Servo - April 2012

Of Instruments, Banks, and Channels

The most basic MIDI module is programmed to reproduce the sound of at least one instrument, but many are capable of synthesizing numerous instruments: acoustic and electric piano; guitars; various woodwind and brass instruments; drums; bongos; cymbals; and much more.

Sound modules have built-in defaults, and for instruments the default is usually an acoustic grand piano. To change the instrument so the module plays something else, you send the “change instrument command,” 0xC0, followed by the number of the instrument you want to use. Instrument numbers span from 0 to 127, with 0 being the default.

For instance, to change to instrument #12 – whatever it might be – you simply transmit a couple of bytes from the controller to the MIDI sound module:

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FIGURE 2. MIDI messages are composed of just a few bytes, an eight-bit command byte (it always has its eighth bit set to 1), and one or more seven-bit data bytes.

0xC0 12

and so on. To combine a command byte with a channel number, simply add them together. Popular programming practice performs this function using binary OR, which on the Arduino is done using the | (pipe) operator: (Before going further, take note that the documentation for the sound module probably lists the instruments starting at 1, but programmatically you specify them beginning with

0xC0|3

// Bits: 11000011

Note: As with instruments, channel numbers in MIDI are typically noted as 1-16, but programmatically are referred to as 0-15. So in the list above, Channel_0 is actually MIDI channel 1, Channel_1 is MIDI channel 2, and so on. This difference really only matters if you’re interfacing with other MIDI equipment — like a MIDI keyboard — where channels are numbered 1 through 16. For programming using an Arduino and a MIDI breakout board like the one in Tunebot, the channel offset numbers make little or no difference.

Understanding Channel Voice Messages

Switching instruments to play something else is an example of modifying the channel voice. Channel voice messages are those that alter the sound on a specific channel, rather than for the entire sound module. There are seven channel message commands in common use:

FIGURE 4. Channel voice bytes are constructed by adding or ORing the four most significant bits (MSB) of a command byte with the four least significant bits (LSB) of the channel number.

For example, the command byte for switching instruments (0xC0) begins with the binary bits 1100. Channel_0 is binary 0000, making the whole command byte:

1100 0000

Other variations are:

11000001
11000010
11000011

Channel_1
Channel_2
Channel_3

FIGURE 3. MIDI devices synthesize one or more instruments that are kept in a repository called a bank. Instruments are played through any of 16 channels.