Re: [linux-audio-dev] cool plugin from Waves

On Friday 04 February 2005 19.52, Jens M Andreasen wrote:
> On fre, 2005-02-04 at 15:56 +0100, Christian Henz wrote:
> > On Sat, Jan 29, 2005 at 12:56:22AM +0100, David Olofson wrote:
> > > On Saturday 29 January 2005 00.00, Jan Depner wrote:
> > > > Now, if you could just do the same with outboard
reverbs... ;-)
> > >
> > > Well, using a recorded impulse response with some sort of
convolution
> > > algorithm works for real rooms, so why not virtual ones?
>
> I think this method will not catch the mild doppler effect arising
> from turbolence? (.. nor the *wild* turbolence, if house is on
> fire!)

I guess not, because that's kind of equivalent to the room changing
shape all the time, I think - and you can't capture that without a
constantly "morphing" impulse response. No idea how you would go
about recording something like that, though.

I *do* have an idea about what kind of problems that would pull in,
though: It would be like looping a sampled waveform, except each
sample is an impulse response... So, you need a nice, click free, non
repetitive sounding loop. :-)

> > I think this is already being done. I remember reading somewhere
that
> > there is a controversy if it is actually legal to 'sample'
outboard
> > reverb units (Lexicon etc.).
>
> Again, will this work at all?

I'm quite sure it can work really rather well.

> The (well-working) designs I have seen, all use time-modulated
> delays in the 'tank'. Every sampled impulse
> response would be slightly different from the previous.

This is a well known way to "cheat" to save cycles.

Most traditional reverbs use a small numbers of allpass filters/delay
lines, hooked up in various ways. Some or all of these form feedback
structures, to generate the reverb tail. With a low number of
filters, there are only so many possible feedback delay permutations,
so you'll almost certainly get some strong resonances. (Usually
*exactly* where you don't want them, of course... *heh*)

If tuned properly, this modulation "trick" reduces the metallic sound
caused by these resonances, so you can get away with an otherwise
insufficient number of filters and/or a suboptimal tuning. (*)

Now, convolution based reverbs don't need this (at least not unless
you want to simplate a room with moving air, that is :-), because
they don't have feedback loops, and thus, they don't have these
undesired and hard to tune resonaces in the first place. If the
impulse is nice, so is the reverb, basically.

        WYHIWYG - What You Hear Is What You Get. :-)

(*) If there is such a thing as an optimal tuning at all, that
    is. To me it seems pointless to define, since it depends
    entirely on what type of sounds you're going to put through
    the reverb, and what kind of results you want. Real rooms
    are not "perfect", so what's desired is not a "perfect"
    reverb, but rather a perfect simulation of an imperfect but
    nice sounding room.

//David Olofson - Programmer, Composer, Open Source Advocate

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Received on Sat Feb 5 00:15:14 2005