MIDI is designed with 16 channels on each port and each of channels 1-9 and 11-16 may be assigned a different voice. This is what happens when you choose the voices in Mozart (or other software) and play the piece back. Each channel accepts 128 different pitches, each a semitone apart, giving a maximum range of just over ten and a half octaves.
Channel 10, however, is different. It is reserved for percussion of indefiniite pitch. It does not, in general, respond to having a voice set, but MIDI signals encoding each of what would be different pitches on the other fifteen channels produce a wide variety of different unpitched percussion sounds.
When you have one or more percussion staves with each line or space representing a different instrument to be hit, shaken or scraped, each note can be output to channel 10 of the MIDI synthesiser with the 'pitch encoding' appropriate for producing the desired percussion sound.
Apart from that MIDI signals to the percussion channel are the same as on any othe channel, consisting of note-on and note-off events in real time to say 'start playing this note now' and 'stop playing this note now', with each note-on event containing its pitch code (now defining the percussion sound) and a 'velocity' defining a volume.
Synthesisers can in general map any pitch encoding on to any percussion sound, but the General MIDI standard prescribes a standard mapping to which most conform by default. The table below defines this mapping.
But to illustrate them more graphically, Jonathan Bryant has kindly supplied the following MOZART example where the parts are all set to channel 10 to get a percussion sound.
The above piece shows all the percussion instruments at their corresponding raw MIDI pitches and was written in an early version of Mozart. Mozart now has more flexible ways to represent percussion of indefinite pitch, but this is still a very useful illustration of the MIDI underlying the play-back.
As seen in the above example, you can write a percussion part on a normal stave and just assign the MIDI percussion channel to hear the sounds on play-back. However the notes are tied to the 'raw MIDI pitches' corresponding to the sounds you wish to produce, and these can be counter intuitive (for example the raw MIDI pitch of the low bongo is higher than that of the high bongo). To overcome this problem, Mozart introduces a percussion stave on which you can assign any percussion instrument to any line or space on the stave. Furthermore it groups different sounds made by the same instrument together. When you play it back, the "pitches" you see on the percussion stave are first converted to the raw MIDI pitch corresponding to the selected instrument and then sent to the synthesiser. NB you still have to ensure that the part is set to play on the percussion channel!
The table below gives the mapping of what would be MIDI pitches on MIDI channels 1-9 or 11-16 and the sounds produced if the MIDI suignals are sent to channel 10, the percussion chanel. It is written with the higher MIDI pitches at the top. [Notes can be numbered from 1 to 128 but at the time of writing the entire range is not yet defined bythe General MIDI standard.]
|77||F'||Low wood block|
|76||E'||High wood block|
|63||Open high conga|
|62||D||Mute high conga|
|60||Middle C||High bongo|
|59||B,||Ride cymbal 2|
|57||A,||Crash cymbal 2|
|51||Ride cymbal 1|
|49||Crash cymbal 1|
|43||G,,||High floor tom|
|41||F,,||Low floor tom|
|36||C,,||Bass drum 1|
|35||B,,,||Accoustic bass drum|
|30||Record scratch 2|
|29||F,,,||Record scratch 1|
Addendum: the first version of this table, covered pitches 35-81. Tim Gill noticed it was incomplete and pointed out the specification of the extended range. Tim's version is a work of art in itself and is in MOZART format. If some of the names in the above table are a little esoteric, then open this file with MOZART and hit the F2 key...