Kurzweil K2500RS, Руководство музыканта

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DSP Functions
Filters
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Filters are widely used in synthesis to change the timbre of a sound by manipulating the
amplitude of speciÞc partials. When using Þlters, you always set a reference point (cutoff or
center frequency) that determines which partials the Þlters affect. HereÕs a quick summary of
the effects of the Þlter functions.
Lowpass Þlters 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 Þlters 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 Þlters, as the name implies, cut the levels of partials in a range between high and low
frequency. Consequently the ÒcutoffÓ frequency is referred to as the center frequency. With
notch Þlters, the levels of partials at the center frequency are cut, while the levels of partials
above and below the center frequency are unaffected.
Bandpass Þlters are the opposite of notch
Þlters; 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 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 octave - for 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 K2vx 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 K2vx 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 K2vx (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 K2vx 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 K2vx.