8 Octave Fixed Filter Bank
In addition to various types of VCF, there is another class of filter that you will occasionally find on powerful synthesisers. These are the fixed filter banks, and they differ from low-pass, high-pass, band-pass or band-reject filters in a number of significant ways. Perhaps the most obvious of these is that, whereas the action of a VCF is determined by a single cut-off frequency and a single resonance value, the fixed filter bank divides the signal into a number of bands and acts upon each of these individually. The more bands that you have, the more control you can exert over the sound.
Basic fixed filter banks often reside within cars or cheap home stereos, where they are called 'graphic equalisers'. On a more professional level, you will find 31 or more fixed filters in a studio quality graphic equaliser. The most flexible of these divide the frequency spectrum into as many as 512 separate bands, and allow you to boost or cut the amplitude in each of these individually. There is, of course, overlap between these bands (because the techniques required to fully separate them without unpleasant artefacts are not practical) but they can be used to sculpt sounds very precisely.
The fixed filter banks in modular synthesisers are less precise than those found in dedicated graphic equalisers, but they fulfill a similar purpose: they allow you to accentuate or reduce the prominence of bands of frequencies, and they are capable of shaping sounds in ways that are impossible using conventional filters. Furthermore, and in common with all the other signal processing tools within modular synths, they can also be used to modify CVs.
The eight knobs on the front panel are attenuators that allow you to reduce the amplitude of the signal in each band after it is boosted. At their fully anticlockwise extreme (MIN), these eliminate the signals in their respective bands. If a knob is in its fully clockwise position (MAX) the full +2.5dB boost is output.