Home Recording Basics: General MIDI
If you compose music for other people to play on their MIDI instruments or if you want to use music from another composer, General MIDI is invaluable to you. General MIDI (GM) is a protocol that enables a MIDI instrument to provide a series of sounds and messages that are consistent with other MIDI instruments.
With General MIDI, you can take a Standard MIDI File (SMF) of a song that was created on one sequencer program, transfer the file to another program, and use that other program to play the exact performance — sounds, timing, program changes, and everything else.
GM instruments contain numerous sound patches that the MIDI community has standardized. Although the quality of these sounds isn’t subject to a uniform standard, their sound type and location (Acoustic Grand Piano on Patch #1, for instance) are the same on all GM-compatible machines.
Not all MIDI-capable instruments follow the GM standards. If this feature is important to you, be sure to find out whether the instrument that interests you is GM-compatible before you buy.
GM standards dictate not only the particular sounds of a synthesizer but also which drum sounds are located on which keys, how many notes of polyphony the instrument has, and how many different channels the instrument can receive and send instructions on. Here are the two levels of GM compatibility:
GM Level 1 compatibility: Level 1 protocols were developed in 1991 and consist of a minimum of 128 instrument patches, 24 notes of polyphony, receiving and sending capability for all 16 MIDI channels, 16-part multitimbrality, and a host of controller and performance messages.
GM Level 2 compatibility: Level 2 was implemented in 1999 and includes more sounds, polyphony, and features. A GM-compatible device has 32 notes of polyphony, 16-channel support, up to 16 simultaneous instrument sound patches, and a host of additional sounds (384, to be exact), including 2 channels of simultaneous percussion sounds. Also added to the GM2 standard are reverb and chorus effects.