Online MIDI Player

A MIDI file holds no sound. The MIDI Association calls MIDI performance instructions, not audio: which key is pressed, how hard, when it is let go, which instrument plays. That is why a whole song often fits in a few kilobytes, and why the audio player in a browser or phone has nothing to decode when you open one.

To hear the file, something has to turn those instructions into sound, the way a keyboard does when the same notes reach it. A MIDI player is that something: it reads the timing, follows every tempo change and plays each note on a synthesizer at the right moment.

General MIDI fixes which instrument each program number is, so a file that asks for a piano gets a piano everywhere. It does not fix the sound; the MIDI Association's own General MIDI Level 1 page says the acoustic characteristics of each sound are not defined. So the same file sounds different in every player.

The page plays the notes as it goes instead of rendering the whole song first. With the browser's Web Audio API it schedules the next second and a half of notes on the audio clock, then keeps topping that up ten times a second, so a long file starts as fast as a short one and seeking is instant.

The sounds come from the same small synthesizer as the MIDI to MP3 Converter on this site: one rough voice for each of the 16 General MIDI instrument families, plus synthesized drums on channel 10. It keeps the page light, with no sound bank to download, but it will not sound like a real orchestra or a large sample library.

The tempo control moves the note timings, not the audio, so a slower or faster song keeps its pitch. Mute and solo act on each track or, for a single-track file, on each MIDI channel, and take effect at once while the song plays.