of Microprocessors

DIGITAL HARDWARE 601
waveform changes during a note are not really possible because addressing the
RAM for writing thoroughly scrambles the read data going to the DAC. This
can be overcome somewhat by using a larger RAM, such as a 2K x 8-bit 6116
or 8K 6164, which can hold 8 and 32 waveforms, respectively, at once. All of
the waveforms can be written ahead of time and then instantly selected as
desired without glitching the output. Note that data input and output of the
6116 are the same pins, so an 8-bit tri-state buffer and an 8-bit latch will
have to be added to separate them.
Variable and Constant Sample Rate
The oscillator and ROM/RAM waveshaper just discussed is an example
ofvariable sample rate digital synthesis. Earlier, it was strongly suggested that
the sample rate should remain constant in a digital synthesis system. This in
fact is true if the system is handling several unrelated signals at once.
However, a dedicated oscillator is handling only one signal so the rule can be
relaxed somewhat with important advantages.
Using the Fig. 17-10 system as an example, we ignore for a moment
the fact that the count-up rate of the most significant 8 bits of the accumulator
divider is not exactly constant. Therefore, if the raw DAC output
is examined, it is found to consist of exactly 256 steps per cycle of the
waveform and each cycle is identical to the previous one. As the division ratio
and hence synthesized wave frequency is changed, the stepped wave is merely
stretched or compressed, but its step-by-step shape remains constant. The
spectrum of such a wave is exactly harmonic, including all of the alias copies
of the intended spectrum. Thus, the alias distortion is purely harmonic distortion
rather than intermodulation distortion and white noise. Furthermore,
and this is the crux of the matter, the quantization noise is also harmonic
distortion! This means that perfectly clean sounding tones can be produced
with 8 and even fewer bit DACs.
Since the reader is not really expected to believe the previous paragraph
immediately, let's discuss the meaning of harmonic distortion. In audio
equipment, the most prevalent measure of quality is harmonic distortion.
Literally, this means that any tone entering the equipment will leave with its
harmonic amplitude relationships altered. Even large amounts (several percent)
of such distortion are inaudible provided the distortion is pure, that is,
no other type of distortion is present, and that the amplitude alteration is
spread out evenly among the harmonics.
However, the mechanism that causes harmonic distortion in audio
equipment does not meet either criteria when several tones are present simultaneously.
First, intermodulation (IM) distortion is inevitable, which causes
easily heard nonharmonic frequencies to occur. In fact, an amplifier with pure
harmonic distortion would be quite an interesting device indeed. Second, the
harmonic portion of the distortion tends to concentrate at high frequencies,