The Musical Instrument Digital Interface is a system for connecting instruments together so that one instrument may control one or more other instruments. Typical applications include causing machines to start and stop in synchronisation, play together in the same tempo, and change presets at the same time. More detailed control is also possible, such as pitch bend and modification of specific parameters such as filter cut-off frequency, but on synths from different manufacturers, design differences can complicate compatibility in these areas.

The MIDI as we know it today really came about as a result of two separate proposals for a universal synthesiser interface, one from Oberheim and SCI in the States, the other from some of the principal Japanese manufacturers. It was the amalgamation of these two ideas that brought about the first MIDI meetings between the major electronic musical instrument companies and the eventual announcement of the first MIDI specification - and the first MIDI keyboard, the Prophet 600 - in April 1983.

Since then, most of the world's leading manufacturers have agreed on a universal MIDI specification (spec 1.0, finalised in August of last year) and incorporated it into a whole host of new electronic instruments.

In theory at least, machines from the same manufacturer should be fully compatible, though there will still be limitations if the machines have different control systems, and of course a MIDI drum machine can't tell a MIDI synth what notes to play.

A fully MIDI-compatible instrument is fitted with three sockets, a MIDI input, a MIDI output, and a MIDI through socket so that several synths may be connected up in such a way that they are all controlled by one master instrument. The specification recommends the use of five-pin 180° DIN sockets for all MIDI connections though XLR connectors are to be found on some products, notably the Octave Plateau Voyetra 8.

The interface is fully digital and operates on a five-volt, current loop system and for reliable operation, the use of screened, twisted pair cable (similar to that used for balanced microphones) is strongly recommended.

Information comes in the form of binary 'words' which specify the type and destination of each instruction, and the operational speed of the system is such that at least three units may be linked together before any timing problems are experienced.

The messages carried by MIDI fall into two distinct categories, System and Channel.

Channel Commands

The Channel system was devised in order that communication could be established between the master instrument and any other specific instrument without causing all the others to respond. Any MIDI instrument may be set up to receive information on any one of 16 channels per instrument: split-keyboard synths may in some cases be made to respond as two separate instruments, for example.

Channel commands are normally used to tell a specific synth what notes to play and at what time to play them, and also to convey information relating to performance controls, the precise details of these depending on the model and type of synth. For example, on instruments which incorporate an after touch facility (for instance SCI Prophet T8 or Yamaha DX7) this information may be conveyed as a MIDI Channel command on a note-for-note basis.

System Commands

These may be divided into three types; System Common, System Real Time and System Exclusive.

System Common

These commands are intended for all units in the system and allow you to select a particular song or sequence, and start at a particular beat within it. There is also a system common message to request analogue synths to tune their oscillators (if such a tuning facility is provided on the synth in use).

System Real Time

System real time messages may be sent at any time and their main purpose is to synchronise the entire system to the clock in the master unit. This enables all units to start and stop in synchronisation and in addition transmits an active sensing message to all machines. In the absence of this message, the other machines will automatically revert to keyboard control.

System Exclusive

These commands precede a manufacturer's identification code which opens up a line of communication to a particular type of instrument. For example, if the number 240 (the system exclusive command) is followed by 67 (the code for Yamaha products) any following instructions will be obeyed only by the Yamaha machines. The system exclusive command is terminated upon receipt of system common command 247, "End of exclusive". Generally, each manufacturer selects a single system exclusive ID code which covers their entire MIDI product range.

MIDI in Action

Because all synthesisers have different facilities (number of notes, number and type of oscillators, FM or subtractive synthesis, type and number of controls and so on) it is impossible (and indeed undesirable) to expect all machines to behave in exactly the same way. MIDI channel voice messages contain varying amounts of information depending on the machine of origin, and may vary from basic key number only to a fully detailed description of all performance controls and key dynamics.

It follows then that only another machine of exactly the same type will be able to interpret all this information. If two machines from different manufacturers are connected, communication will be strictly limited, with perhaps only key number information being understood by the second instrument. Some combinations yield more useful results than others and this subject will be discussed elsewhere in the supplement.

Control Codes

The MIDI specification clearly states that the controllers are not specifically defined. A manufacturer can assign the logical controllers to physical ones as necessary. The controllers allocation table must be provided in the users operation manual. What this means is that the maker of your electronic instrument should provide you with the codes assigned to each feature on your machine that is accessible via MIDI.

For example, the code that enables you to address the cut-off frequency on the synth is unlikely to be the same as the code that addresses the same parameter on your other synth, assuming your second synth even has a cut-off frequency control.

This state of affairs explains why synths from different manufacturers can only communicate at relatively basic levels via MIDI, and to establish anything like full communication, a computer is needed to translate the commands in real time. As this would involve a different software package for every conceivable combination of MIDI machines, it's rather unlikely that everyone's needs will be serviced in the very near future. However, the original concept of MIDI was simply to provide the facility for 'triggering' contemporary instruments from each other as well as some form of pitch control. MIDI does this admirably, and what's more, if used properly and carefully, the system can communicate enormous quantities of information quickly and accurately.

It may not be the answer to all our problems - indeed it may even cause a few new ones - but if it is developed properly, MIDI will become a very powerful and effective aid to modern musicians.