of Microprocessors

618 MUSICAL ApPLICATIONS OF MICROPROCESSORS
sine table as well as greater word length. To equal the dynamic range and
noise performance of the 1024 X lO-bit sine table with a true multiplier, one
would have to go to a 4,096 emry sine table with 16 bits per entry.
A complete block diagram of the generator is shown in Fig. 17-18. A
timing diagram for the generator is given in Fig. 17-19. Note that a
pipeline register has been inserted between the sine ROM and the amplitude
multiplier. The purpose is to isolate the propagation delay of the phase adder
and sine ROM from the delay of the amplitude multiplier. The only side
effect is that the phase memory address for harmonic N is N, whereas the
amplitude memory address is N + 1. This can be avoided by inserting another
pipeline register between the amplitude memory and the multiplier. However,
for minimum cost the comrol computer can simply take the skew into
account when writing into the memories. The harmonic accumulator adds up
the 64 sine waves during a major cycle. At the end of the cycle, its content is
transferred to the output register, which holds it for the entire duration of
the next major cycle, thus giving the DAC stable data.
All timing is derived from an 8-MHz crystal clock. Two clock cycles
make a minor cycle and 64 minor cycles make a major cycle. All of the clock
and clear inputs to the various blocks are assumed to be positive edge
triggered. The first one-quarter of each minor cycle is devoted to external
access to the amplitude and phase memories and the frequency-control register.
This rapid rate of access allows the generator to be connected directly to
the bus of most microcomputers with no wait states or buffer memories
needed. The remaining three-quarters of each minor cycle, which is about
190 osee, is allowed for memory access, adder delay, ete. The parallel
amplitude multiplier must therefore act in approximately 150 nsee.
Rapid, glitchless updating of phase makes possible a control technique
that can be used to simulate inexact tuning of the harmonics. If the comrol
computer periodically increments the phase parameter of a harmonic, the
I _MINOR~I
CYCLE
8-MHz CLOCK
MINOR CYCLE CLOCK (A)
MAJOR CYCLE CLOCK (B)
MINOR CYCLE COUNTER
________----JnL..
60
_
EXTERNAL AOORESS SELECT (C)
EXTERNAL WRITE ENABLE (D)
_-'~_-' '-_---''-_--'"__,IL-lL-ll.-.-
Fig. 17-19. Timing diagram for Fourier series tone generator