From c1ed936c2e682a9a54d5081353bd42929e10d1a7 Mon Sep 17 00:00:00 2001
From: Stan Grams <sjg@haxx.space>
Date: Tue, 3 Mar 2026 01:58:11 +0100
Subject: [PATCH] [fix](trx-backend-soapysdr): restore simple AM demodulation

Revert the recent AM coherent detector experiment and restore the original envelope detector path.

Co-authored-by: Stan Grams <sjg@haxx.space>
Signed-off-by: Stan Grams <sjg@haxx.space>
---
 .../trx-backend-soapysdr/src/demod.rs         |  2 +-
 .../trx-backend-soapysdr/src/demod/am.rs      | 57 ++-----------------
 2 files changed, 6 insertions(+), 53 deletions(-)

diff --git a/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs b/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
index 3a60401..d6796fd 100644
--- a/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs
+++ b/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 coherent detector using a limiter-derived carrier reference.
+    /// AM envelope detector: magnitude of IQ, DC-removed.
     Am,
     /// Narrow-band FM: instantaneous frequency via quadrature.
     Fm,
diff --git a/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod/am.rs b/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod/am.rs
index 10293f0..4320954 100644
--- a/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod/am.rs
+++ b/src/trx-server/trx-backend/trx-backend-soapysdr/src/demod/am.rs
@@ -4,42 +4,12 @@
 
 use num_complex::Complex;
 
-/// 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.
+/// AM envelope detector: magnitude of IQ.
 pub(super) fn demod_am(samples: &[Complex<f32>]) -> Vec<f32> {
-    const EPSILON: f32 = 1.0e-12;
-    const REF_BLEND: f32 = 0.02;
-
-    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);
-    }
-
-    out
+    samples
+        .iter()
+        .map(|sample| (sample.re * sample.re + sample.im * sample.im).sqrt())
+        .collect()
 }
 
 #[cfg(test)]
@@ -83,21 +53,4 @@ 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}"
-        );
-    }
 }