UEE051H3 - STUDIO TECHNOLOGY

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MIDI - the Musical Instrument Digital Interface 

These pages describe the Musical Instrument Digital Interface, more widely known as MIDI.  By reviewing this material you should:

• attain a broad understanding of the MIDI specification at three different levels, namely
• at the level of the electronic circuits which comprise a MIDI interface,
• at the level of the signalling which communicates data between MIDI interfaces, and
• at the level of MIDI messages; their structure and interpretation.
• You should also have a sound understanding of the main types of MIDI message, including Note On, Note Off, Control Change, Program Change and other key messages, including System Exclusive.
• You should also have an appreciation of the system-level capabilities and limitations of MIDI, with an understanding of common problems and their remedies.

For questions, comments or feedback please contact the author Alan Winfield, via:

These course notes are Copyright ©1997-2003 The University of the West of England, Bristol.

Contents:

1. General Introduction
2. The MIDI Specification:
2.1 MIDI Signalling
2.2 MIDI Interface Hardware and Electronics
3. The MIDI Message Structure
3.1 The MIDI Channel
3.2 MIDI Channel Messages
3.2.1 MIDI Messages: Note On and Note Off
3.2.2 MIDI Messages: Polyphonic Key Change
3.2.3 MIDI Messages: Control Change
3.2.4 MIDI Messages: Program Change
3.2.5 MIDI Messages: Overall Key Pressure
3.2.6 MIDI Messages: Pitch Wheel
3.3 MIDI System Messages
4. Bibliography
5. Links to MIDI Internet Resources

1. General Introduction

Since its introduction in the early 1980s, MIDI has become the de-facto standard for interconnecting electronic musical instruments and related devices. Although conceived primarily to allow the keyboard for one synthesiser to control other synthesisers, MIDI has widened in scope so that it now appears as an interface for advanced effects units, audio mixers, sequencers and other peripheral devices. Sequencers may be dedicated devices, but are more often to be found as sequencing software on MIDI equipped Personal Computers. Pre-MIDI synthesisers were often equipped with interfaces to allow them to be controlled by other synthesisers (or vice-versa), but these interfaces were often specific to a particular manufacturer. (A fairly common pre-MIDI interface was the analogue Volts/Octave specification.)

MIDI specifies simplex (one-way) serial communication using current-loop signalling at 31,250 baud, which equates to a maximum of 3,125 bytes per second, or since the majority of MIDI messages need three bytes, a maximum of a little over 1000 midi messages per second. It is important to understand that MIDI is a point-to-point communications link between two devices. MIDI is not a multi-drop 'bus' like ethernet. If more than two devices need to be connected then they need to be daisy-chained together.

MIDI messages vary in length from one byte, for 'system' messages such as 'system reset' and 'system clock'; two bytes for 'program change'; three bytes for 'note on', 'note off' and other common messages, to the multiple-byte 'system exclusive' messages. The first byte in the message determines the message type, and hence the length of the overall message. MIDI is a real-time interface, meaning that when one device sends a MIDI message to another, it intends the message to be acted upon immediately (ie in real-time). This is especially important for 'note on' and 'note off' messages, since any significant delay between receiving the message and carrying out the specified action could destroy the musical timing relationship between notes (ie the rythm). Of course with a serial interface that can only carry one message at a time there will be some MIDI delay; in practice this will be so small as to be subjectively unnoticeable. MIDI is also used to synchronise devices such as sequencers and multi-track recorders, so again an accurate real-time response is important. 

2. The MIDI Specification

3. The MIDI Message Structure

4. Bibliography

1. Penfold, RA, "Advanced Midi User's Guide", PC Publishing 1991, ISBN 1-870775-18-X
2. Rumsey, F, "Midi Systems and Control", Focal Press 1994, ISBN 0-240-51370-3

5. Links to MIDI Internet Resources

An excellent resource for shareware, freeware and demo MIDI software (and all kinds of other computer music software) is at:

http://www.harmony-central.com/Software/

A particularly useful freeware MIDI utility and toolbox is MIDIOX, which can be downloaded from the Yamaha UK website at

http://www.yamaha.co.uk/xg/html/software/s_midiox.htm

The MIDI Manufacturers Association:

http://www.midi.org/

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Last updated 14 October 2003.