Open a .mid file to inspect its structure, watch the notes on a piano roll, and listen with the built-in instant synthesizer. Transpose, change tempo, mute tracks, or swap instruments, then export an edited .mid. A connected MIDI keyboard (or the on-screen keys) plays live too. Everything runs locally ?no uploads.
What is MIDI?
MIDI (Musical Instrument Digital Interface, standardised 1983) is not audio. It is a wire protocol and file format for performance instructions: which key went down, when, how hard, and which instrument should sound. A MIDI file is closer to a player-piano roll than to a recording — it stores the gestures, and any synthesiser that receives them produces the sound. That single distinction explains everything this page does: viewing, transposing, tempo changes and instrument swaps all work because a .mid file contains editable instructions, not fixed waveforms.
Five facts to anchor the rest of the page:
• MIDI files contain zero audio — notes, timings and instrument numbers only
• every note is a number: 60 = middle C (C4), 69 = A4 = 440 Hz, one octave = 12 steps = exactly double the frequency
• velocity (how hard a key is struck) runs 0–127
• a file's internal clock is in ticks, not seconds; tempo is stored as microseconds per quarter note (120 BPM = 500,000)
• General MIDI defines 128 standard instruments, which is why one .mid file sounds similar on any synth
MIDI is instructions, not audio
A .mid file is a stream of short events. Note On says "channel 1, key 64, velocity 100"; a matching Note Off later releases it. Control-change events turn pedals and knobs; program-change events pick instruments. Because the events say what to play but not what it sounds like, the same file played through a 1990s soundcard chip, the WebAudio synth on this page, and a professional sample library produces three very different renditions of the same performance — like the same sheet music played on three different pianos. When you want fixed, portable audio, that is the job of the MIDI to WAV renderer, which bakes one synth's interpretation into a waveform.
Notes, numbers and the piano roll
MIDI names notes by number, with equal temperament baked in: frequency = 440 × 2(n−69)/12. So note 69 is exactly 440 Hz, note 60 is middle C at 261.63 Hz, and any 12-step jump doubles (or halves) the frequency — which is exactly what the transpose slider does when you push it to +12 or −12. The piano roll above is these numbers drawn as a picture: pitch runs vertically (low notes at the bottom), time runs horizontally, and every note becomes a horizontal bar whose length is its duration and whose colour intensity comes from its velocity.
Ticks, PPQ and why tempo is free to change
Inside a Standard MIDI File, time is counted in ticks relative to the file's PPQ (pulses per quarter note — commonly 480, so a quarter note lasts 480 ticks). Tempo lives in a separate event as microseconds per quarter note: 120 BPM is 500,000, and 60 BPM is 1,000,000. Because note positions are stored in musical units rather than seconds, changing tempo rescales when everything happens without moving a single note — the tempo slider above exploits this, and the Export button rewrites the events so any other player reproduces your edits.
Common misconceptions
- MIDI files contain a recording of the music. No. They contain the performance instructions. A 3-minute pop song fits in a few dozen KB because no waveform is stored — which is also why the file sounds different on every synthesiser.
- MIDI sounds cheap by nature. No. The "cheap" reputation comes from the basic waveforms of 1990s soundcard chips. The same MIDI data through a professional SoundFont or orchestral sample library is indistinguishable from a produced track — the instructions are complete; the renderer makes the difference.
- Velocity is just volume. Not only. On a real piano, a harder strike makes the note louder and brighter; good synths model that, mapping velocity into timbre as well as level. MIDI's 0–127 range preserves the performer's touch for the synth to interpret.
Related tools: MIDI to WAV to render MIDI into a fixed waveform, Audio Spectrum to analyse the rendered result, and Audio Tempo / Pitch when you need the same time-stretching freedom on real audio.
