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[perf](trx-backend-soapysdr): increase wfm resampler taps and filter order

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Increase polyphase resampler taps from 16 to 64 for sharper anti-alias
stopband rejection. Upgrade sum/diff lowpass filters from 4th-order to
6th-order Butterworth (three biquad stages) for ~36 dB/octave rolloff,
improving stereo separation by better rejecting the 38 kHz subcarrier
residuals.

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 09:19:11 +01:00
parent 1ba9c9dd3e
commit 700e5c9fab
1 changed files with 26 additions and 15 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+26 -15
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@@ -18,11 +18,13 @@ const AUDIO_BW_HZ: f32 = 18_000.0;
/// Must match AUDIO_BW_HZ so the sum and diff filter paths have identical
/// group delay, which is critical for stereo separation across all frequencies.
const STEREO_DIFF_BW_HZ: f32 = AUDIO_BW_HZ;
/// 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;
/// Stage 2: Q = 1 / (2 cos(3π/8))
const BW4_Q2: f32 = 1.3066;
/// Q values for a 6th-order Butterworth cascade (three 2nd-order stages).
/// Stage 1: Q = 1 / (2 cos(π/12))
const BW6_Q1: f32 = 0.5176;
/// Stage 2: Q = 1 / (2 cos(3π/12))
const BW6_Q2: f32 = 0.7071;
/// Stage 3: Q = 1 / (2 cos(5π/12))
const BW6_Q3: f32 = 1.9319;
/// Q for the 19 kHz pilot notch (~3.8 kHz 3 dB bandwidth).
const PILOT_NOTCH_Q: f32 = 5.0;
/// Narrow 19 kHz band-pass used to derive zero-crossings for switching stereo demod.
@@ -39,7 +41,7 @@ const STEREO_MATRIX_GAIN: f32 = 0.50;
/// 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 = 16;
const WFM_RESAMP_TAPS: usize = 64;
/// Polyphase slots for the WFM fractional FIR resampler.
const WFM_RESAMP_PHASES: usize = 32;
fn build_wfm_resample_bank(cutoff: f32) -> [[f32; WFM_RESAMP_TAPS]; WFM_RESAMP_PHASES] {
@@ -608,15 +610,18 @@ pub struct WfmStereoDecoder {
/// 4th-order Butterworth cascade for L+R (two 2nd-order stages, Q = BW4_Q1/BW4_Q2).
sum_lpf1: BiquadLowPass,
sum_lpf2: BiquadLowPass,
sum_lpf3: BiquadLowPass,
/// Notch at 19 kHz for the mono output path — keeps pilot tone out of mono
/// audio without introducing phase mismatch with the diff channel.
sum_notch: BiquadNotch,
/// 4th-order Butterworth cascade for L-R (matched to sum path for stereo phase accuracy).
diff_lpf1: BiquadLowPass,
diff_lpf2: BiquadLowPass,
diff_lpf3: BiquadLowPass,
/// Quadrature companion of the L-R path used for phase trim / crosstalk adjustment.
diff_q_lpf1: BiquadLowPass,
diff_q_lpf2: BiquadLowPass,
diff_q_lpf3: BiquadLowPass,
/// Gentle high-pass on the stereo-difference path to reduce bass-driven IMD.
diff_dc: DcBlocker,
diff_q_dc: DcBlocker,
@@ -682,13 +687,16 @@ impl WfmStereoDecoder {
pilot_bpf: BiquadBandPass::new(composite_rate_f, PILOT_HZ, PILOT_BPF_Q),
// 4th-order Butterworth: two cascaded biquads with BW4_Q1/BW4_Q2.
// At 19 kHz (pilot): ≈ −12 dB; at 38 kHz (DSB carrier): ≈ −32 dB.
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_lpf1: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW6_Q1),
sum_lpf2: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW6_Q2),
sum_lpf3: BiquadLowPass::new(composite_rate_f, AUDIO_BW_HZ, BW6_Q3),
sum_notch: BiquadNotch::new(composite_rate_f, PILOT_HZ, PILOT_NOTCH_Q),
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),
diff_lpf1: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW6_Q1),
diff_lpf2: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW6_Q2),
diff_lpf3: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW6_Q3),
diff_q_lpf1: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW6_Q1),
diff_q_lpf2: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW6_Q2),
diff_q_lpf3: BiquadLowPass::new(composite_rate_f, STEREO_DIFF_BW_HZ, BW6_Q3),
diff_dc: DcBlocker::new(STEREO_DIFF_DC_R),
diff_q_dc: DcBlocker::new(STEREO_DIFF_DC_R),
dc_m: DcBlocker::new(0.9999),
@@ -780,17 +788,17 @@ impl WfmStereoDecoder {
// identical phase responses, which is required for good stereo separation.
// The notch is applied only on the mono output path where phase matching
// with the diff channel is irrelevant.
let sum = self.sum_lpf2.process(self.sum_lpf1.process(x));
let sum = self.sum_lpf3.process(self.sum_lpf2.process(self.sum_lpf1.process(x)));
// --- 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_dc
.process(self.diff_lpf2.process(self.diff_lpf1.process(x * (cos_2p * 2.0))));
.process(self.diff_lpf3.process(self.diff_lpf2.process(self.diff_lpf1.process(x * (cos_2p * 2.0)))));
let diff_q = self
.diff_q_dc
.process(self.diff_q_lpf2.process(self.diff_q_lpf1.process(x * (-sin_2p * 2.0))));
.process(self.diff_q_lpf3.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
@@ -885,11 +893,14 @@ impl WfmStereoDecoder {
self.pilot_bpf.reset();
self.sum_lpf1.reset();
self.sum_lpf2.reset();
self.sum_lpf3.reset();
self.sum_notch.reset();
self.diff_lpf1.reset();
self.diff_lpf2.reset();
self.diff_lpf3.reset();
self.diff_q_lpf1.reset();
self.diff_q_lpf2.reset();
self.diff_q_lpf3.reset();
self.diff_dc.reset();
self.diff_q_dc.reset();
self.dc_m.reset();