(&e0, &q0, amp_res);
let sum = l.e_orig[0][0] + r.e_orig[0][0];
assert!(
(sum - 2.0 * mono.e_orig[0][0]).abs() < 1e-9,
"amp_res {amp_res} pan {e1v}: {sum}"
);
let qsum = l.q_orig[0][0] + r.q_orig[0][0];
assert!((qsum - 2.0 * mono.q_orig[0][0]).abs() < 1e-9);
}
}
}
/// A pan below the offset weights the left channel heavier (E1
/// counts down from left-dominant to right-dominant).
#[test]
fn coupled_pan_direction() {
let e0 = env(vec![vec![6]]);
let q0 = noise(vec![vec![4]]);
let e1 = env(vec![vec![2]]);
let q1 = noise(vec![vec![2]]);
let (l, r) = dequant_coupled(&e0, &q0, &e1, &q1, true);
assert!(l.e_orig[0][0] < r.e_orig[0][0]);
assert!(l.q_orig[0][0] < r.q_orig[0][0]);
}
}