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[perf](trx-backend-soapysdr): improve wfm stereo resampler and pilot rejection

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Increase polyphase resampler phases from 32 to 128 for finer fractional
sample positioning. Replace Hamming window with Blackman-Harris for ~92 dB
stopband rejection. Add pilot notch on composite signal before diff demod
to prevent 19 kHz × 38 kHz intermod products in the stereo difference
path.

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 09:37:11 +01:00
parent 700e5c9fab
commit 380231a68f
1 changed files with 16 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+16 -5
View File
@@ -43,7 +43,7 @@ const STEREO_DIFF_DC_R: f32 = 0.9995;
/// Fractional-resampler FIR taps for WFM audio reconstruction.
const WFM_RESAMP_TAPS: usize = 64;
/// Polyphase slots for the WFM fractional FIR resampler.
const WFM_RESAMP_PHASES: usize = 32;
const WFM_RESAMP_PHASES: usize = 128;
fn build_wfm_resample_bank(cutoff: f32) -> [[f32; WFM_RESAMP_TAPS]; WFM_RESAMP_PHASES] {
let mut bank = [[0.0; WFM_RESAMP_TAPS]; WFM_RESAMP_PHASES];
let anchor = (WFM_RESAMP_TAPS / 2 - 1) as f32;
@@ -63,7 +63,10 @@ fn build_wfm_resample_bank(cutoff: f32) -> [[f32; WFM_RESAMP_TAPS]; WFM_RESAMP_P
1.0
} else {
let pos = tap_idx as f32 / (WFM_RESAMP_TAPS - 1) as f32;
0.54 - 0.46 * (2.0 * std::f32::consts::PI * pos).cos()
let tw = 2.0 * std::f32::consts::PI * pos;
0.35875 - 0.48829 * tw.cos()
+ 0.14128 * (2.0 * tw).cos()
- 0.01168 * (3.0 * tw).cos()
};
*coeff = sinc * window;
sum += *coeff;
@@ -614,6 +617,9 @@ pub struct WfmStereoDecoder {
/// 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,
/// Notch at 19 kHz on composite before diff demod — removes pilot that would
/// create intermod products when multiplied by the 38 kHz carrier.
diff_pilot_notch: BiquadNotch,
/// 4th-order Butterworth cascade for L-R (matched to sum path for stereo phase accuracy).
diff_lpf1: BiquadLowPass,
diff_lpf2: BiquadLowPass,
@@ -691,6 +697,7 @@ impl WfmStereoDecoder {
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_pilot_notch: BiquadNotch::new(composite_rate_f, PILOT_HZ, PILOT_NOTCH_Q),
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),
@@ -790,15 +797,18 @@ impl WfmStereoDecoder {
// with the diff channel is irrelevant.
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) ---
// --- L-R (diff): 38 kHz demod + 6th-order Butterworth (unblended) ---
// Notch the 19 kHz pilot from the composite before multiplying by the
// 38 kHz carrier to prevent pilot×carrier intermod products.
// 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 x_notched = self.diff_pilot_notch.process(x);
let diff_i = self
.diff_dc
.process(self.diff_lpf3.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_notched * (cos_2p * 2.0)))));
let diff_q = self
.diff_q_dc
.process(self.diff_q_lpf3.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_notched * (-sin_2p * 2.0)))));
// --- Polyphase FIR fractional resampling ---
// This uses a short windowed-sinc bank instead of cubic interpolation
@@ -895,6 +905,7 @@ impl WfmStereoDecoder {
self.sum_lpf2.reset();
self.sum_lpf3.reset();
self.sum_notch.reset();
self.diff_pilot_notch.reset();
self.diff_lpf1.reset();
self.diff_lpf2.reset();
self.diff_lpf3.reset();