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[fix](trx-backend-soapysdr): measure signal strength from peak filtered IQ before AGC

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Replace the DC-component approach (which underreads FM due to carrier deviation) with peak |s|² on the filtered+decimated IQ before AGC is applied. Works correctly for both constant-envelope FM and narrowband modes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
This commit is contained in:
Stan Grams
2026-03-27 15:57:38 +01:00
parent 6440f67d94
commit 56c19ae15d
1 changed files with 13 additions and 12 deletions
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src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp/channel.rs
+13 -12
View File
@@ -673,18 +673,6 @@ impl ChannelDsp {
}
self.mixer_phase = (phase_start + n as f64 * phase_inc).rem_euclid(std::f64::consts::TAU);
// Carrier power: DC component of the mixed signal (narrow-band estimate
// at the tuned frequency). Correlates with the spectrum FFT peak.
// EMA smoothing (α ≈ 0.4) for fast response with light jitter reduction.
{
const SIGNAL_EMA_ALPHA: f32 = 0.4;
let inv_n = 1.0 / n as f32;
let dc_i: f32 = mixed_i.iter().sum::<f32>() * inv_n;
let dc_q: f32 = mixed_q.iter().sum::<f32>() * inv_n;
let carrier_db = 10.0 * (dc_i * dc_i + dc_q * dc_q).max(1e-12).log10();
self.last_signal_db += SIGNAL_EMA_ALPHA * (carrier_db - self.last_signal_db);
}
self.lpf_iq.filter_block_into(
mixed_i,
mixed_q,
@@ -746,6 +734,19 @@ impl ChannelDsp {
return;
}
// Signal strength: peak IQ magnitude of filtered+decimated signal
// BEFORE AGC. For FM (constant envelope) peak ≈ carrier power.
// EMA (α = 0.4) for fast response with light jitter reduction.
{
const SIGNAL_EMA_ALPHA: f32 = 0.4;
let peak_power = decimated
.iter()
.map(|s| s.re * s.re + s.im * s.im)
.fold(0.0_f32, f32::max);
let peak_db = 10.0 * peak_power.max(1e-12).log10();
self.last_signal_db += SIGNAL_EMA_ALPHA * (peak_db - self.last_signal_db);
}
if let Some(iq_agc) = &mut self.iq_agc {
for sample in decimated.iter_mut() {
*sample = iq_agc.process_complex(*sample);