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[fix](trx-backend-soapysdr): match wfm sum/diff filter cutoffs at 18 kHz

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Raise both AUDIO_BW_HZ and STEREO_DIFF_BW_HZ to 18 kHz so the L+R and
L-R filter paths have identical group delay across the full audio band.
The previous mismatch (15.8 vs 14.5 kHz) caused frequency-dependent
phase errors in the stereo matrix that degraded real-world separation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
This commit is contained in:
Stan Grams
2026-03-01 01:31:34 +01:00
parent f9691fee1e
commit a6c3a85c8d
1 changed files with 7 additions and 7 deletions
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src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+7 -7
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@@ -11,14 +11,14 @@ const RDS_BPF_Q: f32 = 10.0;
/// Pilot tone frequency (Hz).
const PILOT_HZ: f32 = 19_000.0;
/// Audio bandwidth for WFM (Hz).
/// 15.8 kHz leaves more guard band below the 19 kHz pilot and reduces
/// top-end artifacts on strong signals while still preserving the useful
/// broadcast audio range.
const AUDIO_BW_HZ: f32 = 15_800.0;
/// 18 kHz matches the diff path cutoff so both have identical group delay,
/// which is required for good stereo separation. The 19 kHz pilot is
/// removed by the notch filter on the mono path.
const AUDIO_BW_HZ: f32 = 18_000.0;
/// 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 = 14_500.0;
/// Wider than the mono path to preserve high-frequency stereo detail; the
/// 19 kHz pilot is already removed by the coherent demodulation step.
const STEREO_DIFF_BW_HZ: f32 = 18_000.0;
/// 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;