Is It Good Enough for GM?

The problem was, if you recorded a piece of music on a MIDI sequencer, the music would only sound the way you expected while it was being played with the same equipment used to create it. If you took the sequencer to your friend’s house, hooked it to your friend’s synthesizer, and hit the Play button, the music would almost certainly sound wildly wrong. The track you recorded using a flute sound, for instance, might be played back by a choir, a distorted electric guitar, or a drum kit.

MIDI defines a set of messages called program change messages, which can be stored in and played back by a sequencer. In your own studio, program change 37 might be assigned to the flute. But 37 is only a number, not a description of a flute sound. When a synthesizer receives program change 37, it will load whatever sound happens to be in memory location 37.

The General MIDI (GM) specification was devised to solve this problem. If you embed program change messages while recording a sequence using a GM-compatible synth or sound module, the sequence should sound very similar when played back using any GM device.

GM is found in most consumer keyboards and many “prosumer” synthesizers. Apple QuickTime, found in all Macintoshes, includes a software-based GM synth, and there’s also one in Windows. There are several software synthesizers that specialize in GM sounds, too (see screenshot above). Even high-end synthesizer workstations often have a bank of GM sounds.

WHAT’S IN GM?

To qualify as a true General MIDI device, a synth has to have a full set of GM sounds. It also has to meet certain minimum specs, and be able to respond to certain MIDI messages.

The General MIDI Sound Set consists of a bank of 128 sound programs. If a GM synth receives a MIDI program change message with a value of 1, for instance, it should cue up a grand piano sound on the channel where the message was received. Any notes that are received on that channel should then play the piano sound. Program change 41 should cue up a violin sound, 115 a steel drum, and so on.

GM synths assign a drum kit to MIDI channel 10. The layout of drum sounds on the keyboard is standardized, so that note 36 (the lowest C on a 5-octave keyboard) plays a kick drum sound, note 38 a snare, and so on.

GM instruments have to be able to play at least 24 notes at once. They have to be able to assign a different sound to each of the 16 MIDI channels. They have to respond to several MIDI messages, including master volume, pan, sustain pedal, pitchbend, and channel pressure (aftertouch).

More details on GM are available on the website of the MIDI Manufacturers Association (www.midi.org).

USING GM

If you have a GM sound source, you can purchase (or in some cases download for free) sequencer files that will play popular tunes. The sequencer files will be in Standard MIDI File (SMF) format, and can be loaded into any sequencer. When you play back the SMF using your GM synth, the sequence will send program changes so as to configure the synth to play the correct sounds. It will also adjust the relative loudness of the various sounds in the sequence.

Standard MIDI Files are used not only for home entertainment but also for backing tracks by solo or small ensemble entertainers and for karaoke. They’re also embedded in some websites and are used in multimedia presentations. And some notation programs can use GM sounds for playback.

Because General MIDI was supposed to create reliable sound playback on different devices, it might seem odd that not all GM synths sound alike. However, many of the GM sound programs, such as 89 (“New Age”) and 95 (“Halo”), are not completely defined. Different manufacturers interpret the names in their own ways. Some manufacturers use earlier GM sounds as models for their new sounds, but others may try to improve the quality of certain sounds.

Also, the definition of GM has been expanded since it was first created. Modern GM synths (which bear the GM2 logo) have more sounds. They also allow various effects, such as chorus and reverb, to be controlled by the SMF. Even things like the depth of a sound’s response to pitchbend messages can be controlled by the SMF.

General MIDI is not often used by professional recording artists and producers, but it has an important place in the consumer and semi-pro world.