MUSICAL INSTRUMENT DIGITAL INTERFACE
Presentation by D.J. Fuller – 13th January,—1993.
History:
Originally electronic musical instruments known as synthesisers, were voltage controlled. Attempts to mix the output of two or more of these instruments resulted in chaos as they used different voltages to produce their notes.
The introduction of micro processors at the end of the 1970s opened the flood gates for new musical machines – polyphonic synthesisers – digital-sequencers – drum machines – digital effects – to a burgeoning market for home computers.
Each company in the business had its own interface, each entirely incomprehensible to the other. At a Music Show in California in 1982 the worlds leading companies in electronic musical instruments reached agreement on a standard which would allow synthesiers to be played in tandem. This agreement said that information should be transferred at a speed of 31.25 kilobits per second (31.25 Baud) and that the equipment should be connected through optical isolation circuits.
The Japanese then took this basic specification and developed it to encompass existing and future technologies and by 1983 a full MIDI specification was agreed by all the main manufacturers. The specification was broad enough to embrace manufacturers identities and the developing technologies.
MIDI was a remarkable achievement when you consider the compatabi1ity factor with other products. Companies were persuaded to adopt a spirit of openess and cooperation without seeming to stifle creativity or competition.
Digital Arithmetic:
Computers only use two digits 0 and 1; all their processing is done in binary arithmetic.
The MIDI standard calls for special number clusters to be held in BYTES. A Byte can be thought of as a container holding numbers ranging from 0 to 255, Sending a BYTE is really just sending a number between 0 and 255.
A BYTE is comprised of 8 BITS, the 8 BITS could be conceived as 8 compartments within the container.
Compartment Location. Decimal Conversion if BIT Compartment Holds 1.
0- 1
1- 2
2- 4
3- 8
4- 16
5- 32
6- 64
7- 128
So by sending 10000101 in BITS the decimal number 133 is transmitted.
Similarly 00000101 = 5 and 11100001 = 225. Thus by sending a succession of 0‘s and l’s in groups of 8 BITS (a BYTE) we can send a series of numbers up to a maximum of 255.
These streams of BITS travel along a single core cable in series at the speed of 31,250 Bits per second.
Midi Messages:
MIDI messages are bursts of electrical pulses which expresses numbers in binary form. Press middle C on a keyboard and off goes a number that means middle C in MIDI and another connected synthesiser will obey the instruction and play the same role.
The MIDI specification contains a whole range of messages and the talk then dealt in detail with some of these. The message for turning a note on and off, the loudness of the note, the note itself and the instrument or voice on which it is to be played.
A demonstration was given using an Atari computer of these messages.
Sequencer:
The MIDI standard generates large number of messages and the control and issue of these is quite complex. The only way to organise these is using a computer and a piece of software that is known as a sequencer.
This enables you to input musical notes relative to time, their volume, the voice or instrument and any other information; display them on the monitor screen, edit them, store them and replay the information byway of output ports on the computer sending appropriate streams of BITS to connected electronic musical instruments to reproduce the sounds.
Demonstration.
The talk concluded with the playing of some musical examples which included virtuoso piano playing, the big band sound and the classics.
