DSP Functions
The DSP Functions
16-7
The use of lowpass, highpass, notch, and bandpass Þlters is often referred to as subtractive
synthesis, since the timbre of a sound is changed by removing certain partials.
Allpass Þlters, 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 Þlter) the cutoff
decrease by a Þxed amount per octaveÑfor example, 6 dB per octave. This curve
of lessening amplitude is called a rolloff.
Poles The number of poles in a Þlter 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 K2600 has one-pole, two-pole, and four-pole Þlters
available. A one-pole Þlter 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 Þlter 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 K2600 is implemented in one of two ways. On
some Þlters, the resonance is Þxed, adding or subtracting a speciÞc amount of dB
to the affected frequencies (the ones near the cutoff). On other Þlters, you can
control the amount of resonance applied. In the case of these Þlters, there will
always be a separate control page for the resonance.
Separation Four of the Þlters in the K2600 (both Four-Pole Þlters, the Double Notch, and the
Twin Peaks) are actually two Þlters combined into one DSP function. For these
Þlters, you will Þnd a control page called Separation. This allows you to shift the
cutoff frequency of the second Þlter, creating a separation in the cutoff frequencies
of the two Þlters. In the case of the Notch and Band Pass Þlters, this can be used to
create two separate notches or band passes. In the case of the four-pole Þlters, it
affects the shape of the roll off. For the four-pole Þlters, 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 K2600 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 K2600.
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