# [PD] Help with reverb design and vanilla's abstractions ([rev1~]/[rev2~]/[rev3~])

Dario Sanfilippo sanfilippo.dario at gmail.com
Thu Feb 7 12:23:22 CET 2019

```Hi, Alexandre.

On Thu, 7 Feb 2019 at 02:26, Alexandre Torres Porres <porres at gmail.com>
wrote:

> Hi, I've read miller's book and I've always wanted to know a bit more
> about reverbs. I also want to better explain these examples to my students.
> One thing I wonder is about the choice of delay lengths for the early
> reflections and also for the recirculating delay lines. They seem quite
> arbitrary, right?
>
> All I could make of it is that these numbers are just broken down so they
> don't become multiples of each other, resulting in a rather complex mesh of
> reflection times.
>

Yes, as far as I understand, the idea is to have an FDN, with whatever
matrix you've chosen, whose impulse response is as close as possible to
white noise. The filters that you put inside the network can then be used
to model different environments.

>
> But also, it seems that a "room size" parameter can be controlled by the
> size of the delay lines, right?
>

There is an algorithm on the website of J. O. Smith to calculate the best
approximation for a desired size while keeping the coprime relationship
between the lengths:
https://ccrma.stanford.edu/~jos/pasp/Prime_Power_Delay_Line_Lengths.html.

My reverb in PD is a 16th order FDN with Hadamard matrix and 1pole lowpass
filters to model the absorption of high frequencies when bouncing on the
walls. For simplicity, I pilot the delay lengths through a signal to change
the exponent of 16 prime bases. Computationally acceptable and I like how
it sounds.

> also some cool references for study (such as "digital reverb modelling for
> dummies").
>

Rocchesso, D. 1997. “Maximally diffusive yet efficient feedback delay
networks for artificial reverberation. ” In Signal Processing Letters, IEEE 4
(9), 252­255.

Rocchesso, D., and J. O. Smith. 1997. “Circulant and Elliptic Feedback
Delay Networks for Artificial Reverberation.” I EEE Transactions on Speech
and Audio Processing 5 (1):51–63.

Ciao,
Dario

> cheers
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