Doepfer A-111-1 High End VCO - Manual (Page 6)

A-111 VCO 2 System A - 100 doepfer
6
Completely different sounds will emerge, though, if the
modulation frequency is in the audio range.
Exponential FM in the audio range
For exponential FM, patch the modulation voltages
into CV input ! or " (see Fig. 7).
Fig. 7: Frequency modulation in the audio range
Thanks to the rapid changes in the modulated VCO’s
pitch, side bands are created: as well as the two
original frequencies, you also get the frequencies crea-
ted by their sum and difference (for instance a modu-
lation frequency of 100 Hz and a carrier frequency of
500 Hz produce side bands at 400 Hz and 600 Hz).
When you try this out for the first time, start off with
sine waves, and slowly raise the modulation frequency
from the sub-audio into the audio range.
If you use waveforms other than sine waves in FM in
the audio range, the sounds that result will be extre-
mely complex and difficult to predict. A sawtooth, for
instance, is like a vast number of sine waves of
different frequencies - all of which will be represented
in the modulated output, so that the final sound will be
a complex mix of the buzzes, noises and tones produ-
ced by all the various sum and difference outputs.
With exponential FM, changes in control voltage pro-
duce proportional changes in the pitch relationship
of the component sounds. This can have unwanted
side-effects. If a 440 Hz VCO sine wave is modulated
by a 2 V
SS
amplitude sine wave ( see Fig. 8), the top
and bottom side-bands are respectively up and down
one octave, at 880 Hz and 220 Hz. You might think
that would be fine - but with modulation in the audio
range, we hear the note half-way between these fre-
quencies - 550Hz - and this is (not surprisingly) out of
tune with the original 440 Hz carrier note.
CV 1 Range
A-110
VCO
Tune CV 2
CV 2
Range
A-110
VCO
Tune