42
When the inputs are AC coupled (Audio position of the switch), any DC component present in them is canceled
before they are fed to the modulator. Thus a sawtooth that starts from zero and goes to +l0vV will instead start at
-5vV and move to +5vV so that its overall positive and negative deviation cancels to zero. Under these conditions
the modulator will generate from any two periodic signals an output signal consisting of the sum and difference
frequencies that can be generated from the frequencies of the two inputs. The input frequencies themselves will
be suppressed.
If both signals are audio-frequency, a large variety of harmonic and inharmonic timbres can be produced from the
modulator, depending on the ratio of the input frequencies and on their own harmonic content. If A is a sine wave
and we represent its frequency by Fa, and B is a complex waveform of frequency Fb with overtones 2Fb, 3Fb, 4Fb,
etc., then the output of the modulator will be a complex waveform with frequency components Fb + Fa, Fb -Fa,
2Fb±Fa, 3Fb ± Fa, 4Fb ± Fa, etc. A moment’s experimentation with the pre-wired sawtooth and sine inputs to the
modulator will demonstrate the complexity of the timbres that can be generated by this simple means.
If, still with AC coupling, one input is subsonic and the other at some audio frequency, there will be an output from
the modulator only when the value of the subsonic input is changing, and the output will be roughly proportional
to the rate of change. If, for example, the subsonic input is a square-wave, the modulator output will be a series
of short, decaying tonebursts—one at each rise or fall in the input signal.
When the inputs are DC coupled, any DC component in either one of the inputs will pass into the modulator and
affect the modulating process. The effect when both inputs are at audio frequency is to allow into the output
waveform some of the input frequencies in addition to the sum and difference frequencies. The effect when one
of the inputs is subsonic is that the modulator operates as a voltage-controlled amplifier: the output amplitude
will be in direct proportion to the instantaneous amplitude of the low-frequency input and will vary as its absolute
value varies. Also, the output phase will reverse when the low-frequency input signal changes from positive to
negative or vice versa.
The AC-coupling time constants are 235 msec, for the left input and 90 msec, for the right input.
4.10
Noise Generator (NG)
The Noise Generator has two manual controls: one for spectral balance (“color”) and one for output level.
The spectral balance is continuously variable from white to red (low-frequency noise output). In the latter case the
output falls off at the rate of 6dB/Octave; the pink noise position approximates a -3dB/Octave slope.
The level control, at minimum, cuts off the output signal completely. At maximum, the output is clipped at 20vV P-P
to produce binary, or two-valued, noise. Clipping begins with the level control approximately half-open input.
