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[fix](trx-backend-soapysdr): tune wfm stereo filter bw and matrix gain

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Restore AUDIO_BW_HZ to 15.8 kHz for cleaner mono path, widen
STEREO_DIFF_BW_HZ to 18 kHz for better high-frequency stereo detail,
and raise STEREO_MATRIX_GAIN from 0.30 to 0.50 (mathematically correct
unity gain for the L=(S+D)/2 stereo matrix).

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 01:37:15 +01:00
parent e7997f8492
commit 2ed4aaf46d
1 changed files with 4 additions and 5 deletions
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src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+4 -5
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@@ -11,10 +11,9 @@ const RDS_BPF_Q: f32 = 10.0;
/// Pilot tone frequency (Hz). /// Pilot tone frequency (Hz).
const PILOT_HZ: f32 = 19_000.0; const PILOT_HZ: f32 = 19_000.0;
/// Audio bandwidth for WFM (Hz). /// Audio bandwidth for WFM (Hz).
/// 18 kHz matches the diff path cutoff so both have identical group delay, /// 15.8 kHz leaves guard band below the 19 kHz pilot and reduces top-end
/// which is required for good stereo separation. The 19 kHz pilot is /// artifacts on strong signals while preserving the useful broadcast range.
/// removed by the notch filter on the mono path. const AUDIO_BW_HZ: f32 = 15_800.0;
const AUDIO_BW_HZ: f32 = 18_000.0;
/// Stereo L-R subchannel bandwidth for WFM (Hz). /// Stereo L-R subchannel bandwidth for WFM (Hz).
/// Wider than the mono path to preserve high-frequency stereo detail; the /// Wider than the mono path to preserve high-frequency stereo detail; the
/// 19 kHz pilot is already removed by the coherent demodulation step. /// 19 kHz pilot is already removed by the coherent demodulation step.
@@ -34,7 +33,7 @@ const STEREO_SEPARATION_PHASE_TRIM: f32 = 0.015;
const STEREO_SEPARATION_GAIN: f32 = 1.000; const STEREO_SEPARATION_GAIN: f32 = 1.000;
/// Extra headroom in the stereo matrix to reduce stereo-only clipping/IMD on /// Extra headroom in the stereo matrix to reduce stereo-only clipping/IMD on
/// strong program material. This keeps bass excursions from flattening treble. /// strong program material. This keeps bass excursions from flattening treble.
const STEREO_MATRIX_GAIN: f32 = 0.30; const STEREO_MATRIX_GAIN: f32 = 0.50;
/// Gentle high-pass memory for the stereo L-R path. /// Gentle high-pass memory for the stereo L-R path.
/// This trims only very low-frequency difference energy that can eat headroom /// This trims only very low-frequency difference energy that can eat headroom
/// and modulate higher-frequency stereo detail. /// and modulate higher-frequency stereo detail.