What is General MIDI?
General MIDI (GM) is a defined standard set of 128 Patches adopted to make sound modules more compatible. The 128 Patches appear in a specific order.
As examples, Patch number 1 on all GM sound modules is always the sound of an Acoustic Grand Piano and Patch number 25 is always a Nylon String Guitar. In this way, no matter what MIDI sound module you use, when you change to a specific Patch number, you always hear the same type of sound. The chart, GM Patches, shows you the names of all GM Patches, and their respective Program Change numbers.
The patches are arranged into 16 "families" of instruments, with each family containing 8 instruments. For example, there is a Reed family. Among the 8 instruments within the Reed family, you will find Saxophone, Oboe, and Clarinet.
A GM sound module should be multi-timbral, meaning that it can play MIDI events upon all 16 channels simultaneously, with a different GM Patch sounding for each channel. This always you to orchestrate and layer GM sounds.
GM also has a "drum module" allowing each Drum Part to trigger MIDI a different drum sound. The assignment of GM drum sounds to MIDI notes is shown in the chart, GM Drum Sounds. The Drum Part is usually automatically set to receive MIDI data on channel 10.
GM makes it easy for musicians to put Program Change messages in MIDI sequencer song files. The messages select the correct instruments on all GM sound modules and play all of the correct instrumentation automatically.
GM also spells out other minimum requirements that a GM module should meet, such as being able to respond to Pitch and Modulation Wheels and to play notes simultaneously with dynamics among the 16 Parts. These standards ensure that MIDI Files play back properly on various types of sound modules, keyboards and drum machines.
MIDI specifications are not limited to the GM standard. Patches can be set up to play entirely different sounds than the GM set and most MIDI sound modules and keyboards offer such programmability, with hundreds, if not thousands, of different sounds. Almost all have a GM option to allow an easy interface.
NOTE: GM does not control the quality of the sound produced. For example, one sound module could use FM synthesis to simulate the Acoustic Grand Piano patch, while another may use 24 bit digital audio waveforms of sampled piano sounds.