of Microprocessors

410 MUSICAL ApPLICATIONS OF MICROPROCESSORS
sample rate increase) have been integrated onto proprietary custom LSI chips
by at least one company making "compact disk" digital audio playback
systems. Although such chips are not generally available in the marketplace,
they show exciting promise for the future. Another drawback of this
technique is that the DAC must now run at a much higher speed. For the
example in Fig. 12-27, where PSI is 40 kHz, the DAC and deglitcher must
now run at 80 ks/s, which requires a more expensive high-speed DAC and
more care in the circuit layout. In·a stereo playback system, two DACs,
rather than one multiplexed two ways, will probably have to be used. Thus,
perhaps the greatest overall advantage of this technique is the elimination of
phase distortion.
A Complete Audio DAC
Figure 12-28 shows a reasonably complete schematic of an inexpensive
yet high-quality audio DAC using the concepts developed in the preceding
sections. It is a 16-bit unit having 14-bit linearity at high signal levels.
Maximum sample rate is 50 ks/s, although the filter shown is set up for a 25­
ks/s sample rate and a lO-kHz audio bandwidth. The deglitcher utilizes the
handy extra analog switch in the 7546, and the low-pass filter incorporates
high frequency compensation for sin(X)/X and deglitcher time constant
losses. Component values in the filter have been rounded to the nearest 1%
standard values. Timing for the DAC and deglitcher is derived from a builtin
crystal oscillator and counter-decoder combination. The counter inputs are
shown connected for a 25-ks/s sample rate (40 J.tsec, 80 counts) but can be
reconnected for any rate from 7.8 to 50 ksls. They could also be connected to
a port on the host for a programmable sample rate. Note that the filter will
need to be changed for different sample rates. The 2-MHz output can be used
to drive the audio ADC to be described later. Total cost of parts using the
AD7546KN DAC module (made by Analog Devices) should be in the $100
range.
For best performance, the circuitry should be constructed over a ground
plane such as a piece of copper-clad Vectorboard. Mount the components on
the copper side and trim the copper from the edge of any hole a component
lead passes through by hand-twisting a small drill bit in the hole. There can
never be too many power-supply bypass capacitors of the 0.1-J.tf monolithic
ceramic variety. The digital circuitry should be kept away from the analog
elements as much as possible. This is made easier by the pinout of the 7546,
which has all digital signals along one side and analog signals along the
other. In particular, the holding register should right at the edge of the
board, and its input data lines should go immediately offboard to the host.
Finally, the entire unit should be shielded, at least by steel front- and
backplates, and mounted in a separate enclosure.
The timing diagram in Fig. 12-29 should be self-explanatory. The WR
pulse to the DAC can be used to inform the host system that a sample has