of Microprocessors

226 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Duty-Cycle Modulation
Probably the simplest and inherently most accurate conversion method
is based on pulse-width or duty-cycle modulation. The fundamental concept
is to generate a rectangular waveform with very precisely controlled voltage
levels and transition points. This waveform is then sent through a low-pass
filter, which effectively takes the long-term average voltage level of the wave
and outputs it as a dc voltage level. Actually, the beauty of the technique is
that the voltage levels are fixed so the on-off times are the only variable.
Thus, the scheme is really one of digital-to-time conversion followed by
time-to-analog conversion.
Figure 7-3 illustrates the operation of a duty-cycle DAC. At integral
multiples of T, the analog switch is flipped upward, which connects the
low-pass filter to the reference voltage source. A variable time (less than T)
later, the switch is flipped back down, which sends zero to the filter. In the
example, the filter sees Vre/ for 25% of the time and zero for the remaining
75%. The time average then is 0.25Vref, which is output by the filter and
load isolating unity gain buffer. This method (and most of the others that
will be studied) gives a multiplying DAC because the output voltage is proportional
to the product of a reference voltage and the digital input expressed
as a binary fraction.
A simple digital counter can be used to control the analog switch in
response to a digital input. At the beginning of a T interval, the counter
would be preset to the digital value to be converted. A stable, highfrequency
clock causes the counter to count down toward zero. As long as the
counter is nonzero, the switch would be up, but as soon as it reaches zero, the
switch would be thrown down until the next cycle.
For high conversion speed, the frequency of the clock should be as high
as possible but not so high that analog switching time becomes a major
portion of a clock cycle. A realistic figure might be 10 MHz. The frequency
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