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Taking Advantage of General MIDI

General MIDI - what is it and where did it come from? Vic Lennard, who is also known as General MIDI having worked his way up from humble Private, explains all with some maps, a codebook and a sharp, pointy stick.


With the General MIDI logo appearing on many of the new sound modules and synths on the market, there's still some confusion about what it is and what advantages it holds for the MIDI studio owner. Vic Lennard sorts out the paper work...


Countless pieces of paper. That's what I remember at the end of each studio session. Mountains of paper with the names of sounds that were used, their patch numbers on the various synths, the drum sounds and their MIDI key numbers, who wants coffee or tea and how many sugars. All with the sole purpose of ensuring that your return to a particular track was as quick and smooth as possible. Did it work? Did it hell! Lennard's Law always ensured that the most important sheet of paper was the one that had been used to mop up the spilt coffee during the intervening session...

Keeping tabs on sounds in a MIDI studio can be a tedious task, but clearly it's pretty imperative if a project is split across various sessions. Enter the General MIDI System Level One (GM)...

Considering that MIDI is now ten years old, it's amazing how far it has progressed in so short a time. Had it gone through the usual procedures involved in standardisation, there's little doubt that it would still be floundering underneath the paperwork on someone's desk. The development of MIDI relies on trust between different manufacturers who need it to ensure their products are compatible.

No, MIDI is not a standard in the true sense of the word; indeed, to have so many different companies working together towards a common goal is well nigh unheard of in other industries. The problem with MIDI is that while the various messages and events are defined, there is no mandatory set of commands which all devices have to be able to send and receive.

If you think about it, such a set would be impossible to create bearing in mind the diversity of MIDI products; synths, sound modules, drum machines, sequencers, lighting boxes, tape recorders... the list is endless, and each item on it has its own special needs.

What General MIDI has done is to ensure that a common set of MIDI facilities exist in any MIDI device carrying the GM logo. The main criteria of GM are detailed in the accompanying boxout, but briefly stipulate that for a sound module to carry the GM badge, it must match incoming MIDI Program Changes to the GM Sound Set of 128 instruments. So, for instance, MIDI Program Change #1 always calls up an Acoustic Grand Piano sound.

As each manufacturer can decide on the type of synthesis they use, different GM sound modules do not (as is sometimes supposed), sound identical - but this should be regarded as an advantage, not a limitation. While the limit of 128 instruments is seen by many to be too tight a restraint, remember that GM is aimed at people with 'budget' MIDI studios who want to use MIDI without having to spend too much time trying to understand it. How many different sounds does the average person use in a MIDI setup? Certainly not as many as their instruments have on offer.

A greater restriction exists in the GM Percussion Map which ensures that a particular MIDI note sent on MIDI channel 10 always plays the same percussion sound. However, only 47 of these are detailed, including only two bass drums and a single acoustic snare; in fact, the latin percussion section is far more comprehensive than more conventional 'kit' sounds. To get around this, most of the current GM modules have more than a single drum kit, and more than the minimum number of drum sounds - Roland's Sound Canvas, for example, has 61 percussion sounds in the Standard Kit as part of their superset to GM called GS.

The problem here is that this goes beyond the GM standard which, effectively, renders it somewhat pointless. If you are writing songs with the intention of having them fully playable on any GM sound module, I'm afraid you'll have to stick to the standard GM Percussion Set.

The biggest problem of all as far as GM is concerned is the thorny question of polyphony. There tend to be two ways of measuring polyphony: notes or voices. If a synth's polyphony is stated as a number of notes, you know exactly how many can be playing at the same time without losing one. Where the polyphony is given in terms of voices, however (and the sound module uses more than one voice to create certain sounds), there is no way of knowing how many notes can be playing before the polyphony is exceeded and notes start to disappear - 'note-stealing' as it is called.

The General MIDI specification states that a GM module must provide a minimum polyphony of 24 dynamically allocated voices which means that they are effectively kept in a central pool, used when needed and then returned for the next notes. Consequently, it is virtually impossible to be certain that every GM song will play on every GM module.

The other possibility mentioned in the specification is a split of 16 voices for melodic instruments and eight for the percussion sounds, and herein lies another problem. To be absolutely certain of a song playing correctly on all GM modules, you have to keep to a lowest common denominator which, in the case of GM, is the split voice option. Let's say that 20 melodic and three percussion voices are in use at a particular point in the song. If the polyphony is 24 voices there is no problem, but if the polyphony is the split 16/8 version then four notes won't play - end of story.


There are many precautions that you can take to ensure that polyphony problems are kept to a minimum. For a start, you can reduce the lengths of all percussion notes to the bare minimum - a tick or two. Why does this help? Well, until a Note Off is sent from the sequencer, the voice(s) in use aren't returned to the central pool. Percussion voices only need to be triggered briefly to play their full length so sending the Note Off as quickly as possible ensures that the maximum number of voices are available at all times.

To say that you should use the sustain pedal sparingly is an understatement. Play a 4-note chord on a synth with the left hand, hit the sustain pedal and play a 5-note melody with the right - that's nine voices used. If you release the sustain pedal slightly after playing the left hand chord on the first beat of the next bar, a total of 13 voices are in use which means that more than half of the available polyphony has been taken up with just one instrument.

It's worth checking in the Event Editor (if your sequencer has one), that Sustain Pedal Off commands are placed before the first beat of the following bar. If you can't do this, then make use of the fact that most GM modules give priority to information on MIDI channel 10 - followed by 1, 2, 3 and so on up to 16 - by putting the keyboard part on a high numbered MIDI channel so that if notes have to be stolen, this will be a prime candidate.

The above scenario leads us round to another sequencer function which causes note-stealing: quantising. This often-used facility is for correcting timing errors in your playing by moving notes to the nearest quantise value, which invariably means that all notes around the first beat of the bar are placed on the first beat. Try leaving some tracks unquantised or manually move some notes to avoid the inevitable congestion which will otherwise occur.

Very often, some notes are masked out by 'stronger' sounds occurring at the same place. For instance, a note being played by a bass synth may match up with one of the notes in the left hand of the piano part, and could mean the latter will not be heard. If this is the case, try taking it out and seeing if it makes any audible difference. If it doesn't, leave it out. Generally, try to use a maximum of three notes in the left hand of a chord - especially if you're using a pad sound at the same time, because there are bound to be overlapping notes.

Finally, the problem with voices and polyphony can be reduced by making sure that, wherever possible, you use sounds which only require a single voice rather than those which need two. There should be a list in the back of the manual giving the number of voices that each instrument uses and you may find that you can replace, say, a string pad with a slightly thinner version and still not detect much of a change in the mix.

With a little forward planning and General MIDI on your side, you should find those countless pieces of paper are reduced to the only one that really matters - the tea and coffee list...

More information on General MIDI can be obtained from UKMA on (Contact Details)

GM Sound Module Criteria

  • Ability to play up to 16 instruments on the 16 MIDI channels
  • MIDI channel 10 to be reserved for key-based percussion following the GM Percussion Map
  • A minimum of 24 simultaneously available voices, possibly with eight reserved for percussion and 16 for other instruments
  • 128 presets each assigned to the MIDI Program Change number in the GM Sound Set
  • Recognition of the following MIDI Control Changes: Modulation (#1), Volume (#7). Pan (#10), Expression (#11), Sustain Pedal (#64), Reset All Controllers (#121) and All Notes Off (#123)
  • Recognition of Channel Aftertouch and Pitchbend
  • Recognition of Pitchbend Sensitivity, Fine Tuning and Coarse Tuning Registered Parameters


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Ensoniq KS-32 MIDI Studio

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Audioshop


Music Technology - Copyright: Music Maker Publications (UK), Future Publishing.

 

Music Technology - May 1993

Topic:

MIDI


Feature by Vic Lennard

Previous article in this issue:

> Ensoniq KS-32 MIDI Studio

Next article in this issue:

> Audioshop


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