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[fix](trx-backend-soapysdr): narrow wfm stereo difference band

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Co-authored-by: OpenAI Codex <codex@openai.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
This commit is contained in:
Stan Grams
2026-02-28 23:37:08 +01:00
parent 9e49bfbcc8
commit 6d466f44d2
1 changed files with 8 additions and 4 deletions
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src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+8 -4
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@@ -15,6 +15,10 @@ const PILOT_HZ: f32 = 19_000.0;
/// top-end artifacts on strong signals while still preserving the useful
/// broadcast audio range.
const AUDIO_BW_HZ: f32 = 15_800.0;
/// Stereo L-R subchannel bandwidth for WFM (Hz).
/// Keep this a bit lower than the mono path because the recovered difference
/// signal is noisier and more prone to high-frequency artifacts.
const STEREO_DIFF_BW_HZ: f32 = 13_500.0;
/// Q values for a proper 4th-order Butterworth cascade (two 2nd-order stages).
/// Stage 1: Q = 1 / (2 cos(π/8))
const BW4_Q1: f32 = 0.5412;
@@ -476,10 +480,10 @@ impl WfmStereoDecoder {
sum_lpf1: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW4_Q1),
sum_lpf2: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW4_Q2),
sum_notch: BiquadNotch::new(composite_rate_f, PILOT_HZ, PILOT_NOTCH_Q),
diff_lpf1: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW4_Q1),
diff_lpf2: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW4_Q2),
diff_q_lpf1: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW4_Q1),
diff_q_lpf2: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW4_Q2),
diff_lpf1: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW4_Q1),
diff_lpf2: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW4_Q2),
diff_q_lpf1: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW4_Q1),
diff_q_lpf2: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW4_Q2),
dc_m: DcBlocker::new(0.9999),
dc_l: DcBlocker::new(0.9999),
dc_r: DcBlocker::new(0.9999),