Lowpass-Feedback Comb Filter

Inspection of `comb.h` in the Freeverb source shows that
Freeverb's ``comb'' filter is more specifically a
*lowpass-feedback-comb filter*
(LBCF^{4.11}--§2.6.5). It is constructed
using a delay line whose output is lowpass-filtered and summed with
the delay-line's input. The particular lowpass used in Freeverb is a
*unity-gain one-pole lowpass* having the transfer function

When , the LBCF reduces to the feedback comb filter (FBCF) of §2.6.2 in which the feedback was not filtered. The overall LBCF transfer function is then

This structure was introduced for artificial reverberation by Schroeder [415] and Moorer [317].

In Freeverb's `comb` section (`comb.h` and
`comb.cpp`), the ``damping''
is set initially to

The lowpass scale-factor is called

Increasing the `roomsize` parameter (typically brought out to a
GUI slider) increases
and hence the reverberation time. Since
is required for dc stability, the `roomsize` must be less
than 1.0714, and so the GUI slider max is typically 1 (
).

The `feedback` variable
mainly determines reverberation
time at low-frequencies at which the feedback lowpass has negligible
effect. The feedback lowpass causes the reverberation time to
decrease with frequency, which is natural. At very high
frequencies--those for which the lowpass gain times
is much less
than 0.5--the reverberation time becomes dominated by the diffusion
allpass filters (which have a fixed feedback coefficient of
).
Thus, in Freeverb, the ``room size'' parameter can be interpreted as
setting the low-frequency T60 (time to decay 60 dB), while the
``damping'' parameter controls how rapidly T60 shortens as a function
of increasing frequency. A lower-limit on T60 is given by the four
diffusion allpass filters.

In terms of the physical interpretation of the filtered-feedback
comb-filter discussed in §2.6.5, Freeverb's
`roomsize` parameter can be interpreted as the square-root of the
low-frequency reflection-coefficient of each wall. That is, when a
planewave bounces back and forth between two walls, the attenuation
coefficient is `roomsize` after one round trip (two wall
reflections). Therefore, a better name in this interpretation would be
`liveness` or `reflectivity`. Since the round-trip delay
is given in samples by the delay-line length, changing the roomsize
requires changing the delay-line lengths in this interpretation.

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Center for Computer Research in Music and Acoustics (CCRMA), Stanford University