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[fix](trx-backend-soapysdr): measure WFM signal strength in IQ domain, not power domain

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The previous carrier power IIR filtered |IQ|² (power), which only smoothed
temporal fluctuations but still integrated noise across the full 180 kHz WFM
channel bandwidth. This caused background noise to read ~-78 dBFS instead of
the expected ~-110 dBFS (~32 dB too high ≈ 10·log₁₀(180kHz/500Hz)).

Move the single-pole IIR lowpass to the IQ domain (filter I and Q separately
at ~500 Hz cutoff), then compute power from the filtered output. This rejects
out-of-band noise before the power measurement, so the meter reads true
carrier level rather than total wideband noise.

https://claude.ai/code/session_01W4WPMB2Lg3hgaY6opsk25f
Signed-off-by: Claude <noreply@anthropic.com>
This commit is contained in:
Claude
2026-03-27 17:07:54 +00:00
committed by Stan Grams
parent 0ad3440a2d
commit 3ca3836702
1 changed files with 25 additions and 18 deletions
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src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp/channel.rs
+25 -18
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@@ -297,15 +297,18 @@ pub struct ChannelDsp {
squelch: VirtualSquelch,
noise_blanker: NoiseBlanker,
last_signal_db: f32,
/// Per-sample IIR state for narrow carrier power measurement (WFM).
carrier_pwr_iir: f32,
/// IIR coefficient for the narrow carrier filter, precomputed from sample rate.
carrier_pwr_alpha: f32,
/// Single-pole IIR states for narrow IQ lowpass (WFM carrier measurement).
/// Filtering the IQ signal (not the power) rejects out-of-band noise so the
/// meter reads carrier level, not total wideband noise power.
carrier_iq_i: f32,
carrier_iq_q: f32,
/// IIR coefficient for the narrow IQ carrier filter, precomputed from sample rate.
carrier_iq_alpha: f32,
}
impl ChannelDsp {
/// Compute the single-pole IIR alpha for narrow carrier power measurement.
/// Uses ~500 Hz cutoff so the meter reads carrier envelope, not wideband noise.
/// Compute the single-pole IIR alpha for narrow IQ carrier measurement.
/// Uses ~500 Hz cutoff so the meter reads carrier level, not wideband noise.
fn narrow_carrier_alpha(channel_sample_rate: u32) -> f32 {
const CARRIER_BW_HZ: f32 = 500.0;
if channel_sample_rate == 0 {
@@ -422,8 +425,9 @@ impl ChannelDsp {
self.iq_agc = iq_agc_for_mode(&self.mode, channel_sample_rate);
self.audio_agc = agc_for_mode(&self.mode, self.audio_sample_rate);
self.audio_dc = dc_for_mode(&self.mode);
self.carrier_pwr_alpha = Self::narrow_carrier_alpha(channel_sample_rate);
self.carrier_pwr_iir = 0.0;
self.carrier_iq_alpha = Self::narrow_carrier_alpha(channel_sample_rate);
self.carrier_iq_i = 0.0;
self.carrier_iq_q = 0.0;
self.frame_buf.clear();
self.frame_buf_offset = 0;
}
@@ -539,8 +543,9 @@ impl ChannelDsp {
squelch: VirtualSquelch::new(squelch_cfg),
noise_blanker: NoiseBlanker::new(nb_cfg.enabled, nb_cfg.threshold),
last_signal_db: -120.0,
carrier_pwr_iir: 0.0,
carrier_pwr_alpha: Self::narrow_carrier_alpha(channel_sample_rate),
carrier_iq_i: 0.0,
carrier_iq_q: 0.0,
carrier_iq_alpha: Self::narrow_carrier_alpha(channel_sample_rate),
}
}
@@ -755,16 +760,18 @@ impl ChannelDsp {
// Signal strength measurement (before AGC).
{
if self.mode == RigMode::WFM {
// WFM: narrow carrier measurement via per-sample IIR on |IQ|².
// FM has constant envelope so IIR converges to carrier power A²,
// rejecting wideband noise that inflates a peak reading.
let alpha = self.carrier_pwr_alpha;
// WFM: narrow-band carrier measurement via IQ-domain lowpass.
// A single-pole IIR on each of I and Q (≈500 Hz cutoff) rejects
// wideband noise *before* computing power, so the meter reads
// carrier level rather than total noise across the 180 kHz channel.
let alpha = self.carrier_iq_alpha;
for s in decimated.iter() {
let pwr = s.re * s.re + s.im * s.im;
self.carrier_pwr_iir += alpha * (pwr - self.carrier_pwr_iir);
self.carrier_iq_i += alpha * (s.re - self.carrier_iq_i);
self.carrier_iq_q += alpha * (s.im - self.carrier_iq_q);
}
self.last_signal_db =
10.0 * self.carrier_pwr_iir.max(1e-12).log10();
let carrier_pwr =
self.carrier_iq_i * self.carrier_iq_i + self.carrier_iq_q * self.carrier_iq_q;
self.last_signal_db = 10.0 * carrier_pwr.max(1e-12).log10();
} else {
// Other modes: peak IQ magnitude with EMA smoothing.
const SIGNAL_EMA_ALPHA: f32 = 0.4;