[fix](trx-backend-soapysdr,trx-frontend-http): make wfm mono mode real

Co-authored-by: Codex <codex@openai.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-server/src/rig_task.rs

@@ -462,6 +462,16 @@ async fn process_command(
             let _ = ctx.state_tx.send(ctx.state.clone());
             return snapshot_from(ctx.state);
         }
+        RigCommand::SetWfmStereo(enabled) => {
+            if let Err(e) = ctx.rig.set_wfm_stereo(enabled).await {
+                return Err(RigError::communication(format!("set_wfm_stereo: {e}")));
+            }
+            if let Some(f) = ctx.state.filter.as_mut() {
+                f.wfm_stereo = enabled;
+            }
+            let _ = ctx.state_tx.send(ctx.state.clone());
+            return snapshot_from(ctx.state);
+        }
         RigCommand::SetWfmDenoise(enabled) => {
             if let Err(e) = ctx.rig.set_wfm_denoise(enabled).await {
                 return Err(RigError::communication(format!("set_wfm_denoise: {e}")));

src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs

@@ -345,6 +345,7 @@ impl Deemphasis {
 #[derive(Debug, Clone)]
 pub struct WfmStereoDecoder {
     output_channels: usize,
+    stereo_enabled: bool,
     rds_decoder: RdsDecoder,
     rds_bpf: BiquadBandPass,
     rds_dc: DcBlocker,
@@ -393,6 +394,7 @@ impl WfmStereoDecoder {
         composite_rate: u32,
         audio_rate: u32,
         output_channels: usize,
+        stereo_enabled: bool,
         deemphasis_us: u32,
         denoise_enabled: bool,
     ) -> Self {
@@ -401,6 +403,7 @@ impl WfmStereoDecoder {
         let deemphasis_us = deemphasis_us as f32;
         Self {
             output_channels: output_channels.max(1),
+            stereo_enabled,
             rds_decoder: RdsDecoder::new(composite_rate),
             rds_bpf: BiquadBandPass::new(composite_rate_f, RDS_SUBCARRIER_HZ, RDS_BPF_Q),
             rds_dc: DcBlocker::new(0.995),
@@ -499,7 +502,7 @@ impl WfmStereoDecoder {
             let blend_i = prev_blend + frac * (stereo_blend - prev_blend);
 
             // --- Deemphasis + DC block + output ---
-            if self.output_channels >= 2 {
+            if self.output_channels >= 2 && self.stereo_enabled {
                 // Apply multiband or single-band stereo blend at audio rate.
                 let diff_denoised = if self.denoise_enabled {
                     // Multiband: attenuates high-frequency diff more aggressively
@@ -521,11 +524,14 @@ impl WfmStereoDecoder {
                 // Mono path: apply the pilot notch here so the 19 kHz pilot tone
                 // does not leak into mono audio.  Phase matching with diff is not
                 // a concern for mono, so the notch can sit anywhere in the chain.
-                output.push(
-                    self.dc_m
-                        .process(self.deemph_m.process(self.sum_notch.process(sum_i)))
-                        .clamp(-1.0, 1.0),
-                );
+                let mono = self
+                    .dc_m
+                    .process(self.deemph_m.process(self.sum_notch.process(sum_i)))
+                    .clamp(-1.0, 1.0);
+                output.push(mono);
+                if self.output_channels >= 2 {
+                    output.push(mono);
+                }
             }
         }
 
@@ -536,6 +542,10 @@ impl WfmStereoDecoder {
         self.denoise_enabled = enabled;
     }
 
+    pub fn set_stereo_enabled(&mut self, enabled: bool) {
+        self.stereo_enabled = enabled;
+    }
+
     pub fn rds_data(&self) -> Option<RdsData> {
         self.rds_decoder.snapshot()
     }

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp.rs

@@ -309,6 +309,8 @@ pub struct ChannelDsp {
     fir_taps: usize,
     /// WFM deemphasis time constant in microseconds.
     wfm_deemphasis_us: u32,
+    /// Whether WFM stereo decoding is enabled.
+    wfm_stereo: bool,
     /// Whether multiband stereo denoising is enabled for WFM.
     wfm_denoise: bool,
     /// Decimation factor: `sdr_sample_rate / audio_sample_rate`.
@@ -410,6 +412,7 @@ impl ChannelDsp {
                     channel_sample_rate,
                     self.audio_sample_rate,
                     self.output_channels,
+                    self.wfm_stereo,
                     self.wfm_deemphasis_us,
                     self.wfm_denoise,
                 ));
@@ -432,6 +435,7 @@ impl ChannelDsp {
         frame_duration_ms: u16,
         audio_bandwidth_hz: u32,
         wfm_deemphasis_us: u32,
+        wfm_stereo: bool,
         wfm_denoise: bool,
         fir_taps: usize,
         pcm_tx: broadcast::Sender<Vec<f32>>,
@@ -477,6 +481,7 @@ impl ChannelDsp {
             audio_bandwidth_hz,
             fir_taps: taps,
             wfm_deemphasis_us,
+            wfm_stereo,
             wfm_denoise,
             decim_factor,
             output_channels,
@@ -497,6 +502,7 @@ impl ChannelDsp {
                     channel_sample_rate,
                     audio_sample_rate,
                     output_channels,
+                    wfm_stereo,
                     wfm_deemphasis_us,
                     wfm_denoise,
                 ))
@@ -544,6 +550,13 @@ impl ChannelDsp {
         self.rebuild_filters(true);
     }
 
+    pub fn set_wfm_stereo(&mut self, enabled: bool) {
+        self.wfm_stereo = enabled;
+        if let Some(decoder) = &mut self.wfm_decoder {
+            decoder.set_stereo_enabled(enabled);
+        }
+    }
+
     pub fn set_wfm_denoise(&mut self, enabled: bool) {
         self.wfm_denoise = enabled;
         if let Some(decoder) = &mut self.wfm_decoder {
@@ -689,6 +702,7 @@ impl SdrPipeline {
         output_channels: usize,
         frame_duration_ms: u16,
         wfm_deemphasis_us: u32,
+        wfm_stereo: bool,
         wfm_denoise: bool,
         channels: &[(f64, RigMode, u32, usize)],
     ) -> Self {
@@ -711,6 +725,7 @@ impl SdrPipeline {
                 frame_duration_ms,
                 audio_bandwidth_hz,
                 wfm_deemphasis_us,
+                wfm_stereo,
                 wfm_denoise,
                 fir_taps,
                 pcm_tx.clone(),

src/trx-server/trx-backend/trx-backend-soapysdr/src/lib.rs

@@ -39,6 +39,8 @@ pub struct SoapySdrRig {
     retune_cmd: Arc<std::sync::Mutex<Option<f64>>>,
     /// Current WFM deemphasis setting in microseconds.
     wfm_deemphasis_us: u32,
+    /// Whether WFM stereo decode is enabled.
+    wfm_stereo: bool,
     /// Whether multiband WFM stereo denoising is enabled.
     wfm_denoise: bool,
 }
@@ -117,6 +119,7 @@ impl SoapySdrRig {
             audio_channels,
             frame_duration_ms,
             wfm_deemphasis_us,
+            true, // wfm_stereo: enabled by default
             true, // wfm_denoise: enabled by default
             channels,
         );
@@ -185,6 +188,7 @@ impl SoapySdrRig {
             center_hz: hardware_center_hz,
             retune_cmd,
             wfm_deemphasis_us,
+            wfm_stereo: true,
             wfm_denoise: true,
         })
     }
@@ -464,6 +468,19 @@ impl RigCat for SoapySdrRig {
         })
     }
 
+    fn set_wfm_stereo<'a>(
+        &'a mut self,
+        enabled: bool,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            self.wfm_stereo = enabled;
+            if let Some(dsp_arc) = self.pipeline.channel_dsps.get(self.primary_channel_idx) {
+                dsp_arc.lock().unwrap().set_wfm_stereo(enabled);
+            }
+            Ok(())
+        })
+    }
+
     fn set_wfm_denoise<'a>(
         &'a mut self,
         enabled: bool,
@@ -483,6 +500,7 @@ impl RigCat for SoapySdrRig {
             fir_taps: self.fir_taps,
             cw_center_hz: 700,
             wfm_deemphasis_us: self.wfm_deemphasis_us,
+            wfm_stereo: self.wfm_stereo,
             wfm_denoise: self.wfm_denoise,
         })
     }