Early Schroeder reverberators consisted of the
- A series connection of several
- A parallel bank of feedback comb filters.
- A mixing matrix.
The all-pass filters provide colourless high-density
echoes in the late impulse response of the
reverberator. These all-pass filters may also be
referred to as diffusers. While all-pass filters are
colourless in theory, perceptually, their impulse
responses are only colourless when they are
extremely short (less than 10 ms or so). Longer
all-pass impulse responses sound similar to feedback
comb-filters. For steady-state tones, however, such
as sinusoids, the all-pass property gives the same
gain at every frequency, unlike comb filters.
The parallel comb-filter bank is intended to give a
psychoacoustically appropriate fluctuation in the
reverberator frequency response. A feedback comb
filter can simulate a pair of parallel walls, so one
could choose the delay-line length in each comb
filter to be the number of samples it takes for a
plane wave to propagate from one wall to the
opposite wall and back.
Thus, one may choose the comb-filter delay-line
lengths more or less arbitrarily, and then use
enough of them in parallel (with mutually prime
delay-line lengths) to achieve a perceptually
adequate fluctuation density in the
frequency-response magnitude. In four such delays
are chosen between 30 and 45 ms, and the
corresponding feedback coefficients are set to give
the desired overall decay time.