The mixed oscillators a classical subtractive synthesizer pass a
filter. The cut-off frequency can be controlled by a controll voltage.
Another parameter of the filter is the resonance (sometimes called
Q-factor). The resonance enhances the frequencies near the cut-off
frequency. There are different types of filters:
Low-pass filter (LPF)
As shown in the Figure below, the frequencies above the cut-off frequency are damped.
If the resonance parameter is not zero, then the frequencies near the cut-off frequence are amplified (indicated by the peak in the Figure below) and the frequencies above the cut-off frequencies are damped.
See the low-pass filter induced changes to single VCO waveforms on an oscilloscope:
low-pass-filter + nanoloop:
High-pass filter (HPF)
Frequencies below the cut-off frequency are made more quiet
Band-pass filter (BPF)
A frequency band passes the filter.
Notch filter (NF)
A band of frequencies is cut out of the spectrum.
Low- and highpass filters can be characterized by the edge steepness.
Typical values are 12 dB/octave or 24 dB/octave. The edge steepness
tells how strong frequencies above the cut-off frequency (in case of
the LPF) are damped. The resonance encances the frequencies near the
cut-off frequency, this means that in the case of a LPF overtones will
be pronounced and in case of a HPF deeper frequencies than the keynote
are tuned up.
spoken a filter has a characteristic curve in the frequency space which
depends on the parameteres cut-off, resonance, edge steepness and
filter-type (LPF, HPF, BPF, NF or in principal any other function curve). The
frequency spectrum of the oscillator mix (obtained by
fourier-transformation of that signal) has to be multiplied with the
caracteristic curve of the filter. After fourier-transformation back
into time-space one obtains the filtered signal.KrassesZeug