of Microprocessors

368 MUSICAL ApPLICATIONS OF MICROPROCESSORS
PARALLEL
DATA FROM
COMPUTER
FIRST-IN
FIRST-OUT
BUFFER
HIGH-RESOLUTION
DAC
DEGLITCHER
AND SLEW
ELIMINATOR
SHARP
LOW-PASS
FILTER
AUDIO
OUTPUT
SAMPLE RATE
PROGRAMMABLE
DIVIDER
o
STABLE CLOCK
OSCILLATOR
Fig. 12-1 High-quality audio DAC system
counter is used for the time base. Such a setup can be expected to exhibit a
jitter of less than 1 nsec if the oscillator is shielded from electrical noise.
Also, the higher crystal frequencies such as 10 MHz tend to reduce jitter
even further.
Because of the stability requirement, connecting the DAC directly to
an output port and then using the computer's interrupt system to time the
samples is unsuitable for anything other than initial experimentation. The
same applies to the direct memory access facility of most computers. Instead,
a first-in, first-out buffer is inserted between the computer and the DAC.
With the buffer, sample pulses from the time base cause an immediate (or
with constant delay) transfer of data to the DAC while the computer has
additional time to provide the next sample to the buffer. Often only one
stage of buffering, which makes it a simple register, is sufficient. In other
cases, FIFO IC chips containing 40 or so stages can bridge gaps in the data
flow up to nearly a millisecond at 50 ks/s. In fact, the computer itself will
tyically be acting as a large FIFO if the data is coming from a mass storage
device.
Other elements in the chain are the high-resolution DAC, low-pass
filter, and combination deglitcher/antislew device. Each of these will be
described in detail in the following sections.
Increasing Dynamic Range
The most difficult and hence most expensive requirement of a highfidelty
audio DAC is the high-resolution DAC module. Earlier it was shown
that the maximum signal-to-noise ratio that can be expected from an N bit
DAC is 6N dB. Note the importance of the word maximum. This refers to the
ideal condition in which the signal exactly fills the full-scale range of the