[feat](trx-backend-soapysdr): improve AM coherent demodulation

Replace the raw AM envelope detector with a limiter-derived coherent detector and update backend tests for the current channel DSP constructor.

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

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

@@ -135,7 +135,7 @@ pub enum Demodulator {
     Usb,
     /// Lower sideband SSB: negate imaginary part before taking real part.
     Lsb,
-    /// AM envelope detector: magnitude of IQ, DC-removed.
+    /// AM coherent detector using a limiter-derived carrier reference.
     Am,
     /// Narrow-band FM: instantaneous frequency via quadrature.
     Fm,

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

@@ -4,12 +4,42 @@
 
 use num_complex::Complex;
 
-/// AM envelope detector: magnitude of IQ.
+/// AM demodulator using a limiter-derived coherent reference.
+///
+/// This is a practical DSP analogue of the patent's split/limit/mix approach:
+/// derive a fixed-amplitude carrier reference from the incoming IQ, smooth that
+/// reference to follow carrier phase/frequency, then mix the original branch by
+/// the conjugate reference and take the in-phase projection.
 pub(super) fn demod_am(samples: &[Complex<f32>]) -> Vec<f32> {
-    samples
-        .iter()
-        .map(|sample| (sample.re * sample.re + sample.im * sample.im).sqrt())
-        .collect()
+    const EPSILON: f32 = 1.0e-12;
+    const REF_BLEND: f32 = 0.08;
+
+    let mut out = Vec::with_capacity(samples.len());
+    let mut carrier_ref = Complex::new(1.0_f32, 0.0);
+
+    for &sample in samples {
+        let mag_sq = sample.re * sample.re + sample.im * sample.im;
+        if mag_sq <= EPSILON {
+            out.push(0.0);
+            continue;
+        }
+
+        let mag = mag_sq.sqrt();
+        let limited = sample / mag;
+        let blended = carrier_ref * (1.0 - REF_BLEND) + limited * REF_BLEND;
+        let blended_mag_sq = blended.re * blended.re + blended.im * blended.im;
+        if blended_mag_sq > EPSILON {
+            carrier_ref = blended / blended_mag_sq.sqrt();
+        } else {
+            carrier_ref = limited;
+        }
+
+        // Project the original signal onto the limiter-derived carrier phase.
+        let mixed = sample * carrier_ref.conj();
+        out.push(mixed.re.max(0.0));
+    }
+
+    out
 }
 
 #[cfg(test)]
@@ -53,4 +83,21 @@ mod tests {
             assert_approx_eq(got, exp, 1e-6, &format!("AM sample {idx}"));
         }
     }
+
+    #[test]
+    fn test_am_tracks_rotating_carrier_reference() {
+        let input: Vec<Complex<f32>> = (0..16)
+            .map(|idx| {
+                let angle = idx as f32 * 0.05;
+                Complex::new(angle.cos(), angle.sin())
+            })
+            .collect();
+        let out = demod_am(&input);
+        assert_eq!(out.len(), input.len());
+        let avg = out.iter().skip(1).copied().sum::<f32>() / (out.len().saturating_sub(1) as f32);
+        assert!(
+            avg > 0.95,
+            "AM rotating carrier: expected strong average coherent output, got {avg}"
+        );
+    }
 }

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

@@ -478,6 +478,7 @@ mod tests {
     #[test]
     fn channel_dsp_processes_silence() {
         let (pcm_tx, _pcm_rx) = broadcast::channel::<Vec<f32>>(8);
+        let (iq_tx, _iq_rx) = broadcast::channel::<Vec<Complex<f32>>>(8);
         let mut dsp = ChannelDsp::new(
             0.0,
             &RigMode::USB,
@@ -490,6 +491,7 @@ mod tests {
             true,
             31,
             pcm_tx,
+            iq_tx,
         );
         let block = vec![Complex::new(0.0_f32, 0.0_f32); 4096];
         dsp.process_block(&block);
@@ -498,6 +500,7 @@ mod tests {
     #[test]
     fn channel_dsp_set_mode() {
         let (pcm_tx, _) = broadcast::channel::<Vec<f32>>(8);
+        let (iq_tx, _) = broadcast::channel::<Vec<Complex<f32>>>(8);
         let mut dsp = ChannelDsp::new(
             0.0,
             &RigMode::USB,
@@ -510,6 +513,7 @@ mod tests {
             true,
             31,
             pcm_tx,
+            iq_tx,
         );
         assert_eq!(dsp.demodulator, Demodulator::Usb);
         dsp.set_mode(&RigMode::FM);