MIDI Standards Explained

If you have been playing around with your AWE32 software for a while, particularly the AWE32 Control Panel, you might have come across a list box which lets you choose General MIDI, GS or MT32. You could have wondered what they stand for, and to "play safe", you have left it in the default General MIDI mode.

What exactly are these selections for? Well, they are actually MIDI standards, which I'll attempt to explain in this article.

General MIDI
MIDI stands for Musical Instrument Digital Interface. When the MIDI specifications were first formulated in 1983, all MIDI instruments had their own instrument mappings. Although the instruments could communicate with each other via the MIDI protocol, a MIDI file produced on a "Brand A" keyboard would not sound the same on "Brand B".

Take for example, MIDI instrument (or more correctly, Program) number 61. On a Korg X3, Program #61 is Full Pipes, a church organ sound. On a Yamaha SY-77, it is "Wide Strings". The problem arises when you create a MIDI file with Korg X3's Program #61, and you play back on a different synthesizer. Very likely, the MIDI file will sound very different from what you have recorded.

The General MIDI standard was thus formed to resolve such inconsistencies by specifying an instrument map of 128 instruments which all General MIDI devices should comply to. The 128 instruments cover most of the common musical instruments and are grouped into 16 groups of 8 variations each, allowing for easy re-arrangement of a MIDI file. The biggest advantage of General MIDI (GM) is that it ensures recordings made on a GM device sound largely the same on another GM device. Unfortunately, GM does not specify how a particular instrument should sound exactly, so you will find that the Acoustic Grand Piano on one GM device sounds different on another, although they will both still be piano sounds.

General MIDI has the following specifications:-

• Minimum 24 voice polyphony.
• All voices are velocity reponsive.
• Middle C (C3) is note #60.
• GM devices respond to all 16 channels.
• Percussion is on channel 10.
• Specific drum sounds have different key assignments.
• The 128 instruments conform to a specific accepted order
• Each channel responds to controllers 1(mod wheel), 7(volume), 10(pan), 11 (expression), and 123 (all notes off).
• Pitch bend sensitivity, fine/coarse tuning.

GS (General Standard)
People soon got tired of GM's limitation of only 128 instruments offered by General MIDI. Roland Corporation came up with a new MIDI standard, a superset of General MIDI known as General Standard, or in short, GS. It is backward compatible with General MIDI, responds to all the GM controllers and adds many extra controllers and sounds (317 as compared to only 128 in GM). In addition, GS has the ability to do basic editing of the sounds by changing its envelope attack and decay rates (see below for explanation).

MIDI Bank Select messages are used to access the extra sounds. The program assignments coincide with the 128 instruments of GM, and are also grouped in 16 groups of 8 instruments in their related families. GS also has a fall-back system, which will reassign the master instrument to a particular channel if it calls for a program/bank which does not exist.

Thus, a GS sound module or sound card like the AWE32 will playback GM sequences correctly, though the reverse may not always be true. Nonetheless, the GS song will still sound largely correct on a GM module, with the exception that the extra GS patches will not be used, and you lose the original feel of the music.

(* The original GS standard has 317 instruments. There is now a new GS standard known as GSmkII, which has 354 instruments).

MT-32
This tone module was introduced earlier by Roland and is based on Linear Alogithm (LA). In short, LA synthesis is a combination of sample-playback and artificial sound synthesis. The theory states that the attack portion of any particular sound has the biggest impression on the listener, and its timbre will let the listener determine what instrument it is. Thus, the attack portion of the sound is played using sample-playback technology, while the decay is artificially generated.

MT-32 is not General MIDI compatible, and is now more or less obsolete. You might still find some MT-32 MIDI files though, and some games still support MT-32. However, though the AWE32 is MT-32 compatible, it is only compatible down to the instrument mapping level. Some MIDI files and games use MT-32 specific System Exclusive (SysEx) messages to program the sounds. In such cases, the AWE32 will ignore the SysEx and playback the sounds using the default MT-32 patch assignments instead. Therefore, if you have a choice, you should select General MIDI or GS in your games for optimum sound performance.

Envelope Attack
- Refers to the start of the note, how quickly (time) and loudly (level) it begins after a key is pressed.

Decay Rates
- Refers to the speed and amount by which the sound drops back to its sustain phase (the main part of the note)

Definition from Classics in Sequence. A Source Book For MIDI Sequencing. By Willaim Lloyd and Paul Terry