of Microprocessors

LOW-COST SYNTHESIS TECHNIQUES 765
SLFO FREQ.
EXTERNAL
MODULATION
CONTROL I - - - - --
R
MODULAIION
SOURCE
SELECT
PITCH
CONTROL
FREQ. RANGE
EXT.
NO ISE0--+-----------,
CLOCK
RATE
CONTROL
CUTOFF
FREQ
I
I
II
AMPLITUDE
TIME
ENVE LOPE
SELECT
SOURCE
SELECT
T
ATTACK DECAY
DECAY TIME
RANGE
ATTACK TIME
I CONTROL
I
I
AUDIO
c~--l--IH---+L-.-----r--,-'r----:I---OOUTPUT
_-.J
Fig. 20-3. SN76477 Complex sound generator block diagram
Sound Generator ICs
In many cases, specialized ICs are used for sound generation in games
and personal computers. While this may not be the lowest cost alternative,
such chips can greatly simplify programming by those unfamiliar with
sound-generation principles and free up the processor for other tasks during
sound generation. Actually, there are only a few sound chips on the open
market to choose from. Many more are custom-designed units used only by
one manufacturer, often in only one product.
One simple and inexpensive sound chip is the Texas Instruments
SN76477. This is really a sound effects rather than a music generator and is
"programmed" by connecting different value resistors and capacitors to
several of its pins. However, the configuration of elements and, hence, the
specific sound produced can be controlled by digital inputs that may either
be a microprocessor output register or mechanical switches. In the block
diagram in Fig. 20-3, it can be seen that the major sound sources are a
voltage-controlled oscillator and a pseudo-random bit noise generator with a
variable low-pass filter. The "super-low frequency" oscillator, which need not
necessarily oscillate at a low frequency, can modulate the VCO frequency and