of Microprocessors

772 MUSICAL ApPLICATIONS OF MICROPROCESSORS
waveform tables to allow the dwell time at each wavetable to be variable and
to prevent a long note from "running off the end" of the table sequence in
memory. Since a typical instrument used 15-30 waveform tables of256 bytes
each, a machine with 64K of memory was a distinct advantage. The included
Fourier series routine would allow instruments to be defined in terms of linesegment
approximations to the envelope shape of each of their significant
harmonics. Thus, arbitrary dynamic spectrum variation was provided for,
which allowed surprisingly accurate emulation of conventional instrument
sounds as well as a tremendous variety of contrived sounds. About the only
defects in the sound produced were a slight background noise level and
occasional clicks on sustained notes in one voice, while the program briefly
stopped sound generation to set up for notes in the other voices. The program
also suffered from the lack of a convenient score entry method.
More recently, the Apple Macintosh computer design also includes an
8-bit audio DAC. Besides having the much more powerful 68000 microprocessor
available for computing sound samples, the DAC hardware
repeatedly reads a 370-byte portion of memory automatically. A fixed
frequency 22.26-kHz sample clock controls the delivery of data to the DAC
rather than the execution time of a program loop. The presence of this clock
and automatic data transfer circuit greatly simplifies sound-generation
programming as well as solving the click between notes defect noted
previously. In addition to the sample clock, an interrupt is available to signal
when the 370-byte "sound buffer" has been completely scanned out by the
DAC and is wrapping around to be scanned out again. Thus sound
generation can operate on an interrupt basis and appear to be almost
automatic. The interrupt service routine would compute 370 new samples as
quickly as possible and return. Any time remaining from then until the next
interrupt (which occurs 16.7 msec later) would be available for interpreting a
music score, computing new waveform tables on the fly, updating the screen,
and other uses. For example, with six voices and an algorithm similar to Fig.
20-2, a new sample would require about 30 microseconds to compute. To
fill the 370 sample buffer then would require about 11.1 msec to which about
100 f.Lsec of interrupt save and restore overhead should be added. This leaves
about 5.5 msec available 60 times per second to perform other tasks.
Although the capability for some truly impressive (for a personal computer)
music generation is there, the author knows of no Macintosh music program
that even begins to exploit it.
In 1980, Atari introduced their model 400 and 800 computers, which
were unique at the time for using three custom integrated circuits along with
a much faster than normal 1. 79 MHz 6502 microprocessor. Two of these
chips were dedicated to the Atari's video graphics display (Atari was best
known at the time for arcade video games), while the third handled the
alphanumeric keyboard, serial I/O bus, and sound. The sound-generator
portion, which is block diagrammed in Fig. 20-7, probably represents the