[sdiy] reverb in csound

Byron G. Jacquot thescum at surfree.com
Tue Jun 9 22:16:54 CEST 2020

Howdy Theo,

I've been in similar territory recently, reading up  and playing with reverb algorithms.  I'm also a big fan of physical/mechanical reverb - chambers, plates, springs, etc.

The first thing I'd suggest is to look over everything that's in Freeverb3 (as well as the original freeverb).  There are a bunch of building blocks (FIR/IIR filters, combs, allpasses, etc), and a number of reverb models built around them.  It's worth noting that the author considered the delay line modulation important enough to make two versions of a number of the algorithms, adding modulation to ones that didn't have it to start.

It's also a little messy - it's got a macro based proto-template thing so the algs can be recompiled for different data types.  There are also some puzzling constructs in there - the JC reverb is definitely not the structure Dr Chowning illustrated.

The second thing to note is that the tuning of the components is somewhat critical - if you build the freeverb type (a Schroder/Moorer with 8 parallel combs, then 4 series allpasses), it's not hard to play with the number of filters...but it's harder to configure them so they're well behaved - to avoid LF beating, dominant eigentones, or just ugly clattery waveguide chatter.  Supposedly the modulation helps alleviate all that.  I'm actually coding up my own modulated delay this afternoon...

Sean Costello has left a trail of breadcrumbs through the years - there's a couple of good AES segments on youtube, notes in his blog, and old notes at music DSP.

-Byron Jacquot

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