GM voice table

The Standard 128 GM Sounds

There are a total of 128 GM sounds, organized into 16 "families" containing 8 sounds per "family" (8 X 16 = 128).

The 16 families are:

PIANO,
CHROMATIC PERCUSSION,
ORGAN,
GUITAR,
BASS,
STRINGS,
ORCHESTRA ENSEMBLE,
BRASS,
REED,
PIPE,
SYNTH LEAD,
SYNTH PAD,
SYNTH EFFECTS,
ETHNIC,
PERCUSSIVE,
SOUND EFFECTS. 

Note that manufacturers usually number their patches from 0-127 rather than from 1-128.

the GS format

The GM format is based on a system developed by synth manufacturer Roland, and they’ve now taken the idea a step further and come up with their GS format. GS is an enhancement to GM in that, instead of having just one bank of standard sounds, you can have several other banks offering variations on those sounds. The arrangement is that the standard GM sound set is in the first bank (bank 0), with up to seven further banks of variations. All of the variation tones of a piano will still sound like a piano, just as all of the variations of organs will sound like organs, but they will offer more variety.

the XG format

Yamaha also introduced their own expanded General MIDI format, which they call XG. Like Roland’s GS mode, this provides several banks of alternative sounds, although most Yamaha XG instruments also support Roland’s GS format. GM and GS tend to be supported mainly by hardware synths rather than by soundcards, although Audigy’s default SoundFonts support both GM and GS variations.

General MIDI - Basic Specs

The GM standard has been embraced by all companies who produce MIDI songfiles, nearly all synthesizer manufacturers, soundcard makers, and music software publishers.

Yet, believe it or not, General MIDI is still an unofficial and largely unregulated standard!

Compliance is strictly voluntary among these companies (however, if they chose to ignore the GM specification, they would lose a lot of business!).

So, General MIDI compliance is considered recommended practice, but it is not a feature that any company is absolutely required to include in their MIDI products. In fact, not every MIDI instrument on the market today will have a General MIDI compatible mode of operation.

It's up to you to make sure that the instrument you wish to purchase sports the GM compatibility logo. That way you know that it will be current with today's MIDI hardware, software, and commercial MIDI songfiles.

Here are the important "minimum features" that a General MIDI compatible instrument will offer...

• A Standard set of 128 sounds to choose from when composing musical soundtracks.
  The sounds are always arranged in a specific numerical order from brand-to-brand.

• The capability of simultaneous playback of up to 16 separate instrument parts.
  The technical term is multitimbral - a basic GM instrument is, therefore, 16 Part Multitimbral.

• A minimum of 24-voice playback (sometimes incorrectly called 24 -note playback).
  The technical term for this feature is polyphony - a GM instrument has at least 24 Voice Polyphony.

• At least one Standard Drum Set or Kit.
  The percussion sounds always have the same note assignments from brand-to-brand.

16 Part Multitimbral
When MIDI synthesizers first became available in the early 1980's, they could only transmit and receive on just one out of a possible 16 MIDI channels. A single channel = a single sound. To achieve an ensemble playback of different sounds, you had to hook several MIDI synthesizers together, each assigned its own personal MIDI channel and instrument sound. Then, for multitimbral playback, the connected synthesizers were triggered for playback all at once.

A GM instrument is literally equivalent to SIXTEEN synthesizers of old! A General MIDI instrument can transmit or receive up to 16 simultaneous MIDI channels through a single MIDI cable receptacle. Each channel can be assigned any one of the 128 GM sounds, except for MIDI channel 10, which is reserved for the Drum Kit.

24 Voice Polyphony
This is the number of actual voices (not necessarily notes) that can be played simultaneously. What's a "voice? Although I have grossly oversimplified this, the explanation still sounds a bit technical, so hang in there.

When you play a note on a MIDI instrument, you trigger an internal circuit called a tone oscillator. Each tone oscillator produces a single "voice" (a sound). So, if you play a single note on a MIDI instrument, you are using a single voice, right? Well, sometimes.

You see, there are some instrument sounds that actually use more than one tone oscillator to produce a particular sound. For example, some of the fuller, lush sounds on a synthesizer may use 2 or more oscillators (per note) to achieve that fullness. Let's see if you followed all that.

If I play a 3-note chord on my MIDI instrument, and I am using an instrument sound that uses one oscillator per note, how many voices have I used up? The answer is, three voices.

Now, if I play a three note chord, and I am using an instrument sound that uses two oscillators per note, how many voices are being used? The answer is, six voices.

You can see that if a feature listed on a MIDI instrument is stated as "24-Note Polyphony," it is only correct if the sounds you use are triggering a single tone oscillator per note. The most accurate way for a manufacturer to state their instrument's polyphony is by voices, not notes i.e.: "24-Voice polyphony."