Well if you think of a piece of sheet music - by itself it doesn't make any noise, however it does contain all of the instructions that a musician (who can read music) needs to replicate the music i.e. how the composer would like it to be replicated. Now when you think about it that's exactly what midi has done - remembered the performance data and as this is stored in a computer (sequencer) it can be played back.
Now there are really big benefits to this and we'll have a look at some of them now....
The sounds can be changed e.g. assuming it was recorded using a synthesiser set to a piano sound
- when the data is played back the synthesiser could be changed to a church organ sound, because midi is only supplying the performance data it doesn't care what sound is being triggered on playback.
The speed of the performance can be changed without altering the key i.e. by speeding up the midi sequencer it simply means play those note numbers quicker.
The key of the music can be altered without changing speed - this is because midi just transposes the midi note numbers up or down by however many notes it wants to.
Can midi do anything else?
Certainly can! for a start it can manipulate the data in many ways and control things like midi lighting rigs. If you think of a note on a synthesiser as simply being a switch - just like a light switch - when you press the note down you turn the switch on and the note sounds and when you let go of the note the switch is flipped off and the note stops sounding. You can see how this relates to a midi equipped lighting rig. When you press the note the light comes on - when you let the note go the light goes off and when you manipulate the velocity you can make the lights fade up or down. Each light has a different note number (just like the synthesiser) so lighting performances can be captured and played back.
Anything else? Oh yes! - programmes like Arkaos (VJ graphics programmes -a 15 day trial can be obtained from arkaos.net)which is on the machine in the pod can be controlled using midi to create complex sequences of videos and computer graphics e.g. Live 8
