DSP Functions
The DSP Functions
16-7
The use of lowpass, highpass, notch, and bandpass filters is often referred to as subtractive
synthesis, since the timbre of a sound is changed by removing certain partials.
Allpass filters, instead of cutting or boosting the partials of a sound, change the phase of the
partials as their frequencies pass through the center frequency.
Filter Terminology
Rolloff Filters do not usually cut all frequencies precisely at their cutoff point. Instead, the
amplitude of the frequencies above (or below, in case of a hi pass filter) the cutoff
decrease by a fixed amount per octave—for example, 6 dB per octave. This curve
of lessening amplitude is called a rolloff.
Poles The number of poles in a filter affect how sharp the rolloff is. The more poles there
are, the sharper the rolloff, meaning that the cutoff will have a more dramatic
effect on the sound. The K2661 has one-pole, two-pole, and four-pole filters
available. A one-pole filter has a 6 dB per octave cutoff; a two-pole is 12 dB per
octave; and a four-pole is 24 dB per octave.
Resonance In a filter that has resonance, the frequencies near the cutoff are given an increase
or decrease in amplitude. If you decrease these frequencies, you are essentially
creating a longer rolloff. But if you increase those frequencies thereby
emphasizing them, it creates a distinctive sound that you will very likely
recognize. Resonance is also sometimes called Emphasis or Q on various
synthesizers. Resonance on the K2661 is implemented in one of two ways. On
some filters, the resonance is fixed, adding or subtracting a specific amount of dB
to the affected frequencies (the ones near the cutoff). On other filters, you can
control the amount of resonance applied. In the case of these filters, there will
always be a separate control page for the resonance.
Separation Four of the filters in the K2661 (both Four-Pole filters, the Double Notch, and the
Twin Peaks) are actually two filters combined into one DSP function. For these
filters, you will find a control page called Separation. This allows you to shift the
cutoff frequency of the second filter, creating a separation in the cutoff frequencies
of the two filters. In the case of the Notch and Band Pass filters, this can be used to
create two separate notches or band passes. In the case of the four-pole filters, it
affects the shape of the roll off. For the four-pole filters, separation set to 0 creates
sharp rolloff of 24dB per octave.
How to Read the Graphs
The graphs show the rolloff curve, using several different values to show how they change the
shape of the curve. Amplitude is always on the vertical axis. Frequency is always on the
horizontal axis. You will notice on several graphs that the curve becomes more dramatic as the
cutoff frequency is set at a higher value. This is because the highest frequency the K2661 can
produce is 20Khz, so as the cutoff is set to higher values, there are fewer frequencies available
before it is past the range of the K2661.
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