feat: stable note tracking, NOTE pitch source and unsure fallback

Producers found the tracked plugins jumpy: the shared tracker switched notes
after 10 ms and retuned every 5 ms, so growls and formant basses flickered
and every tracked plugin acted on wrong notes.

Shared tracker (common-dsp):
- A new note needs a steady read for 40 ms; an octave down 80 ms, an octave
  up 200 ms (an octave-up read still fits the note's period and counts as
  agreeing). Inside a note the tuning only moves past a 0.25 semitone
  deadband. Follow bends keeps the old fast behaviour as an option.
- A trust score turns the note unsure while reads disagree; Tracker::active
  and NoteState report it, and Fallback latches "step back" across note ends.
- Notes end when the window start stays below the gate's release level for
  20 ms, so short gaps between notes are seen without waiting for the gate.
- PitchSource replaces the old (midi, gate, glide) arguments: Audio, MIDI or
  Note mode (a set note, no detection), plus a key lock (minor, major, root
  only) that snaps reads to the key.
- stems.rs: synthetic stems (growls, wobble, reese, formant, 808 slide,
  layered drop, noise riser) and a score; examples/tracker_report scores the
  corpus or real WAV stems. On the corpus the old behaviour makes up to 28
  extra note starts per second; the stable default makes none.

Plugins:
- Anvil, Lathe, Temper, Knurl, Planish, Braze and Forge gain the NOTE pitch
  source, the key lock and Follow bends (shared common-params crate and a
  shared header switch in common-ui).
- Unsure tracking: Anvil brings the original sub back in place of the sine,
  Lathe passes the audio through, Knurl fades its texture out; Temper,
  Planish and Braze hold their last sure tuning. Readouts show the note only
  when it is sure.
- Anvil gains Mode: Replace or Enhance (keeps the original sub and adds the
  sine under it).
- Planish slides its band edges over 6 ms on a new note instead of snapping.
- Ballast defaults to Grid parts, which need no tracking.

plugins/lathe/core/src/lib.rs

@@ -6,7 +6,9 @@
 mod settings;
 use common_dsp::meter::HarmonicMeter;
 use common_dsp::svf::{Svf, SvfCoef};
-use common_dsp::tracker::{Lookahead, NoteEvent, PitchFollower, Tracked, TrackerConfig};
+use common_dsp::tracker::{
+    Fallback, Lookahead, NoteEvent, NoteState, PitchFollower, Tracked, TrackerConfig,
+};
 use common_dsp::{coefficient, db_to_gain, hz_to_midi};
 pub use settings::{Settings, Source, GAIN_MAX, GAIN_MIN, HARMONICS};
 
@@ -22,8 +24,10 @@ const GAIN_MS: f32 = 20.;
 pub struct AnalyzerFrame {
     /// Fundamental the filters are tuned to, 0 before the first note.
     pub hz: f32,
-    /// Sounding note, rounded; None between notes.
+    /// Sounding note, rounded; None between notes and while tracking is unsure.
     pub note: Option<u8>,
+    /// Tracking is unsure, so the audio passes unchanged.
+    pub neutral: bool,
     /// Harmonic levels in dBFS of the delayed input and of the output (index 0 = fundamental).
     pub input: [f32; HARMONICS],
     pub output: [f32; HARMONICS],
@@ -35,6 +39,7 @@ impl Default for AnalyzerFrame {
         Self {
             hz: 0.,
             note: None,
+            neutral: false,
             input: [-120.; HARMONICS],
             output: [-120.; HARMONICS],
             latency: 0,
@@ -64,7 +69,11 @@ pub struct Module {
     bypass: f32,
     meter_in: HarmonicMeter,
     meter_out: HarmonicMeter,
+    /// A sure note sounds: the readout shows it.
     sounding: bool,
+    fallback: Fallback,
+    /// Unsure tracking: every band fades to 0 dB and the audio passes unchanged.
+    neutral: bool,
     /// Smoothed controls start at the next settings instead of gliding from defaults.
     fresh: bool,
 }
@@ -103,6 +112,8 @@ impl Module {
             meter_in: HarmonicMeter::new(rate, 85.),
             meter_out: HarmonicMeter::new(rate, 85.),
             sounding: false,
+            fallback: Fallback::default(),
+            neutral: false,
             fresh: true,
         };
         m.set_settings(settings)?;
@@ -129,6 +140,8 @@ impl Module {
         self.pitch = PitchFollower::default();
         self.hz = 0.;
         self.sounding = false;
+        self.fallback.reset();
+        self.neutral = false;
         for ch in &mut self.state {
             for f in ch.iter_mut() {
                 f.reset();
@@ -143,6 +156,9 @@ impl Module {
     fn targets(&self) -> [f32; BANDS] {
         let s = &self.settings;
         let mut t = [0.; BANDS];
+        if self.neutral {
+            return t;
+        }
         for k in 1..=s.harmonics as usize {
             t[k - 1] = s.effective(k);
             t[HARMONICS + k - 1] = s.between;
@@ -187,6 +203,7 @@ impl Module {
             sample_rate: self.rate,
             input: *self.meter_in.levels(),
             output: *self.meter_out.levels(),
+            neutral: self.neutral,
             ..AnalyzerFrame::default()
         };
         if self.sounding && self.hz > 0. {
@@ -211,8 +228,9 @@ impl Tracked for Module {
         };
         self.pitch.event(self.settings.snap, e);
     }
-    fn process(&mut self, y: [f32; 2], sounding: bool) -> [f32; 2] {
-        self.sounding = sounding;
+    fn process(&mut self, y: [f32; 2], note: NoteState) -> [f32; 2] {
+        self.sounding = note.sure;
+        self.neutral = self.fallback.neutral(note);
         let first = self.hz == 0.;
         let (hz, jump) = self.pitch.step(self.hz_coef);
         let first = first && jump;
@@ -238,7 +256,7 @@ impl Tracked for Module {
             let wet = (self.mix * v + (1. - self.mix) * y[c]) * self.output;
             out[c] = wet * (1. - self.bypass) + y[c] * self.bypass;
         }
-        let hz = if sounding { self.hz } else { 0. };
+        let hz = if note.sure { self.hz } else { 0. };
         let (dry, wet) = if self.channels == 2 {
             (0.5 * (y[0] + y[1]), 0.5 * (out[0] + out[1]))
         } else {
@@ -286,7 +304,7 @@ impl Engine {
     fn set_tracker(&mut self, s: &Settings) {
         self.lookahead
             .tracker
-            .set(s.source == Source::Midi, s.gate, s.smooth > 0.);
+            .set(s.pitch.source(s.source.mode(), s.gate, s.smooth > 0.));
     }
     pub fn reset(&mut self) {
         self.lookahead.reset();

plugins/lathe/core/src/settings.rs

@@ -1,4 +1,5 @@
 //! Host-independent controls. Gains in dB, times in milliseconds.
+use common_dsp::tracker::{Pitch, PitchMode};
 use serde::{Deserialize, Serialize};
 
 /// Editable harmonics: the fundamental is harmonic 1.
@@ -13,12 +14,33 @@ pub enum Source {
     Audio,
     /// Notes come from MIDI note input.
     Midi,
+    /// Note starts are detected, the pitch is the set note: nothing is guessed.
+    Note,
+}
+impl Source {
+    pub fn mode(self) -> PitchMode {
+        match self {
+            Source::Audio => PitchMode::Audio,
+            Source::Midi => PitchMode::Midi,
+            Source::Note => PitchMode::Note,
+        }
+    }
+    pub fn from_mode(mode: PitchMode) -> Self {
+        match mode {
+            PitchMode::Audio => Source::Audio,
+            PitchMode::Midi => Source::Midi,
+            PitchMode::Note => Source::Note,
+        }
+    }
 }
 
 #[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq)]
 #[serde(deny_unknown_fields)]
 pub struct Settings {
     pub source: Source,
+    /// Note mode's note, the key lock and Follow bends.
+    #[serde(default)]
+    pub pitch: Pitch,
     /// Gain per harmonic, index 0 = fundamental.
     pub gains: [f32; HARMONICS],
     /// dB per octave of harmonic number, added on top of `gains`. Harmonic 1 is the pivot.
@@ -45,6 +67,7 @@ impl Default for Settings {
     fn default() -> Self {
         Self {
             source: Source::Audio,
+            pitch: Pitch::default(),
             gains: [0.; HARMONICS],
             tilt: 0.,
             odd_even: 0.,
@@ -78,6 +101,7 @@ impl Settings {
         if !finite {
             return Err("non-finite setting");
         }
+        self.pitch.validate()?;
         let ok = self.gains.iter().all(|g| (GAIN_MIN..=GAIN_MAX).contains(g))
             && (-12.0..=12.).contains(&self.tilt)
             && (-12.0..=12.).contains(&self.odd_even)

plugins/lathe/core/tests/dsp.rs

@@ -137,3 +137,44 @@ fn settings_are_validated_and_serialise() {
     assert!(e.set_settings(bad).is_err());
     assert_eq!(e.settings(), s);
 }
+
+#[test]
+fn note_mode_cuts_the_set_notes_harmonics() {
+    let mut s = Settings::default();
+    s.gains[2] = -12.;
+    s.source = Source::Note;
+    s.pitch.note = 29;
+    let hz = 43.65;
+    let x = saw(hz, 1., 0.5);
+    let (mut e, l) = engine(s);
+    let y = render(&mut e, &x);
+    let a = harmonics(&x[..x.len() - l], hz);
+    let b = harmonics(&y[l..], hz);
+    assert!((b[2] - a[2] + 12.).abs() < 0.7, "{a:?} {b:?}");
+    assert!((b[1] - a[1]).abs() < 1.);
+}
+
+#[test]
+fn unsure_tracking_passes_the_audio_through() {
+    let mut s = Settings::default();
+    s.gains[0] = -12.;
+    s.gains[1] = -12.;
+    s.gains[2] = -12.;
+    let (mut e, l) = engine(s);
+    // A clear note, then noise detection cannot read.
+    let mut x = saw(55., 0.5, 0.5);
+    let mut seed = 7u32;
+    for _ in 0..RATE as usize {
+        seed ^= seed << 13;
+        seed ^= seed >> 17;
+        seed ^= seed << 5;
+        x.push((seed as f32 / u32::MAX as f32 - 0.5) * 0.5);
+    }
+    let y = render(&mut e, &x);
+    let tail = (RATE * 1.0) as usize;
+    let mut worst = 0f32;
+    for i in tail..x.len() {
+        worst = worst.max((y[i] - x[i - l]).abs());
+    }
+    assert!(worst < 1e-3, "{worst}");
+}