feat(dsp): double-talk detection for the NLMS echo canceller (Stage C)

Add Geigel double-talk detection so the adaptive filter stops diverging when
the near-end talks over the far-end (the failure the Stage B demo exposed:
ERLE went negative as the filter mistook near-end voice for echo).

- src/dsp/aec.rs: DoubleTalkDetector — Geigel test (mic level vs a sliding-
  window peak of the far-end, via an O(1) monotonic-deque max) with a hangover
  latch. EchoCanceller — wraps Nlms + the detector, freezing adaptation while
  double-talk is declared; reports the double-talk rate; DTD can be toggled off
  for A/B. 5 aec tests (detector fires on near-end not echo; DTD protects a
  converged filter through double-talk, +15 dB over raw NLMS).
- specview aec: now uses EchoCanceller with tunable --dtd-threshold / --hangover
  / --near-onset / --no-dtd, and prints the double-talk %.

Tuning found with the harness (white far-end, -12 dB echo, near-end onset
mid-call): DTD threshold 0.5 is the sweet spot — late-call ERLE +29.6 dB with
DTD vs -4.0 dB without (a 33 dB swing); 0.35 over-triggers (never learns), 0.7
under-triggers (drifts). Default set to 0.5. Also documents the false-positive
/miss tradeoff and that correlated (pink/speech) far-ends converge slower.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bin/specview.rs

@@ -55,8 +55,9 @@ const USAGE: &str = "specview — spectrogram + AEC measurement tool\n\n\
   specview gen  <kind> -o out.wav [--secs S] [--amp A] [--freq F] [--f0 F] [--f1 F] [--seed N]\n\
                 kinds: sine sweep white pink impulse silence\n\
   specview erle <before.wav> <after.wav> [--show]\n\
-  specview aec  --far far.wav [--near near.wav] [--delay N] [--tail N] [--atten A]\n\
-                [--taps N] [--mu M] [--out cleaned.wav] [--show]";
+  specview aec  --far far.wav [--near near.wav] [--near-onset S] [--delay N] [--tail N]\n\
+                [--atten A] [--taps N] [--mu M] [--dtd-threshold T] [--hangover N]\n\
+                [--no-dtd] [--out cleaned.wav] [--show]";
 
 /// Renders a WAV as a spectrogram plus a numeric summary.
 fn cmd_show(args: &[String]) -> Result<(), String> {
@@ -183,12 +184,19 @@ fn cmd_aec(args: &[String]) -> Result<(), String> {
     let far = wav::read(Path::new(far_path))?;
     let sr = far.sample_rate;
 
-    // Optional near-end (the local voice the canceller must preserve).
+    // Optional near-end (the local voice the canceller must preserve). An onset
+    // can be set so the near-end starts partway through, exercising the classic
+    // "far-end converges first, then double-talk" case.
+    let onset = (flags.f32_or("near-onset", 0.0) * sr as f32) as usize;
     let near = match flags.get("near") {
         Some(p) => {
             let d = wav::read(Path::new(p))?;
             let mut v = d.samples;
             v.resize(far.samples.len(), 0.0); // align length to far-end
+            let cut = onset.min(v.len());
+            for s in v.iter_mut().take(cut) {
+                *s = 0.0; // silence before onset
+            }
             v
         }
         None => vec![0.0f32; far.samples.len()],
@@ -197,15 +205,22 @@ fn cmd_aec(args: &[String]) -> Result<(), String> {
     // Build the echo and the mic signal (echo + near-end).
     let delay = flags.usize_or("delay", 480); // ~10 ms at 48 kHz
     let tail = flags.usize_or("tail", 960); // ~20 ms reverb tail
-    let atten = flags.f32_or("atten", 0.5); // ~ -6 dB echo
+    let atten = flags.f32_or("atten", 0.3); // ~ -10 dB echo (realistic acoustic coupling)
     let path = echo_path::EchoPath::synthetic(delay, tail, atten, flags.u64_or("seed", 1));
     let echo = path.apply(&far.samples);
     let mic: Vec<f32> = echo.iter().zip(&near).map(|(&e, &n)| e + n).collect();
 
-    // Cancel. Default taps comfortably cover the synthetic path length.
+    // Cancel with the full echo canceller (NLMS + double-talk detection).
     let taps = flags.usize_or("taps", (delay + tail).next_power_of_two());
     let mu = flags.f32_or("mu", 0.5);
-    let mut canceller = aec::Nlms::new(taps, mu, 1e-6);
+    // 0.5 is a good operating point for ~-10 to -20 dB acoustic coupling: high
+    // enough that echo-only rarely trips it, low enough to catch near-end talk.
+    let dtd_threshold = flags.f32_or("dtd-threshold", 0.5);
+    let hangover = flags.usize_or("hangover", sr as usize / 50); // ~20 ms
+    let mut canceller = aec::EchoCanceller::new(taps, mu, 1e-6, dtd_threshold, hangover);
+    if flags.present("no-dtd") {
+        canceller.set_dtd_enabled(false);
+    }
     let cleaned = canceller.process(&far.samples, &mic);
 
     // Residual echo (oracle): whatever's left after removing the known near-end.
@@ -225,6 +240,14 @@ fn cmd_aec(args: &[String]) -> Result<(), String> {
         metrics::dbfs(atten),
     );
     println!("  filter: {taps} taps, mu {mu}{}", if has_near { " (with near-end / double-talk)" } else { "" });
+    if has_near {
+        let dtd = if flags.present("no-dtd") { "off" } else { "on" };
+        println!(
+            "  double-talk: detector {dtd}, threshold {dtd_threshold}, flagged {:.0}% of samples{}",
+            100.0 * canceller.double_talk_rate(),
+            if onset > 0 { format!(", near-end onset {:.1}s", onset as f32 / sr as f32) } else { String::new() },
+        );
+    }
     println!("  mic before: {:.1} dBFS rms", metrics::dbfs(metrics::rms(&mic)));
     println!("  residual echo after: {:.1} dBFS rms", metrics::dbfs(metrics::rms(&residual)));
     println!("  ERLE broadband: {broadband:+.1} dB");

src/dsp/aec.rs

@@ -17,6 +17,8 @@
 //! This is the pure, sample-at-a-time core. Double-talk handling and a residual
 //! suppressor build on top of it; integration into the live audio path wraps it.
 
+use std::collections::VecDeque;
+
 /// A single-channel NLMS echo canceller.
 pub struct Nlms {
     /// Adaptive filter coefficients (the estimated echo-path impulse response).
@@ -118,6 +120,146 @@ impl Nlms {
     }
 }
 
+/// Geigel double-talk detector.
+///
+/// The danger with an adaptive echo canceller is *double-talk*: when the
+/// near-end person speaks at the same time as the far-end, the mic contains
+/// near-end voice the filter has no reference for. If it keeps adapting it will
+/// (wrongly) try to model that voice as echo and diverge, wrecking the painstakingly
+/// learned echo path. The fix is to detect double-talk and freeze adaptation
+/// while it lasts (the filter coasts on its last good estimate).
+///
+/// Geigel's classic test: declare near-end speech present when the mic level
+/// rises above a fraction `threshold` of the recent peak far-end level. The echo
+/// is always an attenuated, smeared copy of the far-end, so a mic louder than the
+/// far-end's recent peak (scaled) can only be near-end energy. A `hangover` keeps
+/// the freeze latched for a while after the trigger so brief dips mid-utterance
+/// don't immediately re-enable adaptation.
+pub struct DoubleTalkDetector {
+    /// Sliding window (samples) over which the peak far-end level is tracked.
+    window: usize,
+    /// Detection threshold: `|mic| ≥ threshold · max|far|` over the window ⇒ talk.
+    threshold: f32,
+    /// Samples to keep adaptation frozen after the last trigger.
+    hangover: usize,
+    /// Monotonic-decreasing deque of `(sample_index, |far|)` for O(1) sliding max.
+    mono: VecDeque<(usize, f32)>,
+    /// Running sample counter.
+    i: usize,
+    /// Samples of freeze remaining.
+    hangover_left: usize,
+}
+
+impl DoubleTalkDetector {
+    /// `window` should roughly cover the echo path length (e.g. the filter taps),
+    /// `threshold` is the Geigel fraction (≈0.5–0.7 for ~6 dB echo coupling), and
+    /// `hangover` is how long to stay frozen after a trigger (e.g. ~50 ms).
+    pub fn new(window: usize, threshold: f32, hangover: usize) -> Self {
+        DoubleTalkDetector {
+            window: window.max(1),
+            threshold,
+            hangover,
+            mono: VecDeque::new(),
+            i: 0,
+            hangover_left: 0,
+        }
+    }
+
+    /// Feeds one sample pair; returns `true` if double-talk is currently declared
+    /// (i.e. adaptation should be frozen this sample).
+    pub fn update(&mut self, reference: f32, mic: f32) -> bool {
+        let ax = reference.abs();
+        // Maintain the monotonic-decreasing deque: drop smaller tail entries.
+        while matches!(self.mono.back(), Some(&(_, v)) if v <= ax) {
+            self.mono.pop_back();
+        }
+        self.mono.push_back((self.i, ax));
+        // Evict entries that have fallen out of the sliding window.
+        let window_start = (self.i + 1).saturating_sub(self.window);
+        while matches!(self.mono.front(), Some(&(idx, _)) if idx < window_start) {
+            self.mono.pop_front();
+        }
+        let far_max = self.mono.front().map(|&(_, v)| v).unwrap_or(0.0);
+        self.i += 1;
+
+        // Geigel decision (only meaningful when the far-end is actually active).
+        if far_max > 1e-6 && mic.abs() >= self.threshold * far_max {
+            self.hangover_left = self.hangover;
+        }
+        if self.hangover_left > 0 {
+            self.hangover_left -= 1;
+            true
+        } else {
+            false
+        }
+    }
+}
+
+/// A complete in-process echo canceller: an [`Nlms`] adaptive filter guarded by a
+/// [`DoubleTalkDetector`]. This is the unit the live audio path will wrap — feed
+/// it the far-end reference and the mic, get back cleaned near-end audio, with
+/// adaptation automatically frozen during double-talk.
+pub struct EchoCanceller {
+    nlms: Nlms,
+    dtd: DoubleTalkDetector,
+    /// Whether double-talk protection is active (off = raw NLMS, for A/B testing).
+    dtd_enabled: bool,
+    /// Count of samples the most recent run flagged as double-talk.
+    double_talk_samples: usize,
+    /// Total samples processed (for the double-talk rate).
+    total_samples: usize,
+}
+
+impl EchoCanceller {
+    /// Builds a canceller with `taps` filter length, NLMS `mu`/`eps`, and DTD
+    /// `threshold`/`hangover`. The DTD window is the filter length.
+    pub fn new(taps: usize, mu: f32, eps: f32, dtd_threshold: f32, hangover: usize) -> Self {
+        EchoCanceller {
+            nlms: Nlms::new(taps, mu, eps),
+            dtd: DoubleTalkDetector::new(taps, dtd_threshold, hangover),
+            dtd_enabled: true,
+            double_talk_samples: 0,
+            total_samples: 0,
+        }
+    }
+
+    /// Enables/disables the double-talk freeze (disable to compare against raw NLMS).
+    pub fn set_dtd_enabled(&mut self, on: bool) {
+        self.dtd_enabled = on;
+    }
+
+    /// Fraction of processed samples that were flagged as double-talk (0.0–1.0).
+    pub fn double_talk_rate(&self) -> f32 {
+        if self.total_samples == 0 {
+            0.0
+        } else {
+            self.double_talk_samples as f32 / self.total_samples as f32
+        }
+    }
+
+    /// Processes one sample pair: runs the detector, freezes adaptation if it
+    /// fires, and returns the cleaned output.
+    pub fn process_sample(&mut self, reference: f32, mic: f32) -> f32 {
+        let double_talk = self.dtd_enabled && self.dtd.update(reference, mic);
+        self.nlms.set_adapting(!double_talk);
+        self.total_samples += 1;
+        if double_talk {
+            self.double_talk_samples += 1;
+        }
+        self.nlms.process_sample(reference, mic)
+    }
+
+    /// Cancels a whole buffer pairwise over the shared length.
+    pub fn process(&mut self, reference: &[f32], mic: &[f32]) -> Vec<f32> {
+        let n = reference.len().min(mic.len());
+        let mut out = Vec::with_capacity(n);
+        for i in 0..n {
+            out.push(self.process_sample(reference[i], mic[i]));
+        }
+        out
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -168,4 +310,76 @@ mod tests {
             assert!((a - b).abs() < 1e-6, "near-end should pass through: {a} vs {b}");
         }
     }
+
+    /// Builds a near-end signal that is silent until `onset`, then a loud tone —
+    /// the canonical "far-end converges, then both talk" double-talk scenario.
+    fn near_end_with_onset(len: usize, onset: usize) -> Vec<f32> {
+        use std::f64::consts::PI;
+        (0..len)
+            .map(|i| {
+                if i < onset {
+                    0.0
+                } else {
+                    0.5 * (2.0 * PI * 300.0 * i as f64 / 48_000.0).sin() as f32
+                }
+            })
+            .collect()
+    }
+
+    #[test]
+    fn detector_fires_on_near_end_not_on_echo() {
+        // Realistic ~-12 dB echo coupling (atten 0.25); louder coupling makes
+        // Geigel false-positive on white-noise echo peaks (a real limitation).
+        let far = white_noise(0.5, 20_000, 4);
+        let echo = EchoPath::synthetic(64, 192, 0.25, 5).apply(&far);
+        let onset = 10_000;
+        let near = near_end_with_onset(far.len(), onset);
+        let mic: Vec<f32> = echo.iter().zip(&near).map(|(&e, &n)| e + n).collect();
+
+        let mut dtd = DoubleTalkDetector::new(256, 0.7, 200);
+        let mut early_hits = 0;
+        let mut late_hits = 0;
+        for i in 0..far.len() {
+            if dtd.update(far[i], mic[i]) {
+                if i < onset { early_hits += 1 } else { late_hits += 1 }
+            }
+        }
+        // Echo-only stretch should rarely trip; near-end stretch should trip a lot.
+        let early_rate = early_hits as f32 / onset as f32;
+        let late_rate = late_hits as f32 / (far.len() - onset) as f32;
+        assert!(early_rate < 0.10, "false-positive rate {early_rate:.2} too high");
+        assert!(late_rate > 0.50, "missed double-talk, rate only {late_rate:.2}");
+    }
+
+    #[test]
+    fn dtd_protects_convergence_under_double_talk() {
+        // Far-end runs the whole time; near-end starts at 60%. The filter should
+        // converge during the single-talk opening, then the DTD freezes it so the
+        // late (double-talk) echo stays cancelled. Without DTD it diverges.
+        let far = white_noise(0.5, 60_000, 6);
+        let path = EchoPath::synthetic(64, 192, 0.25, 7);
+        let echo = path.apply(&far);
+        let onset = (far.len() as f32 * 0.6) as usize;
+        let near = near_end_with_onset(far.len(), onset);
+        let mic: Vec<f32> = echo.iter().zip(&near).map(|(&e, &n)| e + n).collect();
+
+        let late = onset..far.len();
+
+        let mut with_dtd = EchoCanceller::new(512, 0.5, 1e-6, 0.7, 1200);
+        let cleaned_d = with_dtd.process(&far, &mic);
+        let res_d: Vec<f32> = cleaned_d.iter().zip(&near).map(|(&c, &n)| c - n).collect();
+        let erle_dtd = erle(&echo[late.clone()], &res_d[late.clone()]);
+
+        let mut no_dtd = EchoCanceller::new(512, 0.5, 1e-6, 0.7, 1200);
+        no_dtd.set_dtd_enabled(false);
+        let cleaned_n = no_dtd.process(&far, &mic);
+        let res_n: Vec<f32> = cleaned_n.iter().zip(&near).map(|(&c, &n)| c - n).collect();
+        let erle_no = erle(&echo[late.clone()], &res_n[late]);
+
+        assert!(
+            erle_dtd > erle_no + 15.0,
+            "DTD should hold the echo path: with {erle_dtd:.1} dB vs without {erle_no:.1} dB"
+        );
+        assert!(erle_dtd > 15.0, "held filter should still cancel echo: {erle_dtd:.1} dB");
+    }
 }