GM, GS, XG - what are they?

What are they, anyway? This question comes up a lot.

The basic MIDI specification is a communications protocol. It defines a whole bunch of "events", and the way those events are transmitted over a MIDI cable. This means that when we press on a key on a keyboard, the keyboard sends a "note event" over the MIDI cable to the sound-generating device, which recieves that "note event", and starts to play the note. Other events include "patch change events". When a controller sends a "patch change event" to a sound-generating device, the device changes to that patch. This allows the controller to switch "instruments", and change the sound from piano to trumpet to whatever.

As people started using MIDI more and more, they realized they had a problem. For example, Synthesizer A might have its patches arranged as 1=Piano, 2=Organ, 3=Trumpet, etc. Synthesizer B might have its patches arranged as 1=Rhodes, 2=Piano, 3=Organ, etc. They would right a MIDI part, and it would sound good when played through Synthesizer A. However, when played through Synthesizer B, the instruments were "all mixed-up".

To fix this problem, people developed the "GM" - "General MIDI" - specification. This specification specified the contents of every patch. For example, the first patch is always a piano. Different GM devices could implement different types or qualities of piano for the patch, but it must be a piano. The rest of the instrument patches, all the way up to 255, are specified in the same way. That means that any MIDI part written for a GM device can be played on any GM device, and the sounds will be played on the right instruments.

The GM specification also includes a standard specification for a drum set. The drum set is essentially a special patch where each note corresponds to a different percussion instrument. The order of instruments in this patch is defined by the GM standard.

The Edirol VSC counts as a GM device. It is capable of playing any GM music, and the music will sound right. The Dreamstation is NOT a GM device. It creates a lot of sounds, very different from the sounds in the Edirol VSC, and no attempt is made to use the Dreamstation to create all the patches specified in the GM specification. Therefore, if you attempt to play a GM song through the Dreamstation, you will get unpredictable and probably ugly results.

As people started using GM more and more, they realized that GM was too limited. The 255 available patches just didn't cut it. So Roland developed an extension to GM called GS. The GS specification is pretty much the same as GM, but it "expands" the available choices. In effect, there are 255 sound "banks". Each of the notes that was specified in the original GM specification can have as many as 255 variations. If you look at the sound choices available in the Edirol VSC, you'll see that it is (I think - maybe someone can confirm this?) actually a GS device. You have three Piano choices availabe in Patch 1 - they show up as "001 001", "001 002", and "001 003" in the Edirol. If you play a GM file through the Edirol, it will use patch "001 001" for all patch 1 piano sounds. The GS specification also has some provisions for turning on and off some basic effects such as reverb, something that must be done in device-specific ways like specialized Sysx or NRPN messages in GM. GS also has a few other differences, like more polyphony (number of simultaneous sounds that are guaranteed to be available before cut-outs occur).

Note that GS is a "superset", or expansion, of GM. A GS device will play a GM file, but a GM device will not play a GS file.

Still not satisfied, Yamaha came up with yet another expansion of GM, called XG. It is basically the same thing as GS, except even fancier. It now has full out effect-bus support built-in, even greater polyphony, and a few other minor differences. Being newer than GS, it is very similar to GS. Unfortunately, it is not a superset of GS. Many patches are the same, but many are different. Therefore, a GS file takes tweeking to make it sound the same when played through an XG device. This sucks, but it was Yamaha's design decision, and it must've seemed like a good idea at the time. However, just like GS, XG is also a superset of GM. So, just like GS devices, XG devices can play GM files, but GM devices cannot play XG files.

Wow, have you ever thought about writing a book, I'd buy a copy.

I spend all my spare time on this forum.

Maybe that's why Garrigus never says anything except "Thanks for the plug, and the book's cheaper on my web site" - if he tried to answer questions on this forum, he'd have no time for writing books...