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[fix](trx-backend-soapysdr): add stereo diff high-pass and headroom

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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 23:51:26 +01:00
parent 420e98de31
commit 635d2b39bb
1 changed files with 15 additions and 4 deletions
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src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+15 -4
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@@ -34,7 +34,11 @@ const STEREO_SEPARATION_PHASE_TRIM: f32 = 0.015;
const STEREO_SEPARATION_GAIN: f32 = 1.006;
/// Extra headroom in the stereo matrix to reduce stereo-only clipping/IMD on
/// strong program material. This keeps bass excursions from flattening treble.
const STEREO_MATRIX_GAIN: f32 = 0.42;
const STEREO_MATRIX_GAIN: f32 = 0.30;
/// Gentle high-pass memory for the stereo L-R path.
/// This trims only very low-frequency difference energy that can eat headroom
/// and modulate higher-frequency stereo detail.
const STEREO_DIFF_DC_R: f32 = 0.9995;
/// Fractional-resampler FIR taps for WFM audio reconstruction.
const WFM_RESAMP_TAPS: usize = 6;
/// Polyphase slots for the WFM fractional FIR resampler.
@@ -419,6 +423,9 @@ pub struct WfmStereoDecoder {
/// Quadrature companion of the L-R path used for phase trim / crosstalk adjustment.
diff_q_lpf1: BiquadLowPass,
diff_q_lpf2: BiquadLowPass,
/// Gentle high-pass on the stereo-difference path to reduce bass-driven IMD.
diff_dc: DcBlocker,
diff_q_dc: DcBlocker,
/// DC blockers on audio outputs — remove carrier-offset DC from the FM discriminator.
dc_m: DcBlocker,
dc_l: DcBlocker,
@@ -487,6 +494,8 @@ impl WfmStereoDecoder {
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),
diff_dc: DcBlocker::new(STEREO_DIFF_DC_R),
diff_q_dc: DcBlocker::new(STEREO_DIFF_DC_R),
dc_m: DcBlocker::new(0.9999),
dc_l: DcBlocker::new(0.9999),
dc_r: DcBlocker::new(0.9999),
@@ -567,10 +576,12 @@ impl WfmStereoDecoder {
// --- L-R (diff): 38 kHz demod + 4th-order Butterworth (unblended) ---
// Blend is applied per-band at audio rate in the emit step below.
let (sin_2p, cos_2p) = (2.0 * pilot_phase_est).sin_cos();
let diff_i = self.diff_lpf2.process(self.diff_lpf1.process(x * (cos_2p * 2.0)));
let diff_i = self
.diff_dc
.process(self.diff_lpf2.process(self.diff_lpf1.process(x * (cos_2p * 2.0))));
let diff_q = self
.diff_q_lpf2
.process(self.diff_q_lpf1.process(x * (-sin_2p * 2.0)));
.diff_q_dc
.process(self.diff_q_lpf2.process(self.diff_q_lpf1.process(x * (-sin_2p * 2.0))));
// --- Polyphase FIR fractional resampling ---
// This uses a short windowed-sinc bank instead of cubic interpolation