General MIDI

First, GM requires that all compliant MIDI instruments meet a certain minimal set of features, such as being able to play at least 24 notes simultaneously (polyphony).

This helps ensure that playback of MIDI files sounds more consistent between different devices compliant with the GM specification.

However, it still leaves the actual sounds of each instrument up to the supplier to implement; one manufacturer's French horn, say, could be brighter, or more mellow, than another's.

[3] Notes recorded on channel 10 always produce percussion sounds when transmitted to a keyboard or synth module which uses the GM standard.

Each distinct note number specifies a unique percussive instrument, rather than the sound's pitch.

Roland GS is a superset of the General MIDI standard that added several proprietary extensions.

Other most notable features were 9 Drum kits with 14 additional drum sounds each, simultaneous Percussion Kits – up to 2 (Channels 10/11), Control Change messages for controlling the send level of sound effect blocks (cc#91-94), entering additional parameters (cc#98-101), portamento, sostenuto, soft pedal (cc#65-67), and model-specific SysEx messages for setting various parameters of the synth engine.

In 1999, the official GM standard was updated to include more controllers, patches, RPNs and SysEx messages, in an attempt to reconcile the conflicting and proprietary Roland GS and Yamaha XG additions.