Tiptop Z2040 4-Pole VCF - Manual (Page 7)

Understanding the VC-FM:
Most VC-filters have a CV input to sweep/modulate the
cut-off frequency, in the Z2040’s case this input is
labeled FM (Frequency Modulation). The Z2040 adds a
unique feature providing the ability to modulate the
magnitude of the FM signal by another signal, this is
done by placing a VCA on the FM line. This simple yet
effective structure provides a variety of modulation
tricks. The most common use will be to modulate the
magnitude of an Envelope or an LFO with another
Envelope and/or LFO. On the other hand, modulating
the FM signal using Audio waveforms will add harmonic
content to the FM signal. (see fig. C)
VC-FM
FM
Volt
time
VC-FM
Volt
time
FM
Modulating a short envelope with a long envelope
Adding audio content to an envelope
audio
Fig. C