Moog Music MF-107 FreqBox - Manual (Page 14)

14
FM
FM stands for frequency modulation. The term modulation refers
to any process in which one waveform is changed in response to the
contour of another waveform. Frequency modulation refers specifically
to modulating the frequency of an oscillator. With a VCO, modulation
is performed with control voltages. A very slowly, repetitively changing
CV can modulate VCO frequency up and down and sounds like vibrato.
If this signal is sped up to an audible rate (faster than 20Hz), the sounds
produced are complex.
When we deal with audio rate FM, we refer to the modulated
oscillator as the CARRIER, and the signal doing the modulation as the
MODULATOR. In the FreqBox, the input signal is the Modulator, and
the VCO is the Carrier. When the Modulator modulates the carrier’s
frequency, new harmonics are generated. They are called sidebands.
Sidebands exist above and below the frequency of the carrier at
intervals determined by the frequency of the modulator. Not only do
the frequency of the Carrier and Modulator combine to create complex
sounds, the amplitude of the Carrier and Modulator also determine how
many sidebands are audible. In a system like the FreqBox, where the
amplitude of the Carrier is constant, the number of audible sidebands are
determined by the amplitude of the Modulator. This means the timbre
of the FreqBox will change with the dynamics of the input signal when
using FM.
When the Carrier and Modulator are harmonically related, as in
octaves or simple ratios, then the sidebands tend to be related, too.
When the Carrier and Modulator are not simply related the results are
quite clangorous, as the siebands aren’t related harmonically to the input
signal. In digital synths, FM can be used for some very predictable
timbres, because these complex relationships can be digitally controlled.
This is not necessarily so in the world of analog, where analog oscillators
are free running and often subject to minor instabilities in tuning.
If this seems complicated, simply remember these points:
- You’ll hear the strongest FM effects when the Carrier is higher in
frequency than the Modulator
- The mellower the Modulator and Carrier sound on their own, the
simpler the FM results will be. Brighter sounding Modulators and
Carriers make for dense and complex FM results.
- The loudness of the modulator affects the strength of the FM results.
- Frequency modulating an analog VCO with an audio signal can