What is MIDI?
Friday, March 23, 2007
By Stephane Brault
The General MIDI (GM) and General MIDI 2 (GM2) standards define a MIDI instrument's response to the receipt of a defined set of MIDI messages. As such, they allow a given, conformant MIDI stream to be played on any conformant instrument. Although dependent on the basic MIDI 1.0 specification, the GM and GM2 specifications are each separate from it. As such, it is not generally safe to assume that any given MIDI message stream or MIDI file is intended to drive GM-compliant or GM2-compliant MIDI instruments.
At heart, these specifications resolve certain ambiguities in the MIDI message protocol. In MIDI, instruments (one per channel) are selected by number (0-127), using the Program Change message. However, the basic MIDI 1.0 specification did not specify what instrument sound (piano, tuba, etc.) corresponds to each number. This was intentional, as MIDI originated as a professional music protocol, and in that context it is typical for a performer to assemble a custom palette of instruments appropriate for their particular repertoire, rather than taking a least-common-denominator approach.
Eventually interest developed in adapting MIDI as a consumer content format, and for computer multimedia applications. In this context, in order for MIDI file content to be portable, the instrument program numbers used must call up the same instrument sound on every player. General MIDI (GM) was an attempt by the MIDI Manufacturers' Association (MMA) to resolve this problem by standardising an instrument programme number map, so that for example Program Change 1 always results in a piano sound on all GM-compliant players. GM also specified the response to certain other MIDI messages in a more controlled manner than the MIDI 1.0 specification. The GM spec is maintained and published by the MIDI Manufacturers' Association (MMA).
From a musical perspective, GM has a mixed reputation, mainly because of small or large audible differences in corresponding instrument sounds across player implementations, the limited size of the instrument palette (128 instruments), its least-common denominator character, and the inability to add customised instruments to suit the needs of the particular piece. Yet the GM instrument set is still included in most MIDI instruments, and from a standardisation perspective GM has proven durable.
General MIDI 2
Later, companies in Japan's Association of Musical Electronics Industry (sic) (AMEI) developed General MIDI Level 2 (GM2), incorporating aspects of the Yamaha XG and Roland GS formats, extending the instrument palette, specifying more message responses in detail, and defining new messages for custom tuning scales and more. The GM2 specs are maintained and published by the MMA and AMEI.
Later still, GM2 became the basis of the instrument selection mechanism in Scalable Polyphony MIDI (SP-MIDI), a MIDI variant for mobile applications where different players may have different numbers of musical voices. SP-MIDI is a component of the 3GPP mobile phone terminal multimedia architecture, starting from release 5.
GM, GM2, and SP-MIDI are also the basis for selecting player-provided instruments in several of the MMA/AMEI XMF file formats (XMF Type 0, Type 1, and Mobile XMF), which allow extending the instrument palette with custom instruments in the Downloadable Sound (DLS) formats, addressing another major GM shortcoming.
That's all folks!
In the weeks to come, we'll show you how to use MIDI softwares to play your favorites songs and even record your own.