Doepfer A-100(~ 40 MB) - Manual (Page 161)

doepfer System A - 100 VCO 2 A-111
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Fig. 8: Exponential FM in the audio range
Whenever you change pitch using exponential FM, the
inevitable side effect of the change will be an unplan-
ned and usually un-musical change in the relative pitch
of the components of the sound.
If the side-effects of exponential FM aren’t wanted,
then you need to use the linear FM input on the A-111.
Linear FM in the audio range
Linear FM is now one of the standard building blocks
of synthesis. Especially after the introduction and in-
stant success of the Yamaha DX 7, in the early 80s,
linear FM was hugely popular throughout the world,
and is partly what people are referring to when they
talk about ‘digital’ sounds. After being superceded in
popularity by ‘sample & synthesis’ technology in the
late 80s - and analog or analog-like instruments in the
90s - it is now appreciated again as a very useful
source of timbres.
With linear FM, changes in control voltage produce
proportional changes in pitch, not in octaves. It’s
a Hz/V rather than V/octave response.
This time, if you modulate a 440 Hz sine wave with a
220 Hz sine wave, the side-bands created will be at
220Hz and 660 Hz, and so the pitch at which we hear
the modulated signal (halfway between 220 Hz and
660 Hz) will be 440 Hz - and thus in perfect tune with
the original carrier frequency.
The relationship between the carrier frequency f
C
and
modulator frequency f
M
is crucial to the timbre.
With identical frequencies for carrier and modulator,
you end up with a timbre which is like a sawtooth put
through a low pass filter (see Fig. 9 on page 8).
With a modulator frequency double the carrier fre-
quency, you end up with something very like a pulse
wave (see Fig. 10 on page 8).
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