of Microprocessors

312 MUSICAL ApPLICATIONS OF MICROPROCESSORS
an output pointer designates the first slot that contains valid data. If both
pointers point to the same slot, the queue is empty and no data is available.
After a deposit or withdrawal from the queue, the pointers are incremented
by four, since each event requires four bytes. Wraparound of the circular
queue from end to beginning is implemented by masking the queue pointers
so that they can never contain a value larger than 31. No test is made for a
full queue so if more than seven events are stacked up due to nonresponse
from the host, they will be lost, Timer interrupts are disabled during queue
operations, but since the timer is an oscillator, no timing errors accumulate.
Actual scanning of the keyboard is slightly unconventional due to the
need for high speed. Since the vast majority of keys will be inactive, the scan
loop is made as efficient as possible for that case. Also the scanning loop is
expanded by a factor of four to decrease the time per key from 15 JLsec to an
average of 12.25 JLsec for a total savings of 167 JLsec. Any key that is not
both in State 0 and contacting the upper bus causes a vector jump to the
approptiate processing routine, which is based on the 2-bit state code kept
for each key. Since a vector jump is used to get to the processing routine,
returning to the proper point in the expanded scan loop is a little tricky but
quite fast. The state processing routines themselves simply implement the
state diagram of Fig. 9--8. Note the convenience of indexed addressing where
the key number currently being considered is always kept in the Y register
and can be used to directly access contact status as well as the state byte for the
current key.
MIDI
One of the centtal issues raised by the use of a separate keyboard
microprocessor is the communication of keystroke data between the keyboard
and the synthesizer's processor. The method just described is highly efficient
and cost-effective when both ends of the intetface (the keyboatd and the
synthesizer) are under the builder's control. The same would be the case with
a manufacturer of keyboard-driven, microprocessor-controlled synthesizers
who makes both the keyboard and the synthesizer. Of course, in these cases,
each interface and data format design would probably be different as the
designer attempts to optimize cost and performance to the problem at hand.
Recently, however, users of commercial keyboard synthesizers have
shown increased interest in connecting them together so that keystrokes on
one synthesizer could also trigger sounds in anothet synthesizer ftom a
different manufacturer. Additionally, a demand has developed for keyboards
purchasable separately from the synthesizer and keyboardless "expander"
synthesizers that can increase the numbet of voices available from a stock
synthesizer. Separate digital sequencers capable of recording a sequence
directly from a keyboard and later playing it through a synthesizer have also
become available. Many owners of personal microcomputers would like to