.MID File Format

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 was adopted by the MIDI Manufacturers Association, now the MIDI Association, in 1988, and is still at version 1.0. It is the usual way to move a song between sequencers, notation programs and keyboards.

There are three types of file. Type 0 holds everything in one track. Type 1 holds several tracks that play together, the first of them carrying the tempo. Type 2 holds independent patterns, each a track of its own; it is rarely used. The type is only the first field of the header and the rest of the layout is the same, so all three are one format. Later additions to the standard, such as new meta events, only add to it, and readers skip what they do not know.

Files use the extension .mid, and sometimes .midi or .smf. A MIDI karaoke file (.kar) is a Standard MIDI File with the words in its text events.

A .mid file dropped on any page of this site opens in the explorer, which shows its chunks and every event, with the notes as a piano roll, the lyrics and a list of the events with their times.

Structure

A Standard MIDI File is a series of chunks, each a four-letter type and a 32-bit length. The header chunk comes first, then one track chunk for each track. All numbers are big-endian.

Each event says how many ticks after the one before it happens. How long a tick lasts comes from the header’s division and the tempo events; the timing section shows how to turn ticks into seconds. Program numbers and percussion keys are named by General MIDI.

00000000  4D 54 68 64 00 00 00 06 00 00 00 01 00 60 4D 54  MThd.........`MT
00000010  72 6B 00 00 00 13 00 FF 51 03 07 A1 20 00 90 3C  rk......Q... ..<
00000020  64 60 80 3C 40 00 FF 2F 00                       d`.<@../.

Telling it apart

The file starts with the four bytes MThd (4D 54 68 64). The explorer’s parser for this format checks that, then that the header chunk is at least 6 bytes long and the type is 0, 1 or 2, and then that every event in every track can be read: a delta time of at most four bytes, a status byte for every event, no system common or real-time status bytes outside an F7 escape, and meta and system exclusive lengths that fit in the track. A file that fails is turned down.

A file that starts with RIFF and has RMID at offset 8 is a RIFF MIDI file, a separate format, even though it carries a Standard MIDI File inside.

Variants

• Standard MIDI File (MThd) – a header chunk and track chunks of timed events, types 0, 1 and 2
• RIFF MIDI file (RMID) – a Standard MIDI File inside a RIFF container, with optional text and a sound bank

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