Midi stands for Musical Instrument Digital Interface and is a lot easier to use than you might think., but first let's establish a few facts:
Midi is not audio - i.e. you can't hear midi as it makes no noise
Midi is only performance data.
Midi is resolution dependent - eh! right what that means is: The quality of the resultant sound triggered by the midi signal depends on quality of the playback device (synthesiser). i.e. connect it to good synth and you get good sound - connect it to a bad synth and the sound is poor. This can be compared with typing a word e.g. CAT into Microsoft word and then printing it out first using an old daisy wheel printer (if you don't know what one of these is - it doesn't matter all you need to know is that it is old and the print looks well crap) and then printing the same word out using a brand new Laser printer - which results in a great looking print. The upshot of this is that the Microsoft word document is also resolution dependent i.e. the quality depends on the external equipment it is connected to.
Midi remembers 3 things:
which notes the performer has pressed (midi note number) and in which order they were played (sequence),
how long each note was held down for (duration)
how hard the keys were pressed (velocity).
A little more detail
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Lets deal with what midi note numbers are first - If you imagine a piano keyboard and then imagine that each note is given a unique note number to identify it you've pretty much cracked it. Middle C on the keyboard (for the musicians among you) has a midi note number of 36 and as you go up the keyboard the notes also increment e.g. C = 36, C# = 37, D=38 etc. all of the way up the keyboard. So when you press the middle C key on the Piano - midi just thinks Aha! he/she just pressed note number 36 - I'd better remember that - and indeed it does!
But not content with that midi also thinks - well while I'm on I think I'll take a note of how long this performer hold each note down before releasing the key - because you never know when that might come in handy.
Anything else midi? Oh yes how hard did they press the notes? (velocity).
