of Microprocessors

480
MUSICAL ApPLICATIONS OF MICROPROCESSORS
OUTPUT
INPUT
AMPLITUDE
CONTROL
Fig. 13-33. Nonlinear waveshaping synthesis calcuations
Figure 13-33 diagrams the calculations needed for digital nonlinear
waveshaping synthesis. Two tables are required; one with a permanent sine
wave and the other with the desired nonlinear function. As with digital FM
synthesis, the sine wave table lookup must be very accurate to avoid
undesired audible distortion. This is because, near the peak of the input sine
wave, small changes in the sine value can produce large changes in the output
from the nonlinear function. This can be seen by looking at the left or right
edges of the curve in Fig. 13-31H in which the slope is very steep. Likewise,
the nonlinear curve table must be large for the same reason. Linear
interpolation into both tables is a must for high-quality synthesis.
Which Is Best?
In this chapter, five distinctly different methods of synthesizing sound
digitally have been covered. Also available are several less well-known and
generally less-useful techniques that have been described in research papers.
There is, of course, no "best" technique for all purposes. Direct waveform and
spectrum synthesis methods are the most general but are difficult to control
and require a lot of computing power to implement. The reduced parameter
methods are easy to control but can be difficult or impossible to adapt to
specific arbitrary requirements. In fact, it is usually advantageous to simply
play with the variables to see if a sound adequately close to the desired result
can be obtained and then either go from there or try a different technique.
Another advantage of reduced parameter methods is that computational
effort, at least theoretically, is reduced. However, the theoretical advantage
may not necessarily be true in practice due to increased accuracy requirements.
In the final analysis, if a synthesizer or program is to rely on just one
technique, it should be one with greater generality such as waveform table
scanning. A really comprehensive synthesis system would ideally provide for
all of those discussed in this chapter plus provision for easily adding others as
they are discovered.