You said MIDI? Part 2
In our august issue, we started a series of chronicals about MIDI. We talked about it’s history and it’s connectors. We’ll continue this month by talking about how MIDI sends data between different instruments and what kind of controllers you can find in MIDI.
First of all, when a key is pressed on a MIDI controller, such as a keyboard, a digital message is sent through a MIDI channel. That simple message will command a slave synthesizer to play a note. The message is followed by two more pieces of information. The first says wich key was pressed (C, A, E, etc…) and the second one indicates how quickly that key was pressed (velocity), wich tells the other instruments the dynamic (loudness or softness) of the note just played. That’s what you call a voice message in MIDI language. Every voice message is sent out just as the action occurs. As an example, pressing a key and releasing a key is not the same action. So the moment a key is pressed, the code indicating that action is sent. The moment a key is released, another code is sent to indicate that movement.
In MIDI, all instruments and devices use simple, eight-bit microprocessors to send and process the data. It makes 256 possibilities and in order for MIDI to get the most mileage out of an eight-bit number system, the messages come in two different flavors, called status and data. Status and data bytes can be distinguished in MIDI by the value of the code number cause the range of available numbers is split in half. Numbers from 0 to 127 are data and numbers from 128 to 255 are status. The status messages describe the kind of information being sent. They tell the other instruments whether the event that just occured was a key press, a pitch wheel, or another type of performance action. When an action occurs, this is always the first code number sent. The status message contain also the channel within it, wich you’ll see later. The data messages follow the status message with actual value for the event. It will indicate wich key was pressed (C, A, E, etc…) and the velocity with wich it was struck. So a note on message has one status message followed by two data messages.
Now, let’s take a look at the channels. All MIDI synthesizer have a way to select the channel number on wich it will send or recieve the data. There’s 16 channels in MIDI. Many MIDI instruments are multitimbral, meaning that they can play several different parts simultaneously (up to 16), each with a different sound and on a different channel. Another possibily with the channels is to split your keyboard in two or more different sections, each assigned to a different channel. That way you can as example, play bass with your left hand and play flute with your right hand. Channels give MIDI a great deal of power and verstility. For the more advanced MIDI musician, the limitation of 16 channels can become restrictive. Fortunatly, it is possible to put together a MIDI system capable of handling more than 16 MIDI instruments or channels. Many sequencers have more than one MIDI output port, each transmitting a seperate MIDI data stream with it’s own set of 16 channels.
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Frank
Levert
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