From the July 1989 issue of Radio And Production

The MIDI Page

MIDI's Sixteen Channels

by Todd Albertson

Have you ever stopped to consider that whenever someone tunes in your station on their receiver, they are still receiving several other stations at the same time? The radio's "RF" filtering system allows the listener to "tune out" all the extraneous signals to concentrate (hopefully) on yours. It may be hard to comprehend, but that listener's receiver is actually tuning out more than just other radio stations. In fact, the receiver is tuning out television signals, short wave, ham, telecommunications, police and fire, CB, and every other signal that happens to be absorbed by the antenna. Now that's powerful!

MIDI receivers are able to do much the same thing on a much smaller scale. Perhaps you have seen a demonstration of a large MIDI system. If so, please tolerate me while I describe one for those who have not:

At the center of our demonstration is a small computer, perhaps it is even tiny (MC 500 or the like). From that computer, one or two MIDI cables run to a series of several keyboards and sound modules. From each sound source (keyboard or sound module) an audio cable (or pair for stereo) runs to a mixing console, and from there the sound is simply amplified as necessary and output to the speakers. The operator hits a button and we are amazed to hear violins, trumpets, guitars, and several electronic sounds. It all sounds as though it were a live performance. Well, surprise! That's exactly what it is! The COMPUTER is giving a live performance on all the keyboards at the same time. This is not a recording, as we have come to think of them, for the sound has not been recorded anywhere. If you were to reach up and unplug one of the keyboards, that sound would disappear instantly. It is truly LIVE.

What we are concerning ourselves with today is the question of how the computer is able to direct one instrument to play one line, while simultaneously giving instructions to another instrument to play another completely different line. What is really mysterious is the fact that we see only one or two cables coming from our computer, but are hearing many more independent parts. Somehow the computer is able to control ALL the instruments through that one (we'll refer to just one now) cable.

The answer is not TOO complicated... The computer does indeed send all the signals down the one cable to all the instruments. The information is passed from one instrument to another with the MIDI THRU ports. Like radio receivers, every instrument receives ALL the transmissions meant for ALL the instruments. It is the "tuning" systems in each instrument that allow it to filter out information meant for other receivers. It is now perhaps more appropriate to think of MIDI reception as analogous to television rather than radio (sorry about that). It turns out that our computer is sending up to 16 different independent parts in each MIDI cable. These are transmitted from the computer on CHANNELS 1 through 16. Got that? There are sixteen MIDI channels. They are referenced with numbers 1 through16. VERY important knowledge! Each instrument is responsible for tuning out the extraneous signals and concentrating on its own. This means the operator has assigned MIDI channels to each instrument with the parameters supplied by the instrument.

Last month, you found out how to "layer" keyboards by using a parameter called "OMNI". When OMNI is turned on, the instrument receives ALL MIDI channels sent to it. It is as though all the "tuning" systems were simply "wide open" on a television. All stations transmitting would get through and you would have a nice mess, thank you; unless... there were only ONE station transmitting. In that case, there would be no problem. The same is true of MIDI. If you only have one transmitting instrument or computer, there is no reason not to use OMNI to receive ALL information sent down the cable; but if you have independent parts, it is usually best to assign MIDI channels to each part, and let the respective instruments respond accordingly.

To test this, set up the transmitter to send on channel 15, and the receiver to channel 16. Be aware that some instruments have complex abilities involving the reception of multiple channels. If there is a MULTI-TIMBRAL mode parameter, turn it off. Also, many modern instruments can transmit on one channel while receiving on another. Make sure you have set the correct parameter. Having studied the Japanese to English gobbledygook in your owner's manual to try to find these parameters, you are probably now ready to burn your instrument in sacrifice to English teachers everywhere. I understand and sympathize! Keep plugging away though and you will find these things eventually. When you finally DO find them and configure them as I've said, you will find that when you transmit on 15, the instrument set to 16 will not respond. Now adjust one of them so that both are set to the same channel, and you are once again able to control one instrument with another.

That's enough for now. "Tune in" again next month (hehe), and we'll begin to look at what "MULTI-TIMBRAL" means! ◙