of Microprocessors

94 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Specialized Modifiers
Other specialized modules are based on the sound modification techniques
described in Chapter 2. Reverberation simulators, usually of the
spring type, are quite popular for enlarging the synthesized sound. Generally,
the only control present determines the mixture of straight and reverberated
sound. Ring modulators are also popular because of their low cost and
distinctive effect. Typically, there are just two signal inputs with level
control knobs and a single output. Frequency shifters are found rarely due to
their very high cost and specialized application.
One "module" (it is usually a free-standing unit) that has recently
become available is a digital implementation of the speed-changing tape
machine described earlier. In one of its modes, it has the ability to change the
frequency of sounds passing through it without altering the harmonic structure
or overall speed of the sound. When connected in a feedback mode, a
single note comes out as an ascending or descending series of notes.
A Typical Patch
Although the evolution of a relatively simple patch was described
earlier, let us look at how a more complex patch might be designed, given a
particular set of requirements. Throughout the discussion, the proportionality
and symmetry properties ofexponential voltage-controlled synthesis
will be emphasized. This will also be used as an opportunity to introduce the
technique of FM timbre synthesis, a simple yet powerful method of producing
quite a wide variety of timbres under the control of only two parameters.
The first step in designing a complex patch, as opposed to fiddling
around and discovering one, is to develop a physical understanding of the
desired effect. In this case, we are trying to develop a timbre (spectrum)
modification technique that can produce as wide a variety of effects as possible
under the control of a minimum number of parameters. In Chapter 1,
while describing frequency modulation for the production of vibrato, it
was mentioned that if the vibrato frequency became high enough and the
vibrato depth (percent modulation) became great enough, the unmodulated
sound would be completely altered into metallic clangs and breaking glass.
However, there are numerous intermediate conditions that produce useful
musical tones. Since there are only two parameters involved, the modulation
frequency and amplitude, and since the range of effects is so great, the
situation bears further investigation.
Frequency Modulation Terminology
Before continuing, some terms must be defined to avoid confusion.
Two signals are involved, the modulating signal; which was die vibrato
waveform, and the modulated signal, which is the tone being modified. The
following terms are really defined only if the modulating signal waveform