Digital Signal Processing/Tunable Filters
We have high-pass and lowpass filters, and it turns out that we can create both filter effects using a single piece of hardware called a tunable filter. Tunable filters rely on a device called an allpass filter to create other types of filter outputs.
A common, although unintuitive, tool in the DSP toolbox is the all-pass filter. As the name implies, the all-pass filter passes all frequency components without attenuation.
The question comes up, if an allpass filter doesn't actually filter anything, why do we bother to study it? The answer is that even though the allpass filter doesn't affect the magnitude response of the system, it will affect the phase response of the system.
Allpass filters have the property that cascading an allpass filter with another IIR filter will produce a linear output for the whole system.