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[fix](trx-backend-soapysdr): remove agc from wfm chain, use fixed output gain

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Replace both IQ AGC and audio AGC in the WFM path with a fixed output
gain of 0.35. AGC pumping on broadcast audio degrades stereo separation
and introduces audible artifacts. The IQ hard limiter already normalizes
input magnitude, and the WFM decoder's internal deemphasis + matrix gain
provides consistent output levels.

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 11:17:14 +01:00
parent c2b287e000
commit f7c10a0185
2 changed files with 10 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+1
View File
@@ -348,6 +348,7 @@ impl SoftAgc {
(x * gain).clamp(-1.0, 1.0)
}
#[allow(dead_code)]
pub(crate) fn process_pair(&mut self, left: f32, right: f32) -> (f32, f32) {
let gain = self.update_gain(left.abs().max(right.abs()));
(
src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp.rs
+8 -19
View File
@@ -428,7 +428,9 @@ fn agc_for_mode(mode: &RigMode, audio_sample_rate: u32) -> SoftAgc {
fn iq_agc_for_mode(mode: &RigMode, sample_rate: u32) -> Option<SoftAgc> {
let sr = sample_rate.max(1) as f32;
match mode {
RigMode::FM | RigMode::WFM | RigMode::PKT => Some(SoftAgc::new(sr, 0.5, 150.0, 0.8, 12.0)),
RigMode::FM | RigMode::PKT => Some(SoftAgc::new(sr, 0.5, 150.0, 0.8, 12.0)),
// WFM uses a hard IQ limiter instead of AGC to preserve phase accuracy.
RigMode::WFM => None,
_ => None,
}
}
@@ -864,27 +866,14 @@ impl ChannelDsp {
}
// --- 4. Demodulate + post-process -----------------------------------
// WFM: full composite decoder (handles its own DC blocks + deemphasis),
// then apply post-audio AGC on the decoded PCM. Other modes use the
// normal stateless demodulator → DC blocker (where enabled) → AGC path.
// WFM: full composite decoder (handles its own DC blocks + deemphasis).
// No AGC — WFM uses IQ hard limiting + fixed output gain to preserve
// stereo separation and avoid AGC pumping on broadcast audio.
const WFM_OUTPUT_GAIN: f32 = 0.35;
let audio = if let Some(decoder) = self.wfm_decoder.as_mut() {
let mut out = decoder.process_iq(decimated);
if !self.wfm_stereo && self.output_channels >= 2 {
for pair in out.chunks_exact_mut(2) {
let mono = self.audio_agc.process(pair[0]);
pair[0] = mono;
pair[1] = mono;
}
} else if self.wfm_stereo && self.output_channels >= 2 {
for pair in out.chunks_exact_mut(2) {
let (left, right) = self.audio_agc.process_pair(pair[0], pair[1]);
pair[0] = left;
pair[1] = right;
}
} else {
for sample in &mut out {
*sample = self.audio_agc.process(*sample);
}
*sample = (*sample * WFM_OUTPUT_GAIN).clamp(-1.0, 1.0);
}
out
} else {