[fix](trx-backend-soapysdr): disable hidden AIS DSP outside AIS modes

Add per-channel processing gating and disable hidden AIS channel DSP unless mode is AIS or MARINE, reducing continuous IQ read-loop CPU load in normal operation.\n\nCo-authored-by: OpenAI Codex <codex@openai.com>

Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -86,6 +86,7 @@ pub struct ChannelDsp {
     iq_agc: Option<SoftAgc>,
     audio_agc: SoftAgc,
     audio_dc: Option<DcBlocker>,
+    processing_enabled: bool,
 }
 
 impl ChannelDsp {
@@ -262,9 +263,14 @@ impl ChannelDsp {
             iq_agc: iq_agc_for_mode(mode, channel_sample_rate),
             audio_agc: agc_for_mode(mode, audio_sample_rate),
             audio_dc: dc_for_mode(mode),
+            processing_enabled: true,
         }
     }
 
+    pub fn set_processing_enabled(&mut self, enabled: bool) {
+        self.processing_enabled = enabled;
+    }
+
     pub fn set_mode(&mut self, mode: &RigMode) {
         self.mode = mode.clone();
         if *mode != RigMode::WFM {
@@ -325,6 +331,9 @@ impl ChannelDsp {
     }
 
     pub fn process_block(&mut self, block: &[Complex<f32>]) {
+        if !self.processing_enabled {
+            return;
+        }
         let n = block.len();
         if n == 0 {
             return;

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

@@ -227,7 +227,7 @@ impl SoapySdrRig {
         let spectrum_buf = pipeline.spectrum_buf.clone();
         let retune_cmd = pipeline.retune_cmd.clone();
 
-        Ok(Self {
+        let rig = Self {
             info,
             freq: initial_freq,
             mode: initial_mode,
@@ -245,7 +245,9 @@ impl SoapySdrRig {
             gain_db,
             max_gain_db,
             ais_channel_indices: Some((primary_channel_count, primary_channel_count + 1)),
-        })
+        };
+        rig.apply_ais_channel_activity();
+        Ok(rig)
     }
 
     /// Simple constructor for backward compatibility with the factory function.
@@ -300,6 +302,18 @@ impl SoapySdrRig {
         }
     }
 
+    fn apply_ais_channel_activity(&self) {
+        let Some((ais_a_idx, ais_b_idx)) = self.ais_channel_indices else {
+            return;
+        };
+        let enabled = matches!(self.mode, RigMode::AIS | RigMode::MARINE);
+        for idx in [ais_a_idx, ais_b_idx] {
+            if let Some(dsp_arc) = self.pipeline.channel_dsps.get(idx) {
+                dsp_arc.lock().unwrap().set_processing_enabled(enabled);
+            }
+        }
+    }
+
     pub fn subscribe_iq_channel(
         &self,
         channel_idx: usize,
@@ -435,6 +449,7 @@ impl RigCat for SoapySdrRig {
                 dsp.set_mode(&mode);
                 dsp.set_filter(self.bandwidth_hz, self.fir_taps as usize);
             }
+            self.apply_ais_channel_activity();
             self.apply_ais_channel_filters();
             Ok(())
         })