Kurzweil K2500 - PERFORMANCE GUIDE REV F PART NUMBER 910251 CHAP 14 Function Manual page 6

DSP Functions
Filters
Filters are widely used in synthesis to change the timbre of a sound by manipulating the
amplitude of specific partials. When using filters, you always set a reference point (cutoff or
center frequency) that determines which partials the filters affect. Here's a quick summary of
the effects of the filter functions.
Lowpass filters cut the levels of all partials above the cutoff frequency without affecting the
partials at or below the cutoff frequency (the low frequencies pass through). Highpass filters do
the opposite; they cut the levels of all partials below the cutoff frequency without affecting the
partials at or above the cutoff frequency.
Notch filters, as the name implies, cut the levels of partials in a range between high and low
frequency. Consequently the "cutoff" frequency is referred to as the center frequency. With
notch filters, the levels of partials at the center frequency are cut, while the levels of partials
above and below the center frequency are unaffected. Bandpass filters are the opposite of notch
filters; they leave the levels of partials at the center frequency unchanged, and cut the levels of
partials above and below the center frequency.
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 K2500 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 K2500 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 K2500 (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 K2500 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 K2500.
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