slowrx 0.5.3

Pure-Rust SSTV (Slow-Scan TV) decoder library — a port of slowrx by Oona Räisänen
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171

//! PD-family mode decoder.
//!
//! PD modes encode each radio "frame" as four channels — Y(odd line),
//! Cr (shared), Cb (shared), Y(even line) — producing two image rows
//! per radio frame with chroma subsampling between them.
//!
//! Translated from slowrx's `video.c` (Oona Räisänen, ISC License).
//! Channel layout: video.c lines 81-93. YUV→RGB matrix: video.c lines
//! 446-451. See `NOTICE.md` for full attribution.

use crate::demod::ChannelDemod;

/// Decode one PD radio frame (`Y(odd)`/`Cr`/`Cb`/`Y(even)`) into two image
/// rows of `image`. Translated from slowrx `video.c:259-486`.
///
/// Closes audit issues:
/// - **#24** time-base alignment: every pixel uses slowrx's exact
///   `Skip + round(rate * (chan_start_sec + pixel_secs * (x + 0.5)))`
///   single-round formula (`video.c:140-142`); `pair_seconds` is folded
///   in here, NOT pre-rounded by the caller, so per-pair rounding
///   error never accumulates.
/// - **#23** FFT-every-N + `StoredLum`: one FFT per working-rate sample
///   produces the latest `Freq`, which fills `StoredLum` at every
///   sample; pixel times read out via `StoredLum[pixel_time]`
///   (`video.c:350-406`). See
///   [`crate::demod::decode_one_channel_into`] for the per-channel
///   inner loop.
/// - **#18** SNR estimator: [`crate::snr::SnrEstimator`] is recomputed
///   every [`crate::demod::SNR_REESTIMATE_STRIDE`] samples
///   (`video.c:302-343`).
///
/// **Deviations from slowrx (deliberate):**
/// - **#44 lifted with hysteresis (0.3.2)**: per-pixel Hann window
///   length is SNR-adaptive (slowrx `video.c:354-367`) plus a 1 dB
///   hysteresis band at each threshold to prevent flip-flop on real-
///   radio SNR fluctuations near boundary values. See
///   [`crate::demod::window_idx_for_snr_with_hysteresis`] and the
///   `SNR hysteresis on adaptive Hann window selection` entry in
///   `docs/intentional-deviations.md`.
///
/// `skip_samples` is the absolute sample index inside `audio` where
/// pair zero's sync pulse begins; `pair_seconds` is `pair_index *
/// line_seconds` (un-rounded); `rate_hz` is the slant-corrected rate
/// from [`crate::sync::find_sync`].
#[allow(
    clippy::cast_precision_loss,
    clippy::cast_possible_truncation,
    clippy::cast_sign_loss,
    clippy::cast_possible_wrap,
    clippy::too_many_arguments
)]
pub(crate) fn decode_pd_line_pair(
    spec: crate::modespec::ModeSpec,
    pair_index: u32,
    audio: &[f32],
    skip_samples: i64,
    pair_seconds: f64,
    rate_hz: f64,
    image: &mut crate::image::SstvImage,
    demod: &mut ChannelDemod,
    snr_est: &mut crate::snr::SnrEstimator,
    hedr_shift_hz: f64,
) {
    let sync_secs = spec.sync_seconds;
    let porch_secs = spec.porch_seconds;
    let pixel_secs = spec.pixel_seconds;
    let septr_secs = spec.septr_seconds;
    let width = spec.line_pixels;

    // PD channel time offsets (seconds from start of line pair):
    // Y(odd) → Cr → Cb → Y(even). Mirrors slowrx video.c:88-92:
    //   ChanStart[n+1] = ChanStart[n] + ChanLen[n] + SeptrTime
    // where ChanLen[n] = PixelTime * ImgWidth.
    // SeptrTime = 0 for the entire PD family (PD120/PD180/PD240, modespec.c),
    // so septr_secs is a no-op for current modes — but having it here
    // prevents a silent break when non-PD modes (Robot, Scottie, Martin —
    // all with non-zero SeptrTime) are added in V2.
    let chan_len = f64::from(width) * pixel_secs;
    let chan_starts_sec = [
        sync_secs + porch_secs,                                     // Y(odd): 0 septr
        sync_secs + porch_secs + chan_len + septr_secs,             // Cr:     1 septr
        sync_secs + porch_secs + 2.0 * chan_len + 2.0 * septr_secs, // Cb:     2 septr
        sync_secs + porch_secs + 3.0 * chan_len + 3.0 * septr_secs, // Y(even):3 septr
    ];

    let row0 = pair_index * 2;
    let row1 = row0 + 1;
    let width_us = width as usize;

    let mut y_odd = vec![0_u8; width_us];
    let mut cr = vec![0_u8; width_us];
    let mut cb = vec![0_u8; width_us];
    let mut y_even = vec![0_u8; width_us];

    let ctx = crate::demod::ChannelDecodeCtx {
        audio,
        skip_samples,
        rate_hz,
        hedr_shift_hz,
        spec,
    };

    let buffers: [&mut [u8]; 4] = [&mut y_odd, &mut cr, &mut cb, &mut y_even];
    for (chan_idx, buf) in buffers.into_iter().enumerate() {
        crate::demod::decode_one_channel_into(
            buf,
            chan_starts_sec[chan_idx],
            pair_seconds,
            &ctx,
            &mut crate::demod::DemodState { demod, snr_est },
        );
    }

    for x in 0..width_us {
        let rgb_odd = crate::demod::ycbcr_to_rgb(y_odd[x], cr[x], cb[x]);
        let rgb_even = crate::demod::ycbcr_to_rgb(y_even[x], cr[x], cb[x]);
        image.put_pixel(x as u32, row0, rgb_odd);
        image.put_pixel(x as u32, row1, rgb_even);
    }
}

#[cfg(test)]
#[allow(
    clippy::cast_possible_truncation,
    clippy::cast_sign_loss,
    clippy::cast_precision_loss,
    clippy::cast_possible_wrap
)]
mod tests {
    /// Verify that with `septr_seconds = 0` the `chan_starts_sec` formula
    /// gives the same values as the pre-#25 direct formula (numeric equivalence).
    /// This confirms the field is a V2 expansion that is a no-op for PD modes.
    #[test]
    fn chan_starts_sec_septr_zero_is_numerically_equivalent_to_old_formula() {
        for spec in [
            crate::modespec::for_mode(crate::modespec::SstvMode::Pd120),
            crate::modespec::for_mode(crate::modespec::SstvMode::Pd180),
            crate::modespec::for_mode(crate::modespec::SstvMode::Pd240),
        ] {
            let sync = spec.sync_seconds;
            let porch = spec.porch_seconds;
            let px = spec.pixel_seconds;
            let septr = spec.septr_seconds;
            let w = f64::from(spec.line_pixels);
            let chan_len = w * px;

            // New formula (with septr_seconds term from slowrx video.c:88-92)
            let new_starts = [
                sync + porch,
                sync + porch + chan_len + septr,
                sync + porch + 2.0 * chan_len + 2.0 * septr,
                sync + porch + 3.0 * chan_len + 3.0 * septr,
            ];
            // Old formula (pre-#25, septr omitted)
            let old_starts = [
                sync + porch,
                sync + porch + w * px,
                sync + porch + 2.0 * w * px,
                sync + porch + 3.0 * w * px,
            ];
            for (n, (n_val, o_val)) in new_starts.iter().zip(old_starts.iter()).enumerate() {
                assert!(
                    (n_val - o_val).abs() < 1e-12,
                    "mode {:?} chan {} new={n_val} old={o_val}",
                    spec.mode,
                    n
                );
            }
        }
    }
}