General MIDI (also known as GM or GM 1) is a standardized specification for electronic musical instruments that respond to MIDI messages. GM was developed by the American MIDI Manufacturers Association (MMA) and the Japan MIDI Standards Committee (JMSC) and first published in 1991. The official specification is available in English from the MMA, bound together with the MIDI 1.0 specification, and in Japanese from the Association of Musical Electronic Industry (AMEI).
GM imposes several requirements beyond the more abstract MIDI 1.0 specification. While MIDI 1.0 by itself provides a communications protocol which ensures that different instruments can interoperate at a fundamental level — for example, that pressing keys on a MIDI keyboard will cause an attached MIDI sound module to play musical notes — GM goes further in two ways. 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). Second, GM attaches specific interpretations to many parameters and control messages which were left unspecified in the MIDI 1.0 specification. For example, assigning one of the 128 possible MIDI Program Numbers selects an instrument. With MIDI 1.0, the assignment could be to an arbitrary instrument; but with GM, a program number assigns a specific instrument name. This helps ensure that playback of MIDI files sounds more consistently 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.
To be GM 1 compatible, sound generating devices (keyboards, hardware or software synthesizers, sound cards) are required to meet the General MIDI System Level 1 performance specification:
|Voices||Allow 24 voices to be available simultaneously for both melodic and percussive sounds (alternatively, allow 16 melodic and 8 percussive voices). All voices respond to note velocity.|
|Channels||Support all 16 channels simultaneously, each assignable to different instruments. Channel 10 is reserved for percussion. Support polyphony (multiple simultaneous notes) on each channel.|
|Instruments||Support a minimum of 128 MIDI Program Numbers (conforming to the GM 1 Instrument Patch Map) and 47 percussion sounds (conforming to the GM 1 Percussion Key Map).|
|Channel messages||Support for controller number 1, 7, 10, 11, 64, 100, 101, 121 and 123; support for channel pressure and pitch bend controllers.|
|Other messages||Respond to the data entry controller and the RPNs for fine and coarse tuning and pitch bend range, as well as all General MIDI Level 1 System Messages.|
GM Instruments must also obey the following conventions for program and controller events:
In MIDI, the instrument sound or "program" for each of the 16 possible MIDI channels is selected with the Program Change message, which has a Program Number parameter. The following table shows which instrument sound corresponds to each of the 128 possible GM Program Numbers. There are 128 program numbers. The numbers can be displayed as values 1 to 128, or, alternatively, as 0 to 127. The 0 to 127 numbering is usually only used internally by the synthesizer; the vast majority of MIDI devices, digital audio workstations and professional MIDI sequencers display these Program Numbers as shown in the table (1–128).
In GM standard MIDI files, channel 10 is reserved for percussion instruments only. 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.
If a MIDI file is programmed to the General MIDI protocol, then the results are predictable, but timbre and sound fidelity may vary depending on the quality of the GM synthesizer. The General MIDI standard includes 47 percussive sounds, using note numbers 35-81 (of the possible 128 numbers from 0-127), as follows:
In MIDI, adjustable parameters for each of the 16 possible MIDI channels may be set with the Control Change (CC) message, which has a Control Number parameter and a Control Value parameter (expressed in a range from 0 to 127). GM also specifies which operations should be performed by multiple Control Numbers.
GM defines several Registered Parameters, which act like Controllers but are addressed in a different way. In MIDI, every Registered Parameter is assigned a Registered Parameter Number or RPN. Registered Parameters are usually called RPNs for short.
Setting Registered Parameters requires sending (numbers are decimal):
The following global Registered Parameter Numbers (RPNs) are standardised (the parameter is specified by RPN LSB/MSB pair and the value is set by Data Entry LSB/MSB pair):
An example of an RPN control sequence to set coarse tuning to A440 (parm 2, value 64) is
Two GM System Exclusive ("SysEx") messages are defined: one to enable and disable General MIDI compatibility mode (for synthesizers that also have non-GM modes); and the other to set the synthesizer's master volume.
Roland GS is a superset of the General MIDI standard that added several proprietary extensions. The most notable addition was the ability to address multiple banks of programs (instrument sounds) by using an additional pair of Bank Select controllers to specify up to 16384 "variation" sounds (cc#0 is Bank Select MSB, and cc#32 is Bank Select LSB). 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. The 14 additional drum sounds are numbered 27-34 and 82-87, bracketing the 47 General MIDI standard sounds numbered 35-81, and are as follows:
Yamaha XG is a superset of the General MIDI standard that added several proprietary extensions. The most notable additions were the 600 instruments and 32 notes polyphony.
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:
Additional melodic instruments can be accessed by setting CC#0 to 121 and then using CC#32 to select the bank before a Program Change.