The MIDI file standard
The MIDI file standard
MIDI files are that you can open on your PC to play a song, but MIDI was created as a wire protocol for live music performance. It’s real-time because when the musician presses a key on their keyboard, it immediately sends a MIDI message to a synthesizer to play a note. Synthesizers can play a whole range of sounds, from piano or violin to the sound of tweeting birds. I was interested in learning a little more about MIDI and wrote this post to give a high-level overview for anyone else interested in learning about it.
A bird’s eye view of the standard
Arguably the most important piece of the standard is the MIDI message; they’re what MIDI instruments send to synthesizers to tell them to play a note. Each message is sent on one of 16 MIDI channels, which can be set up to play the sound of a different instrument. In MIDI, the instruments are called programs. If it supports it, a single synthesizer could also listen on multiple channels, but have each configured to play a different program.
To give you some examples of MIDI messages, two types are “Note On” and “Note Off”, which cause the synthesizer to start or stop playing a note. Each message also comes with additional message data. For Note on/off messages, that data is:
- the pitch of the note
- the velocity of the note. It’s called that because playing a piano key faster makes it louder.
MIDI messages don’t contain any information about when they should be played (they’re always played immediately when they’re received). If you want a MIDI synthesizer to play a whole song, you need something to send all the messages at the right time; for that, you use a MIDI sequencer. A second type of message called meta messages control the sequencer itself and set things like the tempo or time signature.
MIDI files are binary files containing the timing information used by sequencers. The files contain multiple tracks, each of which usually1 corresponds to a single instrument. Each track contains a list of MIDI events, which are a MIDI message paired with a delta time (the time between MIDI messages).
A slightly lower-flying bird’s eye view of the standard
I thought it could be fun to dive a little further into the MIDI file binary format, so I wrote a parser that describes the contents of a MIDI file. Before we take a look at its output, here’s a rough outline of the files’ structure:
Header:
HeaderPrefix # Always 4d 54 68 64 00 00 00 06
SequenceType # Whether tracks should be played simultaneously or synchronously
TrackCount # The number of tracks in the file
DeltaTicksPerCrotchet # The ratio of delta ticks to crotchets. This and the time signature determine the length of a delta tick
Tracks:
- TrackHeader:
TrackHeaderPrefix # Always 4D 54 72 6B
TrackLength # Number of bytes in the track
Events:
- DeltaTime # This is a variable-length encoded
Message # The actual MIDI message to be sent
- DeltaTime
Message
...
- TrackHeader:
...
A MIDI file case study
Now that we know what to expect, let’s look at a specific MIDI file. This one is a bassline that I found on the MIDI wikipedia page. The right-hand side contains the full description, so don’t worry about the left if you’re unfamiliar with hexadecimal numbers:
4d 54 68 64 00 00 00 06 |> MIDI file standard header
00 01 |> 1 (multiple synchronous tracks)
00 02 |> There are 2 tracks
00 f0 |> There are 240 ticks per crotchet
4d 54 72 6b |> Standard track header. Track 1 starting
00 00 00 1c |> The track is 28 bytes long
00 ff 58 04 04 02 18 08 |> Delta time = 0. [Meta] Set time signature to 4/4. Metronome will tick every 1 beat(s). There are 8 demisemiquavers per crotchet
00 ff 51 03 07 a1 20 |> Delta time = 0. [Meta] Set tempo to 120 bpm
00 ff 54 05 40 00 00 00 00 |> Delta time = 0. [Meta] This track's SMPTE offset is [64, 0, 0, 0, 0]
00 ff 2f 00 |> Delta time = 0. [Meta] END OF TRACK
4d 54 72 6b |> Standard track header. Track 2 starting
00 00 00 87 |> The track is 135 bytes long
00 c0 21 |> Delta time = 0. [Channel 0] Switch to program 33
00 b0 07 7f |> Delta time = 0. [Channel 0] Adjust controller 7 to value 127
00 90 2d 4e |> Delta time = 0. [Channel 0] Start A3 with velocity 78
81 01 80 2d 40 |> Delta time = 129. [Channel 0] Stop A3 with velocity 64
81 67 90 30 51 |> Delta time = 231. [Channel 0] Start C4 with velocity 81
78 90 32 4f |> Delta time = 120. [Channel 0] Start D4 with velocity 79
... Snip
05 80 2d 40 |> Delta time = 5. [Channel 0] Stop A3 with velocity 64
81 61 80 30 40 |> Delta time = 225. [Channel 0] Stop C4 with velocity 64
0a 90 2d 4e |> Delta time = 10. [Channel 0] Start A3 with velocity 78
5a 80 2d 40 |> Delta time = 90. [Channel 0] Stop A3 with velocity 64
83 05 ff 2f 00 |> Delta time = 389. [Meta] END OF TRACK
I found it interesting to see the MIDI file standard in action, so here are some notes to give you a little more insight:
- This file has two tracks, though the first one was just used to configure the sequencer with meta-messages.
- Because the meta messages are never sent to the synthesizer, they have no associated channel.
- To give you a sneak peek into MIDI’s binary encoding (see the references for more),
90means “start a note on channel 0” while80means “stop a note on channel 0”. You’ll see those repeated several times on the left side above. - The
Switch to program 33message sets up channel 0 to use the bass guitar program (33). If we switched that to 126, the song would be played by a helicopter instead!
References
I’ve tried to keep the technical details light in this post. If you want any more information on the MIDI standard, these are all great resources to learn a little more:
- recordingblogs.com
- faydoc.tripod.com
- The Java MIDI package docs
- Wikipedia’s handy list of the MIDI program numbers
Footnotes
-
I say usually because it’s also perfectly valid to have a single track that contains multiple instruments, each on a different channel. ↩