.MID

Introduction

A Standard MIDI File stores music as performance instructions, not sound: which note to play, when, how hard and on which instrument. A synthesiser or sound bank turns the instructions into audio, so the same file sounds different on different devices. The format is defined by the MIDI Association and is used to move songs between sequencers, notation programs and keyboards.

Files use the extension .mid and sometimes .midi.

Structure

A MIDI file is a series of chunks, each a four-letter type and a 32-bit big-endian length. The header chunk MThd comes first. Its six bytes give the file format (0 for a single track, 1 for several tracks played together, 2 for independent patterns), the number of tracks and the timing division, usually ticks per quarter note.

Each track chunk MTrk is a list of events. Every event starts with a delta time, the ticks since the previous event, written as a variable-length number of 7 bits per byte. The event is then a MIDI message (note on, note off, control change, program change and so on), a system exclusive message, or a meta event starting FF such as tempo, time signature, track name or end of track.

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