[fix](trx-backend-soapysdr): retune wfm stereo gain

Increase the stereo matrix gain to 1.2 and trim the WFM output gain slightly to rebalance the decoded audio path.

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

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

@@ -33,7 +33,7 @@ const STEREO_SEPARATION_PHASE_TRIM: f32 = 0.0;
 const STEREO_SEPARATION_GAIN: f32 = 1.000;
 /// Extra headroom in the stereo matrix to reduce stereo-only clipping/IMD on
 /// strong program material. This keeps bass excursions from flattening treble.
-const STEREO_MATRIX_GAIN: f32 = 1.00;
+const STEREO_MATRIX_GAIN: f32 = 1.20;
 /// Stereo detection runs every N composite samples to reduce CPU.
 const STEREO_DETECT_DECIMATION: u32 = 16;
 /// Gentle high-pass memory for the stereo L-R path.

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

@@ -869,7 +869,7 @@ impl ChannelDsp {
         // WFM: full composite decoder (handles its own DC blocks + deemphasis).
         // No AGC — WFM uses IQ hard limiting + fixed output gain to preserve
         // stereo separation and avoid AGC pumping on broadcast audio.
-        const WFM_OUTPUT_GAIN: f32 = 0.35;
+        const WFM_OUTPUT_GAIN: f32 = 0.32;
         let audio = if let Some(decoder) = self.wfm_decoder.as_mut() {
             let mut out = decoder.process_iq(decimated);
             for sample in &mut out {