J. MODULATION OF THE OSCILLATOR BANK
The 921A-921B oscillator bank contains three sets of frequency control inputs. The set on the
921A is labelled FREQUENCY. Control signal changes fed to these inputs affect both
921B’s equally. Furthermore, a given control signal change at the 921A FREQUENCY con-
trol input will result in corresponding frequency ratio change (or musical interval change) at
the outputs of the 921B’s. For instance, a one volt increase at a 921A FREQUENCY control
input will double the 921B’s frequencies (increase them by one octave), regardless of how
the 921A’s and 921B’s are being used. This type of frequency control is called exponential.
The other two sets of frequency control inputs are on the 921B’s and are labelled AC and DC
MODULATE. A control signal change fed to one of these in- puts will affect only that of the
connected 921B. Furthermore, a given control signal change at a 921B MODULATE input
will result in a corresponding frequency difference at the 921B output. For instance, if the
921B produces a 1000 Hz tone in the absence of a modulating signal, then a +1 volt signal
will increase the frequency to 2000 Hz (1000 Hz difference), while a -1 volt signal will de-
crease the frequency to zero (also 1000 Hz difference). This type of frequency control is
called linear.
The DC MODULATE input is direct-coupled. That is, it will accept slowly varying wave-
forms without waveform distortion. It is useful for slow or static control signals. The AC
MODULATE input is capacitor coupled. This is, it will not pass slowly varying waveforms
without some distortion or attenuation. It is useful for control signals whose frequency com-
ponents lie within the audio range.
995
904A
902
902
921A
921B
923
921
911
911
921B
MIXER
REV
CONTROLLERS
TRUNK
907A
0
A
B
4
8’
8’
0
0
50
1. Arrange the patchcords and controls as shown above. Run a patchcord from the ATTENU-
ATOR output to a FREQUENCY control input on the 921A.
2. Play the keyboard. Notice that both 921B pitches go up and down while the musical inter-
val between them remains constant.
