Owner`s manual

26
CHAPTER 3 -
The Craft of audio Synthesis
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.)
3.7.1
Linear and Exponential Sensitivity To Control Signals
In designing and constructing a voltage-controlled device, or a digitally-controlled
algorithm, we have to consider how we intend the controlling parameter to relate to the
controlled parameter.
Suppose, for example, we build an oscillator that changes its output frequency by exactly
1000Hz for each rise of 1 volt in electrical pressure at a designated point (a “control input”)
in the oscillator circuitry. This is a linear relationship between the control value and the
frequency. In order to march such an oscillator through a musical scale, we would have
to provide exponentially increasing voltage steps for each rising musical interval. That’s
pretty awkward, and it gets worse very rapidly. The pitch interval produced by, say, a 0.5-
volt control step will depend – completely – on the initial frequency of the oscillator.
Suppose we tune this imaginary linear-responding VCO to 500Hz, and apply a sequence
of 0.5V control steps. What frequency do we get at each step? What musical interval?
And so, instead of linear sensitivity, VCO’s for audio synthesis are designed, almost
universally, with
exponential
sensitivity to control signals. Such a design sets up an
exponential sensitivity to control signals. Such a design sets up an exponential
exponential relationship between control signal and frequency
in order to maintain a
simple linear relation between control signal values and audible pitch changes.
Similar arguments apply in the design of voltage-controlled  lters: it’s more practical
to work with a linear relationship between control-signal and “cutoff timbre”, and
therefore the relationship between control-signal and cutoff frequency really has to be
exponential.
0
+0.5V
+1.0V
+1.5V
+2.0V
+2.5V
500Hz
1KHz
1.5KHz
2.0KHz
2.5KHz
3.0KHz
unison
octave
Fifth
fourth
Major third
Minor third
INITIAL CV
FREQUENCY
INTERVAL