This is an allpass filter implementation taken from the Csound opcode alpass (note: spelling intentionally incorrect here.). In general, allpass filters are unique in that they have unit-magnitude frequency response for the whole spectrum. This particular allpass filter design is used in reverberation design to provide colorless density. For this reason, units for this ugen are in seconds rather than in hertz.

The arguments for allpass are as follows:


The example below shows a typical usecase for an allpass filter. Allpass filters are often used in series. In the example below, three sucessive instances of allpass are created. For an input signal, an impulse is being generated with a broadband noise signal with an exponential envelope. The resulting sound you hear is the "impulse response", which gives you the characteristics of the reverb.

 1 metro 0.001 0.001 0.001 tenvx 0.5 noise * 
 5 0.010 allpass 
 5 0.013 allpass 
 5 0.008 allpass