of Microprocessors

764 MUSICAL "ApPLICATIONS OF MICROPROCESSORS
0061 0748 CA DEX DECREMENT AND CHECK TEMPO COUNT
0062 0749 D005 BNE CHORD2 BRANCH TO TIME WASTE IF NOT RUN OUT
0063 074B C609 DEC DUR DECREMENT AND CHECK DURATION COUNTER
0064 074D DOB3 BNE CHORDO GO RELOAD TEMPO AND CONTINUE IF 0
0065 074F 60 RTS ELSE RETURN IF DURATION RAN OUT
0066 0750 EA CHORD2 NOP EQUALIZE LOOP TIME BETWEEN RELOAD
0067 0751 EA NOP TEMPO AND NOT RELAOD TEMPO
0068 0752 4C0407 JMP CHORD1
Fig. 20-2. Simplified four-voice waveform table scan for 6502 (cant.)
R-C filter or a judiciously selected woofer-type speaker can usually provide
sufficient low-pass filtering. Once an audio DAC is present, whether it be in
a game or a personal computer, the sounds producible are limited only by the
imagination and skill of the programmer.
The most straightforward synthesis algorithm to use is waveform table
scanning. Typically, the waveform table will be 256 entries with 8 bits per
entry and no interpolation. For 6502 and similar microprocessors, the
general scheme illustrated in Fig. 13-4 can be used to scan the waveform
table. In fact, a complete routine capable of producing four simultaneous
tones at a sample rate of8.33 ks/s (for a I-MHz 6502) requires only 85 bytes.
Such a routine is listed in Fig. 20-2. It is called with the 16-bit
frequency parameters for the four voices (zero is used for silence) stored in
FREQI-FREQ4, a duration parameter stored in DUR, and a tempo
parameter stored in TEMPO. The number of samples produced before
returning is the product of TEMPO and DUR. Note that the four voice
samples are mixed by presenting them successively to the D-to-A converter
rather than adding them up. This avoids scaling and as written makes the
fourth voice somewhat louder than the other three due to its increased dwell
time. In effect, the low-pass filter performs the mixing function. With a
smooth waveform stored in the table (to avoid aliasing of high harmonics),
the routine is very effective in producing four-part organ music with just a
hint of roughness due to the short table and programming shortcuts taken.
In fact, including a Fourier series routine to compute the waveform table
(perhaps from random harmonic amplitudes) requires just a couple of
hundred additional bytes.
Other synthesis techniques have also been used in a similar context.
Most 8-bit microprocessors are capable of producing one or two FM or
nonlinear waveshaping voices. The VOSIM alogrithm has even been used to
produce remarkably good synthesized speech, in real time, with nothing
more than the audio DAC and some exceptionally clever programming. The
increasing use of 16-bit internal architecture microprocessors with multiply
instructions in even cost-sensitive products will extend further what can be
done with two to three dollars worth of sound generation hardware.