[fix](trx-backend-soapysdr): restore wfm agc and cutoffs

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

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

@@ -11,13 +11,14 @@ const RDS_BPF_Q: f32 = 10.0;
 /// Pilot tone frequency (Hz).
 const PILOT_HZ: f32 = 19_000.0;
 /// Audio bandwidth for WFM (Hz).
-/// 17 kHz preserves more top-end while still leaving guard band below the
-/// 19 kHz pilot.
-const AUDIO_BW_HZ: f32 = 17_000.0;
+/// 15.8 kHz leaves more guard band below the 19 kHz pilot and reduces
+/// top-end artifacts on strong signals while still preserving the useful
+/// broadcast audio range.
+const AUDIO_BW_HZ: f32 = 15_800.0;
 /// Stereo L-R subchannel bandwidth for WFM (Hz).
 /// Keep this a bit lower than the mono path because the recovered difference
 /// signal is noisier and more prone to high-frequency artifacts.
-const STEREO_DIFF_BW_HZ: f32 = 15_800.0;
+const STEREO_DIFF_BW_HZ: f32 = 14_500.0;
 /// Q values for a proper 4th-order Butterworth cascade (two 2nd-order stages).
 /// Stage 1: Q = 1 / (2 cos(π/8))
 const BW4_Q1: f32 = 0.5412;
@@ -220,6 +221,14 @@ impl SoftAgc {
         (x * gain).clamp(-1.0, 1.0)
     }
 
+    pub(crate) fn process_pair(&mut self, left: f32, right: f32) -> (f32, f32) {
+        let gain = self.update_gain(left.abs().max(right.abs()));
+        (
+            (left * gain).clamp(-1.0, 1.0),
+            (right * gain).clamp(-1.0, 1.0),
+        )
+    }
+
     pub(crate) fn process_complex(&mut self, x: Complex<f32>) -> Complex<f32> {
         let gain = self.update_gain((x.re * x.re + x.im * x.im).sqrt());
         let mut y = x * gain;

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

@@ -690,12 +690,29 @@ impl ChannelDsp {
         }
 
         // --- 4. Demodulate + post-process -----------------------------------
-        // WFM: full composite decoder (handles its own DC blocks + deemphasis).
-        // All other modes: stateless demodulator → DC blocker (where enabled) → AGC.
-        // WFM bypasses post-audio AGC so the deemphasized stereo path is
-        // heard directly; all other modes use the normal post-demod AGC path.
+        // WFM: full composite decoder (handles its own DC blocks + deemphasis),
+        // then apply post-audio AGC on the decoded PCM. Other modes use the
+        // normal stateless demodulator → DC blocker (where enabled) → AGC path.
         let audio = if let Some(decoder) = self.wfm_decoder.as_mut() {
-            decoder.process_iq(&decimated)
+            let mut out = decoder.process_iq(&decimated);
+            if !self.wfm_stereo && self.output_channels >= 2 {
+                for pair in out.chunks_exact_mut(2) {
+                    let mono = self.audio_agc.process(pair[0]);
+                    pair[0] = mono;
+                    pair[1] = mono;
+                }
+            } else if self.wfm_stereo && self.output_channels >= 2 {
+                for pair in out.chunks_exact_mut(2) {
+                    let (left, right) = self.audio_agc.process_pair(pair[0], pair[1]);
+                    pair[0] = left;
+                    pair[1] = right;
+                }
+            } else {
+                for sample in &mut out {
+                    *sample = self.audio_agc.process(*sample);
+                }
+            }
+            out
         } else {
             let mut raw = self.demodulator.demodulate(&decimated);
             for s in &mut raw {