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6.2.3. Slope
A filter's
slope
or
rolloff
is a measure of how much audio is attenuated at a certain
frequency beyond the cutoff.
A filter doesn't just mute all audio past the cutoff; there are filters like that, but they're not
very musical. Instead, as you move past the cutoff, audio is steadily attenuated more and
more. This is measured in how many decibels quieter an audio signal is when you go one
octave past the cutoff, written
dB/octave
.
Here's an example: a Low Pass filter is set to a cutoff frequency of 1 kHz. We measure that
one octave above that (2 kHz), a given signal is 12 dB quieter than at the cutoff frequency,
and two octaves beyond it (4 kHz), it's 24 dB quieter. That means our filter has a slope of 12
dB/octave.
MiniFreak's Analog Filter has a fixed 12 dB/octave slope, but the digital Multimode Filter [p.35]
in Osc 2 has a wide variety of available slopes, including some that are quite unusual in
analog synths.
6.2.4. Resonance
Nearly every synth's filter circuit will resonate at the cutoff frequency, even if only an
inaudibly tiny bit. The steeper the slope, the greater the possibility for high amounts of
resonance, which can be very useful for a variety of tonalities. Resonance is sometimes
called
emphasis
or simply
Q
, an audio engineering term that describes the
quality factor
(width) of a peak.
As the Resonance control on the Analog Filter is turned up, a resonant peak around the cutoff
frequency will form. The higher the peak, the narrower it becomes; the sound is perceived
as becoming more aggressive and sharp. The tonality of the peak will change with the cutoff
frequency.
On many filters, if the resonance is turned up all the way, the filter will start to
self-oscillate
,
making a sound even with no input signal. The filter itself becomes a sound source, with
the peak width so narrower and louder that it's effectively one frequency: a sine wave. This
adds a hollow, whistling tone to whatever the oscillators are doing, or can be used on its
own.
Try this yourself: turn the Resonance all the way up and turn down the Volume for both Oscillators
to 0. You'll hear a whistle every time you press a key. However, it will be the same pitch for every note,
which isn't that useful... fortunately there's a solution to that.
Arturia - User Manual MiniFreak - The Analog Filter 42
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