of Microprocessors

414 MUSICAL ApPLICATIONS OF MICROPROCESSORS
I
0
I
I
SAMPLE
RATE
INPUT
( VALUE
SHOWN
IS FOR
25 ks/sl
0
DO 74161
START
0
DC
TO SAR
0 DB QD (FIG. 7-331
0 DA
lD
2.0 MHz
ClR
ClK
QC
CET QB
CEP QA
AUDIO
ANALOG
INPUT o----+-=-lIN OUTI-7.---------_~OUTPUT
7 V rms SMP.II TO ADC
20 V _ 14 HOLD II (FIG.7-331
p p
S/H CAP
5,000 pF
V+ I POLYSTYRENE
Fig. 12-32. Front-end for 12-bit audio ADC
A 12-Bit Audio A-to-D Converter
Although the last few years have seen a very large price decrease in 16­
bit DAC modules suitable for audio use, the same has not been true for ADC
modules. This is even true at the 12-bit level where a converter fast enough
for audio use still costs $100 or more. Figure 12-32 shows a timing generator
and sample-and-hold circuit that may be added to the fast ADC circuit in
Fig. 7-33 to allow sampling audio signals at rates up to about 80 ks/s. The
SMP-ll (Precision Monolithics) in the SAH circuit normally has dynamic
characteristics unsuitable for deglitchers but it performs well in ADC
sampling. The 2-MHz clock for the successive approximation register also
operates the timing generator, which properly sequences SAH and successive