### 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

`comb`section (

`comb.h`and

`comb.cpp`), the ``damping'' is set initially to

`feedback`in the source, and it is set initially to

`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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Freeverb Allpass Approximation

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Freeverb Main Loop