of Microprocessors

314 MUSICAL ApPLICATIONS OF MICROPROCESSORS
is usually sent over a shielded twisted-pair of wires as a switched current with
a one represented by no current and a zero represented by 5 mAo An
optoisolator (a light-emitting diode close-coupled to a photocell in the same
case for electrical noise isolation) is normally used for detecting the current at
the receiver.
Figure 9-lOA shows a typical MIDI transmitter and receiver circuit.
Any serial interface IC can encode/decode the serial bit stream, although a
6850 is typically used with 6502 and 68000 microprocessors. Note that the
31.25 K bits/sec. data rate, which is not a standard serial communication bit
rate, was selected to be 500 KHz divided by 16, which can be accomplished
internally by the 6850. This eliminates the need for a costly "baud rate
generator" IC, since it can usually be generated by dividing the microprocessor
clock by a small integer (2 for the 6502 or 16 fot the 68000).
Figure 9-lOB shows a typical connection pattern between a MIDI
controller, such as a keyboard, and several MIDI-equipped synthesizers. The
controller, which may have other panel controls as well, has two-way
communication with the primary synthesizer. Thus, if necessary, the
controller can interrogate the status of the ptimary synthesizer. The "MIDIthru"
connection on each synthesizer simply provides a buffered copy of the
signal received by the "MIDI-in" connector. Thus, data sent from the
controller to the primary synthesizer can also be "heard" by the other
synthesizers. Addressing information in the data bytes (called "channels" in
MIDI terminology) determines which synthesizer acts on the data to produce
sound.
The complete MIDI communication protocol is rather complex and
subject to revision, so only its most basic provisions will be covered here.
Readers desiring the full, current specification should check the Bibliography
at the back of the book.
MIDI-equipped synthesizers operate in one of three communication
modes. In "mono" mode, the 4 address bits associated with commands address
actual oscillators or voices in the synthesizer. If one synthesizer has fewer than
16 voices, then two or more can be used up to a total of 16 voices. This mode
offers the most flexibility because the MIDI controller can refer directly to
voices, which may be patched quite differently, by number. "Poly" mode,
which is most useful for live performance keyboard playing, generally assigns
each synthesizer in the chain to one channel addresses. Each synthesizer will
then only respond to MIDI commands addressed to its channel. If the
synthesizer is indeed polyphonic (has several voices), then it is capable of
receiving several keystrokes at once, which it assigns to internal voices
according to whatever its key assignment algorithm would be if a keyboard
was connected directly. A synthesizer in "omni" mode in effect ignores the
channel address and responds to everything in the MIDI command stream.
The MIDI data format itself is much like an 8-bit microprocessor
instruction; there is an operation or command byte (inexplicably called a