Carrier/modulator model

Earlier we saw how to
use ring modulation
to modify the spectrum of a periodic signal, placing spectral peaks in
specified locations (see Figure
5.4, Page ). To do
so we need to be able to generate periodic signals whose spectra have maxima
at DC and fall off monotonically with increasing frequency.
If we can make a signal with a formant at frequency zero--and no other
formants besides that one--we can use ring modulation to displace the formant
to any desired harmonic. If we use waveshaping to generate the initial formant,
the ring modulation product will be of the form

where (the

Much earlier in Section 2.4 we introduced the
technique of
*timbre stretching*,
as part of the discussion of wavetable synthesis. This technique, which is
capable of generating complex, variable timbres, can be fit into the
same framework. The enveloped wavetable output for one cycle is:

where , the phase, satisfies . Here is a function stored in a wavetable, is a windowing function, and and are the wavetable stretching and a modulation index for the windowing function. Figure 6.3 shows how to realize this in block diagram form. Comparing this to Figure 2.7, we see that the only significant new feature is the addition of the index .

In this setup, as in the previous one, the first term specifies the placement of energy in the spectrum--in this case, with the parameter acting to stretch out the wavetable spectrum. This is the role that was previously carried out by the choice of ring modulation carrier frequency .

Both of these (ring modulated waveshaping and stretched wavetable synthesis)
can be considered as particular cases of a more general approach
which is to compute functions of the form,

where is a periodic function describing the carrier signal, and is a periodic modulator function which depends on an index . The modulation functions we're interested in will usually take the form of pulse trains, and the index will control the width of the pulse; higher values of will give narrower pulses. In the wavetable case, the modulation function must reach zero at phase wraparound points to suppress any discontinuities in the carrier function when the phase wraps around. The carrier signal will give rise to a single spectral peak (a formant) in the ring modulated waveshaping case; for wavetables, it may have a more complicated spectrum.

In the next section we will further develop the two forms of modulating signal we've introduced here, and in the following one we'll look more closely at the carrier signal.