[perf](trx-server): reduce headless scheduler audio cpu

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

src/trx-server/src/main.rs

@@ -575,6 +575,12 @@ fn spawn_rig_audio_stack(
                 loop {
                     match sdr_rx.recv().await {
                         Ok(frame) => {
+                            let has_audio_clients = rx_audio_tx_sdr.receiver_count() > 0;
+                            if !has_audio_clients {
+                                let _ = pcm_tx_clone.send(frame);
+                                continue;
+                            }
+
                             let pcm_frame = match sdr_channels {
                                 1 => frame,
                                 2 => {

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

@@ -155,6 +155,7 @@ impl SdrPipeline {
                 wfm_deemphasis_us,
                 wfm_stereo,
                 fir_taps,
+                channel_idx == 0,
                 channel_squelch_cfg,
                 pcm_tx.clone(),
                 iq_tx.clone(),
@@ -233,6 +234,7 @@ impl SdrPipeline {
             self.wfm_deemphasis_us,
             self.wfm_stereo,
             fir_taps.max(1),
+            false,
             VirtualSquelchConfig::default(),
             pcm_tx.clone(),
             iq_tx.clone(),

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

@@ -169,6 +169,7 @@ pub struct ChannelDsp {
     audio_agc: SoftAgc,
     audio_dc: Option<DcBlocker>,
     processing_enabled: bool,
+    force_mono_pcm: bool,
     squelch: VirtualSquelch,
 }
 
@@ -269,6 +270,7 @@ impl ChannelDsp {
         wfm_deemphasis_us: u32,
         wfm_stereo: bool,
         fir_taps: usize,
+        force_mono_pcm: bool,
         squelch_cfg: VirtualSquelchConfig,
         pcm_tx: broadcast::Sender<Vec<f32>>,
         iq_tx: broadcast::Sender<Vec<Complex<f32>>>,
@@ -348,6 +350,7 @@ impl ChannelDsp {
             audio_agc: agc_for_mode(mode, audio_sample_rate),
             audio_dc: dc_for_mode(mode),
             processing_enabled: true,
+            force_mono_pcm,
             squelch: VirtualSquelch::new(squelch_cfg),
         }
     }
@@ -356,6 +359,10 @@ impl ChannelDsp {
         self.processing_enabled = enabled;
     }
 
+    pub fn set_force_mono_pcm(&mut self, enabled: bool) {
+        self.force_mono_pcm = enabled;
+    }
+
     pub fn set_squelch(&mut self, enabled: bool, threshold_db: f32) {
         self.squelch.set_enabled(enabled);
         self.squelch.set_threshold_db(threshold_db);
@@ -548,7 +555,7 @@ impl ChannelDsp {
                 }
                 *sample = self.audio_agc.process(*sample);
             }
-            if self.output_channels >= 2 {
+            if self.output_channels >= 2 && !self.force_mono_pcm {
                 let mut stereo = Vec::with_capacity(raw.len() * self.output_channels);
                 for sample in raw {
                     stereo.push(sample);

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

@@ -203,6 +203,13 @@ impl SdrVirtualChannelManager {
             hidden,
         });
 
+        if hidden {
+            let dsps = self.pipeline.channel_dsps.read().unwrap();
+            if let Some(dsp_arc) = dsps.get(pipeline_slot) {
+                dsp_arc.lock().unwrap().set_force_mono_pcm(true);
+            }
+        }
+
         Ok(pcm_rx)
     }