Chord Coach

SAMPLE PROJECT / REFERENCE IMPLEMENTATION. Built by the course developer before the term to prove this submission path works. Not student work.

The page reads an audio file, finds its tempo and beats, and works out which chord is sounding over each one: a windowed FFT becomes a chroma - how much of each of the twelve notes is present - and each beat's chroma is matched against a hundred and eight chord templates. A small Viterbi search picks the best sequence rather than the best chord for each beat on its own, which is what stops a passing note becoming a chord of its own - and a seventh is only allowed to rename a chord if it is really being held, which is what stops a melody touching the seventh on its way past from turning a plain G into Gmaj7. The bass is followed in a band of its own, heard through its harmonics because down there an FFT bin is wider than a semitone, which is how A/C# is told from A, and the overtones every plucked string adds are subtracted back out before anything is named.

None of that is presented as the answer. Everything the analysis finds - every chord, the tempo and where it changes, the key, the time signature, where bar one is - arrives as a suggestion, drawn faded, and becomes part of the chart only when somebody has listened and confirmed it or typed the right one. A detector that is right nine times in ten is wrong somewhere on every page, and a band reading a chart on Sunday needs to know which parts were checked. When nothing on the chart is a guess it is finished, and the practice tools open: a speed control that keeps the pitch, an A-B loop, transposing, a capo, and the chord playing now and next as guitar diagrams. Finished charts go into a shared archive with the song's title and artist, so the next person to want that song already has it.

It is reference project G: the signal-processing path, and the honest answer to "can it just take a YouTube link?" - it cannot, and the page says why.

Working out chords by ear is slow, and the tools that claim to do it for you upload your recording and hand back a chart nobody has checked. This drafts one in the browser, on your own file, marks every guess as a guess, and leaves the deciding to the person who can hear the song.

Guitarists working out a song from a recording, and students who want to see that "the computer worked out the chords" is arithmetic they can read.

A YouTube link cannot be analysed: a browser will not let a page read the audio of a video from another site, and pulling the audio down on a server would break YouTube's terms. The page explains this rather than hiding it.

Nine kinds of chord - major, minor, 7, m7, maj7, sus4, sus2, dim, aug - and inversions, but no ninths or altered chords: every template added makes every other chord easier to mistake. Anything can still be typed in by hand.

The archive is a folder of charts, not a service that accepts uploads; a new chart is exported and sent in. It holds chord grids with a title and an artist - no audio, no lyrics, no sheet music - and the charts that ship with it are hymns in the public domain. The recording itself is never uploaded, copied or hosted, and the song is named - title, artist, CCLI number - rather than reproduced.

Dense mixes, heavy distortion and piano voicings confuse it; free-time passages are walked through at the tempo around them; chord changes are assumed to fall on beats. The beat model runs by itself only where the browser lends it a graphics card - without one it is offered, and takes minutes on the processor. Analysis takes a few seconds for a whole song. Drafts live in the browser's local storage, so they are per-browser unless the chart is saved to a file.

The chord-sheet reader reads chord sheets - chord names over lyrics - and not printed staff notation, which is a research problem nobody has solved well enough to hand to a class as though they had. It is about nine megabytes and loads only when somebody photographs a sheet.

analyse.js takes samples and a sample rate and returns a chord chart: it touches neither the page nor the Web Audio API, which is why tests/chord_test.mjs can run the whole detector under Node with no browser. worker.js is four lines around it, and exists because several seconds of analysis on the main thread is a page that looks broken. chords.js knows about guitars and nothing about audio: open shapes are a table, everything else is the E and A barre shapes moved up the neck.