More and more modules and keyboards are now General MIDI compatible. GM is supposed to make your life easier - but what the hell is it all about? Paul White explains.

Somewhere between the big bang and the death of the universe, man invented MIDI. Unlike many other of man's inventions, such as nuclear weapons, genetically engineered diseases and fast food (the last two may be considered interchangeable), MIDI was, on the whole, a good thing. Over the years we've come to terms with MIDI's little foibles and eccentricities, and every time we seem in danger of outgrowing it, another chunk gets added to the protocol to further extend its power and usefulness.

General MIDI-compatible equipment has been with us for two years now, but I get the feeling that most 'serious' MIDI users consider it to be some form of distracting and largely irrelevant 'consumer' addition. I confess that I tended to lean towards that way of thinking until clients started to arrive at my studio clutching GM MIDI song files which they wanted turned into usable backing tapes for their solo club gigs. Assigning appropriate sounds from a variety of non-GM modules takes time, and though a little judicious mapping can help here, a dedicated GM module makes life a lot easier. So, for the benefit of the uninitiated, what is General MIDI, or GM, as it is known to its friends?

GENERAL MIDI

Without getting in too deep, General MIDI is another stage of standardisation which has been added to the existing MIDI protocol to enable manufacturers to build synthesizers and synth modules that exhibit a specified degree of compatibility in certain areas. Specifically, General MIDI sets out strict rules for patch mapping, drum note mapping, multitimbrality and polyphony. In a nutshell, the aim is to allow a MIDI sequence recorded using one GM module to be played back on any other GM module without the need to remap patches, move drum note allocations or worry about running out of parts or polyphony. This doesn't mean that all GM synths have to sound exactly the same, but it does mean that, for example, a piano preset on one machine must be in the same patch location as a similar piano preset on any other GM machine.

MIDI COMPATIBILITY

Normally, whenever you record a piece of music using a sequencer, you insert patch change commands at the start of the piece, and at any subsequent locations in the piece where a sound is required to change. That way, whenever you play back the sequence through the same instruments and modules, the correct sounds will automatically be called up with no intervention on your behalf. However, if you play the sequence back on a friend's MIDI system, you may well find that all the sounds are completely wrong because the patches are not only completely different in sound, but any that are suitable are also stored in different patch locations. In the case of programmable instruments, this kind of chaos is difficult to avoid because most users have their own system for storing their edited sounds. Furthermore, the factory presets that come with most MIDI instruments were not stored in any particular order prior to the introduction of GM.

Another potential stumbling block is the drum and percussion part. Is your friend's system set up with the drum part on the same MIDI channel and, if so, are the drum sounds mapped to the same notes? Roland have had their own more or less standard drum mapping system for some time now, but some other manufacturers have not been so well organised. Furthermore, you may have written the piece using drum sounds that don't have any close equivalent on your friend's drum machine.

Let's say you're lucky and you get all the sounds mapped out. There's still a good chance you'll come unstuck when it comes to controller information; perhaps one of the modules doesn't respond to Controller 7 (Master Volume), or perhaps the pitch bend ranges are set differently to those on your own system.

The problem was that, although MIDI was a wonderful system, for many applications there was too much that was still optional. General MIDI was devised to provide a solution for use in situations where a high degree of compatibility was essential, for example when replaying prerecorded MIDI song files. Now, with the introduction of general MIDI, musicians can play each other's GM song files and be confident that they will sound more or less the way they should, even though there are subtle subjective differences between one manufacturer's GM machine and another. This has opened up the market for commercially available MIDI song files, and though these hold little attraction for the serious composer, they are very effective as live backings and may also be used to gain recording, mixing and effects processing experience.

POLYPHONY AND MULTITIMBRALITY

The problem with polyphony and multitimbrality is that you can never have enough of it! GM MIDI instruments provide the ability to play back 16 parts on 16 MIDI channels, but is a total polyphony of 24 notes sufficient to go round? If not, note robbing takes place and previously played notes start to drop out. The answer has to be that more would be nice, but we'll have to be grateful for what we get. What's more, where a synth uses two voices to make up a sound, the actual polyphony is further reduced. In theory, that means that a GM song file written for one GM machine could still come unstuck if played back using a different GM module claiming that number of voices and polyphony mean the same thing.

ROLAND'S ENHANCED GS FORMAT

Much of the present GM format owes its existence to Roland's own protocols, so it's hardly surprising that Roland have gone one step further and devised an enhanced version of General MIDI which they call GS. Realising that many users wouldn't be satisfied with 128 preset sounds, Roland have designed their GS machines to offer several alternative banks of sounds, the basic GM set (Capital Tones) being the first bank (Bank 0). There are up to seven 'Variation Tones' based on each of the Capital Tones and these are arranged so as to have the same program change numbers as the tones from which they are derived. In other words, all the variation tones of a Piano Capital Tone will still be pianos, though they will all be subtly different. Further banks are provided for sounds known as Sub-Capital Tones, which are less obviously related to the Capital Tones.

A Bank Change (in Roland's case based on Controller 0) allows the user to switch between the various banks. Conventional program change commands are then used to select the sounds within each bank — a neat way to get around MIDI's limitation of being able to directly address only 128 patches.

EDITING

Because General MIDI is based on the concept of the same sounds always being in the same place, it stands to reason that any attempt at editing the sounds will risk negating any advantages the system has. Some modules allow editing, though the patch name remains unchanged, while others retain the GM sounds as presets.

COMMERCIAL MIDI SONG FILES

Over the past couple of years, companies have sprung up marketing GM song files covering all musical styles from pop to classical. These have many applications, from general interest to song analysis, but the area that has attracted the most interest is as an alternative to backing tapes for cabaret performers. Providing the originators of the disk pay MCPS (Mechanical Copyright Protection Society) royalties and the performer plays in a venue registered with PRS (Performing Rights Society), nobody loses out. A huge advantage of MIDI song files over pre-recorded backing tapes is that the key can be changed at the touch of a button, making life rather less difficult for the solo entertainer. The overall sound quality is generally better too; with pre-recorded tapes, you either have to play the original night after night and risk wearing it out, or copy it onto another cassette with the consequent loss of quality.