It defines a standardized group of sounds and the minimum requirements for General MIDI compatible synthesizers or sound modules, so that a specially prepared sequence or MIDI file that is sent to the instrument via MIDI will play back the correct sound types, regardless of make and model of the instrument.
MIDI identifies sounds by their program change number. Before the General MIDI standard was introduced, the same MIDI program change number often addressed totally different types of sound in any two synthesizers or sound modules from different manufacturers, eg a flute type sound in one instrument and a piano type sound in the other.
With the introduction of General MIDI standard compatible instruments this changed. These instruments use the same program change numbers for the same types of instruments.
So, if the person that prepared a sequence or MIDI file wants the melody to be played by a “piano”, he can use a certain program change command embedded into the sequence to automatically select a piano sound in any GM compatible sound module. The GM standard does not specify in great detail how that piano should sound. It is simply assumed that the manufacturer reproduces an acoustic piano within the capabilities of the instrument.
General MIDI supports all 16 MIDI channels. Each channel can play a variable number of voices (thus be polyphonic). Each channel can play a different instrument (or sound, or program). A minimum of 24 fully dynamically allocated voices are simultaneously available for both melodic and percussion sounds.
Furthermore, in GM compatible instruments, percussion and drum instruments which are key-based always use MIDI channel 10 and specific note numbers are reserved for specific drum sounds.
There are a number of other MIDI messages that GM compatible instruments should respond to. Among these are the MIDI controller events for Volume (Controller 7) and Pan (controller 10). By using these controllers it is possible to create a MIDI Mix for a piece of music.
This is a variation of General MIDI introduced by Roland. It defines additional standard procedures for selecting alternate drum kits and sound variations, and for setting a number of other parameters in Roland GS compatible instruments.
This is a variation of General MIDI introduced by Yamaha. It defines additional standard procedures for selecting alternate drum kits and for setting a number of other parameters in Yamaha XG compatible instruments.