of Microprocessors

13
Digital Tone
Generation
Teehniques
Because of its total generality, there can be difficulty in knowing where to
start in designing a direct digital synthesis system. Most often, though, the
fundamental concepts of analog synthesis, which have been proven through
years of use, form the basis for a digital synthesis system. Thus, digital
equivalents of tone generators, sound modifiers, and control mechanisms are
incorporated into the system, hopefully with improved characteristics and
flexibility. In fact, some direct synthesis software systems simulate a complete
"voltage"-controlled synthesizer along with provisions for simulated
patch cords! In this chapter, the digital equivalents of analog tone generators
will be described followed later by other tone-generation techniques that are
practical only in the digital domain.
One of the many strengths of digital tone generation is that the frequency
and amplitude of the resulting waveforms are extremely accurate and
stable with time. The user need have no concern whatever about the unpredictable
results that frequency and amplitude errors can create. Also, the
exact phase between two or more digitally generated tones can be controlled
to an equal degree ofprecision. Of course, if slight relative errors are desired
for, say, an ensemble effect, they will actually have to be added in.
Although the previous chapter may have seemed to be preoccupied
with vanishingly low noise and distortion figures for the overall sound,
individual tones need not be of such high quality. For example, a slight
amount of harmonic distortion on a single tone is completely inaudible,
while even the effect of a larger amount is simply a slight change in timbre.
The equivalent amount of distortion in an ensemble of sounds would be very
objectionable because of the accompanying intermodulation distortion. Also,
super-signal-to-noise ratios for a single tone are not needed because subsequent
signal processing, which is what would change the amplitude, also
processes the noise, thus retaining whatever the tone's SiN ratio is independent
of signal level. These are important points because, as will be shown later,
extra effort and computation time are necessary for ultra-low-distortion tone
generation.
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