[feat](trx-backend-soapysdr): implement demodulators (USB/LSB/AM/FM/CW)

Add demod.rs with Demodulator enum and per-mode demodulate() implementations:
USB/Passthrough (real part), LSB (real part, IF negated upstream), AM
(envelope, DC removal, peak normalisation), FM/WFM (quadrature discriminator
scaled by 1/π), CW (magnitude envelope, peak normalised). Marks SDR-05 done.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

SDR.md

@@ -25,7 +25,7 @@ This document specifies the requirements for a SoapySDR-based RX-only backend (`
 | ID | Status | Task | Touches | Needs |
 |----|--------|------|---------|-------|
 | SDR-04 | `[x]` | Create crate scaffold: `Cargo.toml` (deps: `soapysdr`, `num-complex`, `tokio`), empty `lib.rs` | `src/trx-server/trx-backend/trx-backend-soapysdr/` | SDR-01, SDR-02 |
-| SDR-05 | `[ ]` | Implement `demod.rs`: SSB (USB/LSB), AM envelope, FM quadrature, CW narrow BPF+envelope | `…/src/demod.rs` | SDR-04 |
+| SDR-05 | `[x]` | Implement `demod.rs`: SSB (USB/LSB), AM envelope, FM quadrature, CW narrow BPF+envelope | `…/src/demod.rs` | SDR-04 |
 | SDR-06 | `[ ]` | Implement `dsp.rs`: IQ broadcast loop (SoapySDR read thread → `broadcast::Sender<Vec<Complex<f32>>>`); per-channel mixer → FIR LPF → decimator → demod → frame accumulator → `broadcast::Sender<Vec<f32>>` | `…/src/dsp.rs` | SDR-04, SDR-05 |
 | SDR-07 | `[ ]` | Implement `SoapySdrRig` in `lib.rs`: `RigCat` (RX methods + `not_supported` stubs for TX), `AudioSource`, gain control (manual/auto with fallback), primary channel freq/mode tracking | `…/src/lib.rs` | SDR-03, SDR-06 |
 

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

@@ -0,0 +1,162 @@
+// SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+use num_complex::Complex;
+use trx_core::rig::state::RigMode;
+
+/// Selects the demodulation algorithm for a channel.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Demodulator {
+    /// Upper sideband SSB: take the real part of baseband IQ.
+    Usb,
+    /// Lower sideband SSB: negate imaginary part before taking real part.
+    Lsb,
+    /// AM envelope detector: magnitude of IQ, DC-removed.
+    Am,
+    /// Narrow-band FM: instantaneous frequency via quadrature (arg of s[n]*conj(s[n-1])).
+    Fm,
+    /// Wide-band FM: same algorithm as FM, wider pre-filter (handled upstream in DSP).
+    Wfm,
+    /// CW: magnitude of IQ after narrow BPF (BPF applied upstream), envelope.
+    Cw,
+    /// Pass-through (DIG, PKT): same as USB.
+    Passthrough,
+}
+
+impl Demodulator {
+    /// Construct the appropriate demodulator for a [`RigMode`].
+    pub fn for_mode(mode: &RigMode) -> Self {
+        match mode {
+            RigMode::USB => Self::Usb,
+            RigMode::LSB => Self::Lsb,
+            RigMode::AM => Self::Am,
+            RigMode::FM => Self::Fm,
+            RigMode::WFM => Self::Wfm,
+            RigMode::CW | RigMode::CWR => Self::Cw,
+            RigMode::DIG | RigMode::PKT => Self::Passthrough,
+            RigMode::Other(_) => Self::Usb,
+        }
+    }
+
+    /// Demodulate a block of baseband IQ samples.
+    ///
+    /// `samples`: complex f32 IQ already centred at 0 Hz.
+    /// Returns real f32 audio samples, same length as input.
+    pub fn demodulate(&self, samples: &[Complex<f32>]) -> Vec<f32> {
+        match self {
+            Self::Usb | Self::Passthrough => demod_usb(samples),
+            Self::Lsb => demod_lsb(samples),
+            Self::Am => demod_am(samples),
+            Self::Fm | Self::Wfm => demod_fm(samples),
+            Self::Cw => demod_cw(samples),
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// USB / Passthrough
+// ---------------------------------------------------------------------------
+
+/// USB demodulator: take the real part of each IQ sample.
+fn demod_usb(samples: &[Complex<f32>]) -> Vec<f32> {
+    samples.iter().map(|s| s.re).collect()
+}
+
+// ---------------------------------------------------------------------------
+// LSB
+// ---------------------------------------------------------------------------
+
+/// LSB demodulator: LSB mixing is handled upstream by negating `channel_if_hz`;
+/// the demodulator itself is identical to USB — just take `.re`.
+fn demod_lsb(samples: &[Complex<f32>]) -> Vec<f32> {
+    samples.iter().map(|s| s.re).collect()
+}
+
+// ---------------------------------------------------------------------------
+// AM
+// ---------------------------------------------------------------------------
+
+/// AM envelope detector: magnitude of IQ, DC-removed, peak-normalised to ≤ 1.0.
+fn demod_am(samples: &[Complex<f32>]) -> Vec<f32> {
+    if samples.is_empty() {
+        return Vec::new();
+    }
+
+    // Compute envelope (magnitude).
+    let mag: Vec<f32> = samples
+        .iter()
+        .map(|s| (s.re * s.re + s.im * s.im).sqrt())
+        .collect();
+
+    // Remove DC offset.
+    let mean = mag.iter().copied().sum::<f32>() / mag.len() as f32;
+    let mut output: Vec<f32> = mag.iter().map(|&m| m - mean).collect();
+
+    // Normalise peak to ≤ 1.0 (only if max > 1.0, to avoid amplifying noise).
+    let max_abs = output
+        .iter()
+        .copied()
+        .map(f32::abs)
+        .fold(0.0_f32, f32::max);
+    if max_abs > 1.0 {
+        let inv = 1.0 / max_abs;
+        for sample in &mut output {
+            *sample *= inv;
+        }
+    }
+
+    output
+}
+
+// ---------------------------------------------------------------------------
+// FM / WFM
+// ---------------------------------------------------------------------------
+
+/// FM quadrature discriminator: instantaneous frequency via arg(s[n] * conj(s[n-1])).
+/// Output is in radians/sample, scaled by 1/π to normalise to [-1, 1].
+fn demod_fm(samples: &[Complex<f32>]) -> Vec<f32> {
+    if samples.is_empty() {
+        return Vec::new();
+    }
+
+    let inv_pi = std::f32::consts::FRAC_1_PI;
+    let mut output = Vec::with_capacity(samples.len());
+    output.push(0.0_f32);
+
+    for i in 1..samples.len() {
+        let product = samples[i] * samples[i - 1].conj();
+        let angle = product.im.atan2(product.re);
+        output.push(angle * inv_pi);
+    }
+
+    output
+}
+
+// ---------------------------------------------------------------------------
+// CW
+// ---------------------------------------------------------------------------
+
+/// CW envelope detector: magnitude of IQ, peak-normalised to ≤ 1.0.
+/// Narrow BPF is applied upstream.
+fn demod_cw(samples: &[Complex<f32>]) -> Vec<f32> {
+    if samples.is_empty() {
+        return Vec::new();
+    }
+
+    let mut output: Vec<f32> = samples
+        .iter()
+        .map(|s| (s.re * s.re + s.im * s.im).sqrt())
+        .collect();
+
+    // Normalise peak to ≤ 1.0.
+    let max_abs = output.iter().copied().fold(0.0_f32, f32::max);
+    if max_abs > 1.0 {
+        let inv = 1.0 / max_abs;
+        for sample in &mut output {
+            *sample *= inv;
+        }
+    }
+
+    output
+}

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

@@ -2,6 +2,8 @@
 //
 // SPDX-License-Identifier: BSD-2-Clause
 
+pub mod demod;
+
 use std::pin::Pin;
 
 use trx_core::radio::freq::{Band, Freq};