Controlling One Module By Means Of Another - M-Audio Wayoutware TimewARP 2600 User Manual

So, just for example, if you modulate a 10-component carrier signal with a 10-component program signal, the
signal resulting from the modulation will have not less than 100 spectral components. This can get very messy;
the most useful thing you can do with such a signal, before you do anything else, is filter it to get rid of some of
the fuzz.
3.6.5.2 Amplitude Modulation
Suppose we modulate the amplitude of a 1000Hz sine wave with a 5Hz sine wave. The result is indistinguishable
from what we would get if we mixed three sine waves, at 995Hz, 1kHz, and 1005Hz. They are the same signal.
The 995Hz component of the output is the lower sideband resulting from the modulation, and the 1005Hz
component is the upper sideband.
3.6.5.2.1 Ring Modulation
While a VCA responds only to a positive-going signal at its amplitude-control input, a ring modulator responds
to both positive and negative levels at both of its inputs. Its output is simply the product, arithmetically, of the two
inputs. If you are new to audio synthesis, draw a couple of signal graphs—it doesn't matter what they are—on the
same timebase and vertical scale, and use a pocket calculator to work out the result of multiplying the two signals
together. That's what a ring modulator does. (The expression "ring modulator" describes the appearance of the
analogue circuit design that's required for the multiplication.)
In the frequency domain, the difference between this and ordinary AM is only that the carrier signal components
are suppressed. Once again, if you work out the arithmetic, a single-frequency carrier, modulated by a single-
component program, generates a three-component AM spectrum but only a two-component ring-modulation
spectrum.
What's useful about this? Well, since the carrier is almost always periodic (it comes from an oscillator, right?), it
has a harmonic spectrum. Suppressing this spectrum lets you hear just the sidebands, which can be completely
inharmonic if you're careful about the ratio of the two input signal frequencies.
3.6.5.2.2 Frequency Shifting
It's theoretically possible not only to suppress the original carrier, as in ring modulation, but to isolate the lower
and upper sidebands and make them available separately. The arithmetic here is fascinating, because the end
result (for once) is in one-to-one correspondence with the input: for each component of the program signal, there
is a component in the output at C-p (or at C+p). In other words, the final spectrum has only as many components
as the original program did. This is called frequency shifting. Picture the entire program signal spectrum shifted
up or down by some fixed frequency.
The important thing to remember about this is that it's not pitch shifting—which would have to be accomplished
by frequency multiplication—but frequency shifting. It's an addition or subtraction process, and it really messes
up any harmonic relationships that might have existed in the original spectrum.
3.6.5.3 Frequency Modulation
The spectrum resulting from amplitude modulation always has three components for every one component of
the program signal: the carrier itself, and two sidebands. In Frequency Modulation, however, the number of
sidebands depends on the modulation depth. It is possible from only two sine waves to generate a spectrum
with dozens or even hundreds of components. Modulating one sawtooth with another can produce a spectrum
so complex that it sounds almost like a noise generator. In such a patch, you will usually reach for a filter to take
the edge off the resulting spectrum.
What happens is this: as the depth of modulation increases, the number of sidebands does too, without limit. The
additional sidebands come in at—guess what?—integral multiples of the program frequency.
For this reason, the most useful FM techniques involve only sine-wave carrier and program signals.
3.7

Controlling One Module by Means of Another

The modulation methods we've just described can be accomplished with voltage-controlled or digitally-controlled
equipment. For example, to set up an AM effect, feed the carrier signal into a VCA audio input, and the program
signal into one of the amplitude-control inputs.
Likewise, to set up an FM method, route the program signal into one of the frequency-control inputs of a VCO. (See
section 4.3 for news of some TimewARP 2600 extensions relating to this.)
27
TimewARP • User Guide