Chessynth

01The claim

Plenty of things turn data into sound. The bar Chessynth sets itself is higher and much easier to fail: a listener who does not play chess should be able to hear what happened, and a listener who does should be able to point at the move that did it.

So the mapping is not a sonification of coordinates. It is an arrangement decision per chess fact, chosen because that is what the fact means. A knight is a horn; when the knight is captured the horn stops for the rest of the game. Nothing re-instantiates it, because the piece is not coming back. That single rule does more work than any pitch mapping: the orchestra thins as the board thins, and an endgame is audibly a smaller ensemble.

02How a move becomes music

A game arrives as PGN, ideally with clock comments. It is then processed once, ahead of playback, and once per ply during it.

PGN  →  timeline      legal moves, captures, checks, castling,
                       promotions, a per-ply piece census, and the
                       real think time between moves

     →  analysis      Stockfish 18 (WASM, shipped locally) scores
                       each position: evaluation, forced mates,
                       blunders and their severity

     →  arrangement   the whole game is planned before a note
                       sounds — recognised opening, intro length,
                       section arc, where the climax belongs

     →  per ply       six signals update, the bar is composed,
                       and the ply's chess events fire their voices

The number of musical bars a ply occupies is derived from how long the player actually thought, relative to their own median for that game. A long think is a long bar. This is why replaying a real blitz game feels hurried and a classical game breathes: the tempo of the music is the tempo of the thinking, not a setting.

03The six signals

Everything continuous about the music is driven by six numbers, each owned by exactly one place in the code. They are the only channel between the chess and the arrangement.

They are deliberately not independent of the chess and deliberately independent of each other. tension reads the position, so it stays high while a loaded position sits still; an event accumulator would have decayed. scramble reads only the clock, so a game with no clock record reports zero and is unaffected rather than guessing.

04Pieces as instruments

In the orchestral family each piece type owns a part. The assignment is by register and behaviour, not by material value.

A capture is scored by what was taken, in four brackets — a pawn gets a triangle and a short note with nothing underneath; a minor piece a damped stroke with a bassoon on the chord; a rook low brass; the queen the full stroke. The losing side's instrument then goes silent permanently.

Chess the music actually reacts to

• Recognised openings set how long the introduction holds — a King's Indian coils for a quarter of the game, a King's Gambit is alight by move three.
• Castling on opposite wings changes the character: both kings are targets and the arrangement stops being symmetrical.
• Tactics each have a figure — fork, absolute and relative pin, skewer, discovered check, double check, battery. A fork holds rather than restating itself; a double check strikes in two voices at once.
• A forced mate floors the tension and starts a clock figure whose rate accelerates as the distance closes.
• Blunders are graded by the engine's severity, not flagged as a binary.

05The eighteen modes

Three families. They are not presets over one engine: the orchestral and film-score families run a different scheduler from the club family.

Classical — four modes

Soundtrack — two modes

Club — twelve modes

Detroit, Bleep, Dub Techno, Minimal, Melodic, Hardgroove, Acid, Industrial, Hard Techno, Schranz, Dark Techno and Peak Time, 120–165 BPM. Each names its own hats, reverb send, delay and drive — the hats alone distinguish most techno subgenres before anything else happens. Here the pieces are stems: the kick, the bassline, the acid riff, the pad. Losing a piece removes its stem from the arrangement, so the mix thins toward the endgame.

06Synthesis

Every sound is generated at runtime through the Web Audio API. There are no samples anywhere in the application: 72 voice functions build their own oscillators, noise buffers, filters and envelopes. A triangle is three inharmonic sine partials through a bandpass; a kick is one oscillator swept in frequency with the attack coming from saturation downstream rather than a separate click layer.

The instruments are level-matched by measurement, not by ear. An offline render of each voice is A-weighted through a biquad cascade and compared, because A-weighting is where the ear actually is — a bright voice at the same nominal gain is not the same loudness. The same bench measures decay time, which is how a triangle that read as quiet but rang for 1.8 seconds was found and shortened.

Determinism, and why it matters twice

Given the same game and the same seed, Chessynth produces the same audio, sample for sample, on any machine. That buys two things at once: the offline WAV render calls the same scheduler as live playback, so the exported file is the track rather than a re-performance of it — and two players in a live match stay in sync without streaming any audio, because each end renders the same thing from the same moves.

07The ledger

The failure mode for a system like this is not a bad sound. It is two chess facts quietly driving the same musical parameter, so neither is legible and the arrangement turns to mush. Chessynth carries an allocation ledger: 74 rows, one per musical sub-parameter, each naming the single signal that owns it and the function that writes it.

It is enforced rather than aspirational. A checker parses the source, matches it against the ledger, and fails if a shared audio node has two owners or if a documented row has drifted from the code. Twenty-six test suites sit behind that, and the assertions are mutation-tested — an assertion that still passes when the behaviour it guards is reverted is deleted and rewritten.

08The music log

The explainability claim is testable, which is the point of making it. Menu → Copy music log exports one entry per ply: the move in algebraic notation, the mover's clock, the chess evidence, what was captured and which instrument that cost, the recognised opening and the tactics, and every signal that moved by more than a threshold — with what that signal controls written next to it.

9. Qxb5  white 0:42  [capture]
    queen takes bishop — black loses Bass · light-sq bishop, for good
      minor piece (330) → a damped stroke, bassoon on the chord
    FORK — the fork figure, which HOLDS rather than restating itself
    tension 0.12→0.71 ↑ filter and dynamic  ·  pace 0.36→0.54 ↑ note density
    > section ROLL → BUILD (4-bar window; motto opens the next period)

A judge can therefore check the central claim directly rather than taking it on trust: play a passage, notice something, and read what the board did.

09Ways to play

10Stack & footprint

• One HTML file, ~1.1 MB. No framework, no build step, no bundler, no dependencies at runtime.
• Web Audio API for all synthesis; OfflineAudioContext for rendering and for the measurement benches.
• Stockfish 18 lite (7.3 MB WASM) shipped locally, not called out to a service. GPL text travels with it.
• WebSocket relay for room signalling, WebRTC for party audio.
• No account, no telemetry, no third-party requests in normal play.
• Desktop and mobile; the interface folds its entry points into one control once a game is running so the deck is not pushed off a phone screen.

11What it does not do

Stated plainly, because a jury will find these anyway.

• The film-score family is the newest and is still being tuned. It is included because it is the strongest demonstration of the phrase composer, not because it is finished.
• 2-player is same-machine today. The wire protocol is written and measured — 326-byte handshake, 27-byte move — and party mode already runs over a real relay, but the internet transport is not yet wired to the 2-player button.
• Party mode fails on symmetric NAT. A relay is a different feature with a different bill, and it is not pretended otherwise.
• It is not a chess trainer. The engine is there to hear the game, not to coach it.