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TimewARP • User Guide
English
3.6.4.2
High-Pass Filters
Any device or mechanism that passes along faster motions better than slower ones is a high-pass filter.
Take the drinking straw from your water glass. Seal the end of it with your thumb and dip it back into the water.
Notice that you can pump it up and down in the glass, fast or slow, but the water never leaks into the straw as
long as you hold your thumb over the end. Think of the up and down motion of the water at the bottom end of
the straw as “the signal.”
Now start letting a little air leak into the straw as you move it up and down. The water level at the bottom end of
the straw no longer stays down when the straw “signal” goes down—it starts to come up again. And then when
you draw the straw back up, the water leaks back down more or less rapidly depending on the position of your
thumb at the top of the straw.
This is a high-pass filter. In the time domain, it constantly “leaks” its output signal level back to zero, at a rate
related to the cutoff frequency and slope. In the frequency domain, it passes all spectral components higher than
the cutoff frequency, and attenuates those below the cutoff frequency by an amount proportional to the cutoff
slope.
3.6.4.3
Cutoff Slope
This is the rate at which a filter attenuates spectral components, as a function of their frequency. It is usually a
multiple of 6dB/octave.
3.6.4.4
Feedback and Resonance
It is usually possible, with any signal-processing device or system of devices, to mix some of its output signal back
into the input signal. This may be intentional, or it can happen by accident; everybody who has ever worked with
a PA system has experienced the terrible screech of a system “in feedback.”
In filter modules for audio synthesis, it is common to provide controllable feedback. With just a little, the filter
response begins to peak around its cutoff frequency; as the feedback level increases, the peak gets stronger.
Eventually—just as happens with a PA system—the filter falls into oscillation. Regardless of the input signal, it
“screams” a sine-wave at its cutoff frequency. In this state it is no longer behaving as a filter at all; it has become
an oscillator.
Systems in feedback have been very well studied in physics. Their behavior can be described mathematically.
Whereas feedback in a PA system can be unpredictable and uncontrollable, feedback in a filter module for audio
synthesis can be—and is—a controllable and useful feature.
3.6.4.5
Lag Processors
A lag processor is a low-pass filter intended specifically for processing sub-audio signals. It introduces a “lag” in
the output signal wherever the input shows a sharp change in value. How much of a lag depends on the “cutoff
frequency” of the processor.
