Operation Manual

6.16 Comb 82
6.16 Comb
The Comb filter enhances frequency components located at
harmonic intervals. The frequency response of the filter is com-
posed, as shown in Figure 36, of resonances around frequency com-
ponents located at multiples of a fundamental frequency (hence its
name). The effect of the filter is to color the sound with a change
of apparent pitch. This module has one output, the filtered signal,
and three inputs. The first input is the signal to be filtered. The sec-
ond and third inputs are modulation signals used to vary the tuning
(resonance frequency) of the filter.
The algorithm implemented in this module sends the input signal into a variable delay line. The
output of this delay is then re-injected in this delay line with a gain factor.
depth = 0.99
depth = 0.5
depth = 0
frequency
gain
3f2ff
Figure 35: Frequency response of a Comb filter
Tuning
The coarse and fine knobs and the range switch are used to tune the resonance frequency of the
filter. The variations in resonance frequency caused by changes in the modulation signals are
relative to this level. The length of the filter delay line is calculated as the inverse of the resonance
frequency (a period). When the two knobs are in their center position (green LEDs on for the
coarse knob), the range switch is set to 8 and there is no modulation signal, the filter resonance
frequency has a value of 261.6 Hz, which corresponds to the C3 key on a piano (middle C). The
range switch transposes the resonance frequency one or two octaves up or down. The reading on
the counter indicates the resonance frequency, in Hertz, of the filter. The length of the delay line
can be modulated by using the modulation inputs of the module. The amount of variation of the
resonance frequency obtained with the modulation inputs depends on the adjustment of the mod1
and mod2 gain knobs. The total modulation signal is the sum of the two inputs each multiplied
by the gain corresponding to its respective mod knob. When the knobs are in the center position