GM/GM2 (Sound format)

General MIDI was developed by the MIDI Manufacturers Association (MMA) as a standardized format for the sound arrangement of a MIDI.

In a General MIDI instruments are allocated to single tracks. This guarantees that a song is being carried out equally on devices that are compatible with GM formats.

The requirements for a General MIDI are 24 sounds that can be played simultaneously and contain at least 16 melody and 8 percussion voices.

The quality and implementation of single sounds differs from manufacturer to manufacturer of GM compatible sound modules. Therefore no quality standard is given for the musical reproduction.

General MIDI Level 2

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. Here's a quick overview of the GM2 changes in comparison to GM/GS:

• Number of Notes – minimum 32 simultaneous notes
• Simultaneous Percussion Kits – up to 2 (Channels 10/11)
• Up to 16384 variation banks are allowed, each containing a version of the 128 Melodic Sounds (the exact use of these banks is up to the individual manufacturer.)
• 9 GS Drum kits are included
• Additional Control Change introduced, called "Sound Controllers 1–10"
• and many more

More detailed information could be found at wikipedia.