Midi Channels
What is Midi?
    The first thing that you should realize about midi (Musical Instrument Digital Interface) is that it is only a message, much like the words on this screen.  Midi does not contain musical sounds like tapes and CD's do, but it contains a message that other midi compatible instruments read and decipher.  Midi basically tells the other instrument or computer to "Play a loud middle C using sound 53- Play a soft D using sound 93, etc."  So midi is just a series of these messages that tell another instrument which notes to play and how to play them.
    If you are still confused about the difference between tapes, CD's, digital audio and Midi, then this little story might help. Let's say a friend gives you a cake that has been baked and is ready to be eaten.  Of course before you eat it you can add some more frosting, or put a little more sugar on it, but overall it remains the same cake.  This is like CD's, tapes, and digital audio.  They are the final product, ready to be heard.  You can't really change how the CD, tape or digital audio really sounds, just like how you can not really change the cake that someone gave you into something else.  You can run your tape, CD, or digital audio through an effects box and add echo or other special things just like you can add a little sugar to the cake, but overall the tape, CD, or digital audio remains the same.  Now let's change the story a little bit, instead of your friend giving you a baked cake, lets say he gives you the recipe for the cake.  Now you can do anything you want with recipe, you can double it, totally take out the sugar, mix some vegtables in it, or use chickens instead of eggs.  You can change the cake in any way that you can think of, in fact you don't even have to make it a cake anymore.  This is just like midi.  With midi, you have total control over every aspect of the sequence.  For example, if you send me a midi, I can load it in my sequencing program and go in and change the volume of a single note in the sequence, or I could transpose the whole sequence up by half a step, or I could make the chorus repeat again.  With midi's you can change anything you want about the sequence.  Does that make a little more sense?
    One consequence of this is that each time a midi song (sequence) is played on a different midi instrument, it will sound a little different unlike CD's and tapes that sound the same on every CD and tape player.  The notes will all be the same but the timbre (the sounds of the instruments) will be a little different.  So if you play a midi sequence on your computer and then copy it and take it to a friend's house and play it, it will most likely sound different.  This is becuase your computer's sound card contains a slightly different set of sounds than your friend's sound card.  Each sound card in a computer has it's own GM (general midi) sound set which is bascially a group of sounds that all midi manufactors have agreed on.  This agreement was needed because like I said before a midi message tells the other instrument which notes to play and what sounds to play them with.  So if there was no standard, you might take the midi sequence over to a friend's house and the part that has suppose to be played by a piano might be played by a flute or a drum.  For example, if the midi message said "Play a loud middle C using sound 53- Play a soft D using sound 93", the midi instrument will pull up its sound number 53 and 93 so play the part with.  If sound number 53 and 93 were not the same patch (sound) on each keyboard, it would be impossible to trade midi's with others unless they had the exact same equipment as you did.  In the GM sound bank, which contains 127 different sounds, the first patch (sound number 1) is always an Acoustic Grand Piano, the second patch (sound number 2) is always a Bright Acoustic Piano, etc.  So although the sounds are generally the same, each manufactor still makes the sounds in a different way so although sound number 1 is the Acoustic Grand Piano it will still sound slightly different on different midi instruments.
    This does present a problem for GM sequences.  Let's say I want to make a song using some really weird or different sounds, like an air raid siren that goes up and down slowly.  Well, remember I said that the GM bank contains 127 sounds?  None of those sounds are an air raid siren that goes up and down.  So if you want to make a song that uses an air raid siren, then you will not be able to save your song in GM format, you will have to save it as a digital audio file if you want everyone else to hear it on the internet.  This is also the reason that you will never hear a GM sequence that has someone singing words on it.  So digital audio is very important for creating new and interesting songs.
    Got all of that?  Well now onto midi channels, the things which allow you to play more than one patch at a time.  In order to have different patches (sounds) play at the same time, the midi instrument send and recieve information on different midi channels, which the next section will explain.

Midi Channels

      In order to have one instrument tell another to use the first patch to play a C and use the second patch to play a E, the instruments must make use of midi channels.  In midi there is normally 16 (1-16) different midi  channels and each midi channel can be set to use a certain patch.  The midi message must be transmitted on a certain midi channel and then the recieving instrument must be set to recieve midi information on that channel.  So in order to have different patches play at the same, you must have two midi channels becuase each channel can only play one patch at a time.  So in order to get a piano, a bass, a guitar, and some drums to all play at the same time, you would have to set up four midi channels and have each channel set to use a different patch.  For example, you would set channel one to use the a piano patch, channel two to use a bass patch, channel three to use a guitar patch, and channel four to use a  drum patch.  Midi channels are also used in layering, which is having two sounds play at the same time (layering the sounds).  For example, you can set your keyboard up so that when you press a key a piano and string sound plays.  To do this you would need to set one midi channel to use the piano patch, and another midi channel to use the string patch.  Then when you press a key, the midi information that is generated would be transmitted on both midi channels making both sounds play.
    Well, I probably need to go into more detail on that, but things get complicated.  Perhaps during the summer I will add more to this page.  In the mean time, if you need more clarification, please email me at  Romero@pharmacy.arizona.edu