The acronym MIDI stands for Musical Instrument Digital Interface. A Musical Instrument is a machine that makes sounds which humans have decided to call music. Digital means information that is encoded in numerical form, i.e. numbers, while Interface means a machine which facilitates communication between two or more systems.

In practical terms, MIDI is a standard way for all sorts of modern musical equipment to talk to each other. This equipment commonly consists of things like keyboards, computer sequencers, synthesisers, and samplers, but it also includes mixers, tape recorders, effects generators, guitars, drum kits, wind instruments etc.

The MIDI Standard was designed in the early 80's by a partnership between Roland and Sequential Circuits, two of the largest synthesiser manufactures of the time. This came about because of pressure from keyboard players, who wanted a universal interface standard for all their synthesisers to comply to. They were fed up with different synthesiser corporations using their own communications standard which were incompatible with those of other corporations.

After the publication of the MIDI standard in 1984, other musical equipment manufactures quickly began to implement it in the designs of their products and MIDI became a world wide standard.

A major advantage of MIDI over old analogue interface standards, such as CV (Control Voltage), is that it is possible to transfer up to sixteen channels of data down one cable, as opposed to CV's one channel per cable.

Another major advantage of MIDI is that it enables computers equipped with MIDI to be used to write music and control musical equipment. This is done with programs called sequencers. They can give a very high degree of control over music, impossible through conventional means.

Another advantage of MIDI is that it is now a world wide standard, insuring that practically all professional electronic music equipment will be compatible with it.

Having sixteen channels to transfer MIDI data can also be a limitation when you want to use more than sixteen channels. However, this problem can be got around by using two or more midi interfaces each giving sixteen channels.

Another limitation of MIDI is that you can not use it to transfer real time digital audio.

MIDI information is transferred by sending a digital signal down a wire from one system to another. This digital data takes the form of binary numbers, physically transferred by sending zero volts for zero or off and plus five volts for one or on.

Certain binary numbers convey certain types of information, for example a certain binary number will tell the device that a note on a keyboard has been pressed. This is called a note on event and the binary numbers sent through MIDI will also tell the receiving system which note has be pressed and how quickly it was pressed.

A common misconception sometimes made by those new to MIDI is that analogue or digital audio data is somehow transmitted over MIDI cables. It is only possible to transfer binary data over MIDI. Although this may include sample data, you cannot use MIDI to send audio information in real time.

If a sampler, synthesiser or sound module is multi-timbral then it can generate two or more different instrumental sounds at the same time. These sounds are normally accessed over MIDI, therefore, the multi-timbral capacity of an instrument is normally limited to sixteen instruments to match the sixteen available MIDI channels.

If an instrument is polyphonic it can generate two or more voices or sounds at once. For example, a sampler that is thirty-two voice polyphonic can play thirty-two different samples at once.

The terms multi-timbral and polyphony are often confused. They do not have to be linked, for example my Roland Super JX-10 is only two part multi-timbral, but it is twelve voice polyphonic.

General MIDI is a standard for unifying the set of instruments used in different manufacturers keyboards, sound modules and computer sound cards. It consists of 128 instruments and several drum kits.

The idea is that if you put together a MIDI file using one manufactures General MIDI compliant device, you can play it back on another device and it will sound, if not exactly the same, at least all the instruments will be the same.