cochlea

A headless audio engine for agents. Write a score as data, render it offline to deterministic PCM, then listen through numbers — loudness, onsets, pitch, key, spectrograms — and assert what you heard. Compose → render → probe → verify, with no human ear (and no audio device) in the loop.

What the agent sees: the mel spectrogram of examples/scores/first_light.ron — the score used in the example below — after render and probe. No PCM in sight.

use cochlea_score::*;

let score = Score::new(SampleRate(48_000), Ppq(960))
    .time_signature(4, 4)
    .tempo(Ticks(0), Bpm(120.0))
    .track("lead", Instrument::preset("saw_lead"))
    .note("lead", bar(1).beat(1), Dur::quarter(), Pitch::A4, Vel(96))
    .automate("lead", Param::CUTOFF_HZ,
        keys![(bar(1), 400.0, ease_in_out()), (bar(3), 4_000.0)]);

let rendered = cochlea_render::render(&score)?;
rendered.write_wav("mix.wav")?;

use cochlea_verify::{VerifyExt, Tol, Ms, Cents, Db};
let report = rendered.verify(&score)
    .true_peak_below(-1.0)
    .pitch_matches_score("lead", Cents(10.0))
    .monotone("lead", Param::CUTOFF_HZ, bar(1)..bar(3))
    .silent_after(bar(5))
    .run();
assert!(report.passed);

Or entirely from the command line, score as RON:

cochlea render score.ron --out mix.wav --stems stems/ --verify
cochlea probe input.wav --json report.json --spectro spec.png
cochlea probe input.wav --digest --window-ms 500
cochlea probe input.mp3 --from 42.0 --to 44.5      # zoom into a window, any format
cochlea diff a.wav b.wav --tier2 --spectro delta.png
cochlea lint score.ron
cochlea spectro input.wav --out spec.png --annotate  # draw beats/onsets/pitch on the image
cochlea import song.mid --out score.ron              # SMF -> score, timing exact
cochlea transcribe solo.wav --out score.ron          # audio -> score, the inverse of render
cochlea reference    # the full score-authoring reference, generated from the live preset bank

cochlea probe works on any WAV, plus FLAC (decoded bit-exact), mp3, and ogg — still without ffmpeg, and with no score required. That's the front door: point it at audio you didn't render, and you get the same JSON report and spectrogram an agent uses to review its own work.

How an agent listens

compose → render → probe (JSON) → spectrogram (one vision call) → verify

1. compose a score as data (RON, or the Rust builder above).
2. render it to deterministic PCM — cochlea render score.ron --out mix.wav.
3. probe the mix into a compact JSON report (loudness, onsets, pitch, key, silence, clipping) — cochlea probe mix.wav --json report.json. No image, no audio: the agent reads numbers.
4. look, when numbers aren't enough — cochlea spectro mix.wav --out spec.png renders one small PNG the agent reviews in a single vision call instead of reasoning about raw samples.
5. verify — cochlea render score.ron --verify runs the score's embedded assertions and exits nonzero on failure. An agent can retry on its own, without a human confirming "yes, that sounds right."

When something in the middle of a long file needs a closer listen, every read tool takes --from/--to. Probe just bars 17–19, or draw a spectrogram of just the drop. The cut is frame-exact, report times are relative to it, and source.start_ms records where it came from. That turns the whole stack into a zoom lens instead of a whole-file-only report.

The economics are the point here, not an afterthought. The first_light render above is 7 seconds of 48 kHz 32-bit-float PCM, which is 2.7 MB. A 3-minute piece at the same settings runs about 66 MB. You would not hand that to an agent as text, and reading it sample by sample is worse.

Its probe report is a few KB of JSON instead. Here is schema v5, trimmed to the interesting fields. Note pitch.melody: the piece's bass line and melody read back as note events, which is the read-back half of the compose loop.

{
  "schema_version": 5,
  "source": { "sample_rate": 48000, "channels": 2, "duration_ms": 7035.708333333333, "start_ms": 0.0 },
  "loudness": { "integrated_lufs": -22.700454879284784, "true_peak_dbtp": -15.910817022082783, "lra": 10.607660373688798 },
  "onsets": { "count": 6, "times_ms": [1077.33, 2149.33, 2346.67, 3221.33, 4538.67, 5034.67] },
  "pitch": { "voiced_ratio": 0.9847560975609756, "median_f0_hz": 110.00194603797897,
             "melody": [ { "name": "A2", "start_ms": 0.0, "end_ms": 1045.3, "cents_off": 0.1 },
                         { "name": "E2", "start_ms": 1077.3, "end_ms": 2112.0, "cents_off": 0.3 },
                         { "name": "F#2", "start_ms": 2154.7, "end_ms": 3178.7, "cents_off": 0.2 },
                         { "name": "E2", "start_ms": 3210.7, "end_ms": 4384.0, "cents_off": 0.3 },
                         { "name": "E5", "start_ms": 4394.7, "end_ms": 5813.3, "cents_off": -0.4 } ] },
  "timbre": { "mfcc_mean": [-37.64, 14.47, -4.44, 1.24, "..."], "mfcc_std": ["..."], "frames": 656 },
  "key": { "tonic": "E", "mode": "major", "confidence": 0.8093960265638273 },
  "tempo": { "bpm": 55.97014925373134, "confidence": 0.6633739386089712, "stability": 0.3333333333333333,
             "candidates": [ { "bpm": 55.97014925373134, "salience": 0.6633739386089712 },
                             { "bpm": 112.5, "salience": 0.21588204941945222 } ] },
  "rhythm": { "grid_alignment": 0.8333333333333334, "grid": "straight", "offbeat_ratio": 0.4, "clear_rhythm": true },
  "stereo": { "width": 0.02967719705208343, "correlation": 0.9981362354107913, "balance": -0.0016380539212361243 },
  "structure": { "section_count": 1, "confidence": 0.0 },
  "silence": { "trailing_ms": 2485.708333333333 },
  "clipping": { "clipped_samples": 0, "true_peak_over_0dbtp": false }
}

And the spectrogram is one small image. Here's the title_cue demo — a pad whose cutoff_hz automation sweeps 250 Hz → 5000 Hz across bars 1–3:

The dark band at the top of the frame narrows as the sweep runs, letting more high-frequency energy through over time. An agent reads that straight off the image. The demo's Monotone(track: "pad", param: "cutoff_hz", ...) assertion checks the same thing numerically.

To get a whole piece in one image no matter how long it is, --sheet tiles the spectrogram into a contact sheet instead of one long strip. Two bars per tile here, via --bars-per-tile 2:

Reading audio without a context window

probe --digest skips JSON and prints a deterministic text summary instead: one line per feature dimension, then a windowed timeline capped at about 40 rows. Here's real output for the drum_groove demo — 20.8 seconds and four tracks, with the rhythm, stereo, and structure dimensions all in one screenful:

cochlea digest: 20.755s  2ch  48000Hz
loudness: integrated=-24.06  momentary_max=-22.42  true_peak=-5.95  lra=1.61
key: A# minor (conf 0.54)  pitch: voiced=23%  median=63.8Hz (C2 -42.8c)
melody: 6 notes  C2 C2 C2 C2 A1 A1
tempo: 110.3bpm (conf 0.79, stability 1.00)  alts: 54.9bpm(0.89), 36.6bpm(0.79)
rhythm: clear=true  grid_align=0.98 (straight)  offbeat=0.56
stereo: width=0.07 corr=0.99 bal=-0.01
structure: 1 section
onsets: count=58  rate=2.79/s
silence: leading=0ms  trailing=2545ms
clipping: clipped=0  over_0dbtp=false
timeline: window=1000ms  bucket=1x  rows=21
   idx        t(s)     rms   peak  ons     f0  flags
     0   0.000-1.000   -25.55  -7.36    4    64.0  -
     1   1.000-2.000   -25.61  -8.37    3    63.4  -
     ...

Tempo and rhythm are reported as separate axes, because they change independently. A drum solo can hold a rock-steady pulse while its pattern turns unrecognizable, and that difference is exactly what an agent needs to see.

Here the tempo reads 110.3 BPM, matching the authored 110, and stability 1.00 says the speed never moves across the piece. The alts list surfaces the genuine half-tempo reading at 54.9 BPM — which is actually the stronger raw peak, with the octave prior breaking the tie toward the beat. Metrical ambiguity like that is data an agent can weigh, rather than a coin flip hidden inside the detector.

The rhythm line then says how the hits relate to that pulse: 98% of onsets sit on the beat-subdivision grid and 56% land on off-beat subdivisions, so clear=true. That's an eighth-note hat groove, with its syncopation reported as a number.

Under the pre-0.2.0 metric this same groove read clear_rhythm=false at confidence 0.01. Layering hats, kick, snare, and pad across three metrical levels diluted every lag's share of a mass-fraction score. The grid-based rule asks the right question instead.

The (straight) tag is the grid hypothesis test: alignment is measured against both straight sixteenths and eighth-note triplets, and the report carries whichever more hits land on. A shuffle or swing take reads grid_align=1.00 (triplet) — recognized as an aligned triplet rhythm — instead of being force-fit to sixteenths and scored sloppy.

cochlea diff compares two files in feature space instead of byte-for-byte — "did my change do what I meant," not "is the file bitwise equal." Real output diffing first_light.wav against title_cue.wav:

verdict: different (duration, loudness, onsets, key)
duration     a->b +1264.3 ms
loudness     integrated -5.95 LU  true_peak +5.70 dB  lra -8.88 LU
onsets       matched=0  mean_offset=-  max_offset=-  unmatched_a=6  unmatched_b=5
pitch        delta +0.5 cents
key          a=E major (conf 0.81)  b=A minor (conf 0.86)  changed=true
segments     max_abs_rms_delta 120.99 dB at idx=7
tempo        bpm -24.01 bpm  stability -0.33
rhythm       clear_rhythm_changed=false  grid_align -0.03  grid_changed=true
timbre       mfcc_distance 4.00
stereo       width +0.14  correlation -0.08  balance -0.01
structure    section_count +0

The timbre row is an MFCC spectral-shape distance, with c0 excluded since that's just loudness. The same instrument re-rendered measures around 0, while swapping a sine for a saw at matched loudness measures well above it. It's the "did the re-render keep the instrument's character" axis, which loudness and pitch can't see.

Add --spectro delta.png and the diff also renders a signed difference heat map: red where B is louder, blue where it's quieter, black where nothing changed. A moved onset shows up as a blue/red vertical pair, and a brightened sweep as a red wedge. What changed becomes visible structure, not just a number.

Diff a render against itself, or a re-render of the same score, and the verdict reads byte-identical instead — the determinism contract above, checked from the outside. --tier2 turns that verdict into a gate: exit 0 for byte-identical or Tier-2-equivalent, exit 1 otherwise, so a CI job or an agent can catch a regression without ever reading a raw sample.

Agents as MCP clients

cochlea-mcp is a stdio MCP server built on the same libraries the CLI uses. It exposes twelve tools — render_score, probe_audio, spectrogram, lint_score, probe_digest, loudness_timeline, beat_grid, audio_diff, import_midi, export_midi, transcribe_audio, and score_reference — each a thin wrapper over the matching library call. Any MCP client gets the same compose → render → probe → spectrogram → verify loop as tool calls, rather than as shelled-out subprocesses.

cargo install cochlea-mcp
claude mcp add cochlea -- cochlea-mcp

What makes it agent-native rather than a CLI in a trenchcoat:

• It teaches itself. score_reference returns the complete authoring reference — the RON grammar, the live preset catalog with every automatable parameter (generated from the same registry that validates scores, so it can't go stale), all verify: assertions, and a worked example the test suite itself renders. An agent connected cold can compose without ever seeing this repo.
• It shows, not points. spectrogram returns the image inline as MCP image content (base64 PNG, size-capped), so a client with no filesystem access still gets the one-vision-call review; out_path is optional. annotate: true draws the detected beats, onsets, and pitch onto the image, and audio_diff can return the signed difference heat map the same way.
• It zooms. probe_audio and spectrogram take from_s/to_s — lean into 42.0–44.5 s of a long file the way a human replays a bar, instead of paying for whole-file analysis every call.
• It can be confined. cochlea-mcp --root DIR refuses any read or write that resolves (canonically — symlinks and .. included) outside DIR, before touching the filesystem.

Full tool schemas, arguments, and the JSON-RPC framing are in docs/mcp.md.

Install

All nine crates are on crates.io:

cargo install cochlea        # the CLI: render / probe / diff / lint / spectro / reference
cargo install cochlea-mcp    # the MCP stdio server
cargo add cochlea-features   # or any crate as a library dependency

Or from source: git clone https://github.com/richer-richard/cochlea && cd cochlea && cargo install --path crates/cli.

Concepts

Score IR (cochlea-score). A score is plain data: tracks, notes, per-parameter automation, a tempo map of step changes, and an optional master section. It serializes to RON (version: 1) and round-trips both ways under test.

Positions read the way you'd say them — bar(3).beat(2) — and durations are exact fractions: Dur::quarter(), "3/16", with dotted and triplet sugar. A position that doesn't land on the tick grid is an error, not something quietly rounded. cochlea import reads Standard MIDI Files with timing intact: SMF ticks land on the grid verbatim, and GM programs become labeled preset guesses.

Integer time is ground truth. Everything is ticks at 960 PPQ. BPM is converted once, up front, to integer nanoseconds per quarter note. Turning ticks into samples is exact rational u64/u128 arithmetic (fenestra-anim's mul_div), applied once when events are scheduled. Nothing accumulates floating-point seconds, nothing reads a wall clock, and a property test holds it drift-free across 10⁹ ticks.

Synth (cochlea-synth). Eleven presets built on fundsp. Eight are subtractive: sine, saw_lead, square_bass, chord_pad (genuinely stereo — its detuned saws pan apart), noise_hat, pluck, kick, and snare. Three are not: fm_bell (harmonic FM with an automatable brightness), marimba (a modal struck bar), and organ (an additive drawbar). There's also a reverb insert.

Each instrument declares its automatable params with a name, unit, range, and default. Scores are validated against that registry, and the same registry generates the authoring reference, so the docs can't drift from the code. All noise comes from a counter-based RNG keyed on (seed, sample_index) — random access, with no stateful generator anywhere.

Renderer (cochlea-render). Audio is rendered in 64-sample blocks, split at event boundaries, so note timing is sample-accurate while automation runs at control rate (~1.3 ms at 48 kHz). Tracks render independently, which is both the parallelism unit and where stems come from for free. Voice allocation and oldest-note stealing are pure functions of the schedule.

The master bus sums stems at f64 in fixed track order, then applies the score's optional master stage: an output gain, and a brick-wall lookahead limiter whose sample-peak ceiling holds exactly (offline, lookahead is just a forward window maximum — no delay line). That's the tool for hitting a LUFS target while leaving TruePeakBelow headroom. With no master section, the mix is byte-equal to the sum of the stems, both by definition and by test.

Features (cochlea-features). One schema-versioned JSON report, covering:

• loudness — integrated LUFS, momentary max, true peak, and LRA, via ebur128;
• onsets from spectral flux, and YIN pitch with cents deviation;
• a quantized melody: note events an agent can diff against what it wrote;
• an MFCC timbre digest, and chroma plus Krumhansl-Schmuckler key;
• tempo and rhythm as separate axes — tempo gives pulse clarity, octave alternatives, and windowed stability, while rhythm gives grid alignment (with a straight-vs-triplet hypothesis test), offbeat ratio, and a calibrated clear_rhythm;
• stereo width, correlation, and balance;
• structure boundaries via Foote novelty, plus silence, tail, and clipping.

On top of that: a windowed segment timeline, an LLM-sized text digest, a feature-space diff between two files, and frame-exact windowing (Audio::window) behind every --from/--to.

Spectro (cochlea-spectro). Mel spectrogram PNGs, with an HTK filterbank, viridis colors, a time ruler, and bar markers. It can draw analysis overlays on the image (beat grid, onsets, pitch), render a signed A→B difference heat map, and tile a whole piece into a contact sheet so an agent can review it in a single vision call.

Verify (cochlea-verify). The assertion DSL shown above. The same assertions embed in score RON under verify:, and cochlea render score.ron --verify runs them, exiting nonzero with a machine-readable JSON failure report.

Determinism, precisely scoped

Audio is a fold, not a map: filters and delays carry state, so per-sample purity is not the contract. The contract is three tiers:

Tier 1 is bought with a specific set of choices:

• libm for every transcendental in a DSP path. The std float methods are banned by clippy config, not by convention.
• No fast-math and no implicit FMA. mul_add is banned too.
• Denormals are honored everywhere. Flushing them is a realtime performance hack, and x86 and aarch64 can't even do it uniformly. We render offline and eat the rare slow tail.
• Fixed summation order, and an f64 master bus.
• Voices tick sample by sample. fundsp's SIMD block path provably diverges from its scalar path, so it's banned.
• Analysis FFTs use FftPlannerScalar, which does no runtime CPU dispatch.

The full audit trail — per fundsp node family, ebur128's internals, rustfft's dispatch — lives in docs/determinism.md.

Feature accuracy (synthesized ground truth, 48 kHz)

ffmpeg-free by design

cochlea reads WAV, FLAC, mp3, and ogg/vorbis using hound and symphonia, both pure Rust. It writes plain WAV, and renders PNGs on the CPU with rustfft, a hand-rolled mel filterbank, a viridis LUT, and image.

There are no subprocess calls, no system codecs, no GPU, and no audio device. The whole pipeline is a pure Rust dependency graph, and CI blocks GUI, GPU, and device crates from ever entering Cargo.lock (deny.toml).

Decoding comes with two different promises, and it's worth being clear about which is which. WAV and FLAC decode bit-exact — FLAC is lossless by spec, and it's checked against WAV twins in-tree. mp3 and ogg are analysis input only: reproducible for a given build, but a lossy codec already threw the original samples away, so there's no exactness claim left to make.

Assertion cookbook

use cochlea_verify::{VerifyExt, Tol, Ms, Cents, Db};

rendered.verify(&score)
    // Mix-level loudness and headroom:
    .integrated_lufs(-14.0, Tol(0.5))     // streaming-loudness target
    .true_peak_below(-1.0)                 // intersample-safe headroom
    // Timing: did the hit land where the score says?
    .onset_at("drums", bar(17).beat(1), Ms(5.0))
    // Intonation: does every note read as written? (monophonic tracks)
    .pitch_matches_score("lead", Cents(10.0))
    // Was the sweep *written*? (authored curve, block-rate — a score lint)
    .monotone("lead", Param::CUTOFF_HZ, bar(1)..bar(3))
    // ...and did it audibly *happen*? (rendered stem's spectral centroid)
    .brightness_rises("lead", bar(1)..bar(3), 1.3)
    // Do the hits land on the detected beat grid?
    .grid_alignment_at_least(0.9)
    // Click detection away from note boundaries:
    .no_discontinuity("lead", Db(40.0))
    // Does the piece actually end?
    .silent_after(bar(64))
    .run();

The same assertions embed in score RON:

verify: [
    IntegratedLufs(target: -14.0, tol: 0.5),
    TruePeakBelow(dbtp: -1.0),
    OnsetAt(track: "drums", at: (17, 1), tol_ms: 5.0),
    PitchMatchesScore(track: "lead", tol_cents: 10.0),
    Monotone(track: "pad", param: "cutoff_hz", from: (1, 1), to: (3, 1), direction: Rising),
    BrightnessRises(track: "pad", from: (1, 1), to: (3, 1), min_ratio: 1.3),
    NoDiscontinuity(track: "lead", db: 40.0),
    SilentAfter(at: (64, 1)),
    TempoIs(bpm: 110.0, tol_bpm: 2.0),
    HasClearRhythm(expected: true),
    GridAlignmentAtLeast(min: 0.9),
]

cochlea render score.ron --verify runs them; failures come back as JSON ({"passed": false, "checks": [...]}) and a nonzero exit.

To actually hit a loudness target rather than just assert it, give the score a master bus — gain to push, a limiter to hold the ceiling:

master: Master(
    gain_db: 4.0,
    limiter: Limiter(ceiling_db: -2.0),   // sample-peak ceiling holds exactly
),
verify: [
    IntegratedLufs(target: -14.0, tol: 0.5),
    TruePeakBelow(dbtp: -1.0),   // ~1 dB headroom over the ceiling: true peak is inter-sample
]

Four worked demos live in demos/:

• metronome — sample-exact scheduling and onset tolerances.
• chord_pad — harmony reads back as written.
• title_cue — a four-bar cinematic sting that asserts a LUFS target, a monotone filter sweep, click-freedom, and silence after the fade.
• drum_groove — a 110 BPM eight-bar groove on the real kick and snare patches, hats panned right and snare left. It asserts detected tempo, HasClearRhythm(true) with grid alignment ≥ 0.9, stereo width, loudness range, and section count.

drum_groove is also the fixture that motivated splitting tempo from rhythm: the old single confidence metric read it as rhythm-less at 0.01, despite getting the BPM spot on.

Workspace

crates/
  score      # IR: ticks, tempo map, bar/beat math, notes, automation, master, RON form, MIDI import
  synth      # Patch trait over fundsp, eleven presets, param registry, counter RNG
  render     # block engine, voices, stems, f64 master sum + gain/limiter, WAV out
  features   # LUFS/true peak, onsets, pitch+melody, timbre, chroma/key, tempo, rhythm, stereo, structure
  decode     # WAV + FLAC (bit-exact) + mp3 + ogg (analysis) -> Audio, pure Rust
  spectro    # mel spectrogram -> PNG, overlays, diff heat maps, contact sheets
  verify     # assertion DSL + RON-embeddable specs + JSON reports
  cli        # the `cochlea` binary
  mcp        # MCP stdio server (agents call compose/render/probe/verify as tools)

features and spectro depend on neither score nor synth — enforced in CI — which is why probe works on arbitrary audio files with no score in sight.

License

MIT OR Apache-2.0, at your option.