Just to make sure, I'm pretending to know it all about reverbs ... ;-)
But, yes, I think one of the basic goals of designing a good reverb is
to get an pretty flat frequency response, so that all frequencies have
more or less the same reverberation. That is not necessarily what you
want in the end (maybe you want the higher frequencies to deacy
faster), but if you have a controlled flat response to begin with it
is much easier to model the decay any way you want. Improper use of
e.g. comb filters can lead to ringing at certain frequencies, and this
tend to sound "metallic".

Now to Doug's question about phase shift in allpass filters, the way I
understand it, you can use the feedback feature of an allpass filter
to create reverberation, and the (somewhat) flat frequency response
helps avoid ringing. This is not enough to create a nice reverb in
itself, as you will get ringing sounds with allpasses too if you turn
up the feedback high. So you combine several alpasses, each with
slightly different phase shifts in different frequencies. The combined
"messing up" of the phases in the different alpass filters helps even
out the ringing frequencies due to cancellations and overlaps. Also,
each allpass will contribute only a little bit to the desired decay
time, putting them in series lets them reverberate into each other,
and this "reverb on reverb" (with slightly different settings) method
adds up to a longer reverb tail.
This is a fairly big topic, and it will take some time to dig into the
details. Reading Julius Smith's online book(s) will help you get
started understanding of the basic concepts. Then, practical
experimentation will probably take you somewhat further.


best
Oeyvind

2008/12/1 Ludo Smissaert <ludo@ludikidee.com>:
> Oeyvind Brandtsegg wrote:
>>
>> The allpasses are IIR, that is, they use feedback, this way you can
>> "lengthen" the sound without attenuating any frequencies, (trying a
>> simple explanation on something a little bit more complex). The comb
>> filters in freeverb are feedforward afaik, I'm not sure why feedback
>> is not used there as well, except maybe the metallic "comb" sound may
>> get too present in the output with significant feedback. Good luck, if
>> you go for this reverb architecture (well, any architecture really) ,
>> good results lay in fine tuning the different filters in relation to
>> each other. It is very easy to get a metallic ringing reverb even with
>> a good architecture like this one...
>
> ... dropping in to this discussion. Are you saying the art of designing a
> good reverb is to get it beyond 'metallic ringing'. I 've always found the
> classic Csound reverbs a bit metallic and I thought designing your own
> filter means to get it beyond 'metallic'.
>
> Furter, I recall a collection of very steep ("brick wall, 4 pole allpass" ?)
> filters on Csound.com, in the early days of version 3.58. Never tried it
> out.
>
> Ludo
>
>
>
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