
Natural Sounding Artificial Reverberation (1962) [pdf] - sctb
http://charlesames.net/pdf/MRSchroeder/artificial-reverb.pdf
======
dspig
While Schroeder's algorithm sounds like reverberation, these days it might not
be confused for 'natural sounding'. Well, to be fair there is worse sounding
reverberation in real life (garages).

A better sounding algorithm is to convolve the signal with decaying random
noise - impractical in Schroeder's day - but that's sort of what his and later
algorithms are approximating.

Edit: Improvements on Schroeder's algorithm include feedback delay networks
(where the comb filters don't just feed back to their own input, but to the
others too, and/or nesting allpass filters inside each other. Both of these
increase the density of echoes faster.

~~~
bch
Does that mean convolve (e.g.) white noise w some decay model (or apply adsr
envelope?), and then again convolve your subject w that resultant decayed
random noise?

Do my definitions for “random” and “decay” even fit your thought?

~~~
dspig
Yes, take a few seconds of white noise, apply an exponentially decaying
envelope, and convolve with the input signal.

Of course there is a lot more you can do: high frequencies decay faster in a
real room, the envelope could be a more complex shape, or just skip to near-
perfect and convolve with an impulse response measured from a real room.

~~~
tzahola
Why does white noise work? Is it because the spectral density of both the
Dirac impulse and white noise are flat, and our ears don't care about phase?

~~~
dspig
It doesn't have to be white - pink noise works the same just sounds more
muffled/bassy. It's more that there is no distinct pattern in the time domain
for the ear to pick out, which would add echoes or metallic ringing. The same
as a boxy room or shower cubicle will ring/resonate at certain frequencies,
but a bigger more complex shaped room will have an effectively random pattern
of reflections for a given source and listener position.

------
EEGuy
Not an expert in the field, but it's possible to convolve a signal on the
recorded impulse response of a room [1] or a 'Slinky' toy [2], or that of a
subjectively 'desirable' piece of audio equipment [3].

The result is as if your signal were played in that room ( with all its reverb
) or through that thing ( with its 'desirable'-or-otherwise characteristics
modeled in its impulse response ).

Thinkable in 1962, but not thought possible. Now it's part of an industry.

\-----

[1]
[https://en.wikipedia.org/wiki/Convolution_reverb](https://en.wikipedia.org/wiki/Convolution_reverb)

[2]
[http://www.openairlib.net/auralizationdb?page=1](http://www.openairlib.net/auralizationdb?page=1)

[3] [https://www.soundonsound.com/techniques/convolution-
processi...](https://www.soundonsound.com/techniques/convolution-processing-
impulse-responses)

~~~
acjohnson55
The downside of convolutional reverb is the lack of parameterization. You're
kind of stuck with one fixed geometry of source, receiver and surfaces. It can
also be expensive to apply in real-time processing.

~~~
jerrre
A lot can be done post-processing the impulse response (volume envelopes,
timestretching, combining with other parts, etc)

As for the efficiency: a modern laptop can easily run ~100 channels of multi-
second convolution reverb in realtime on 44.1/48kHz sample rate and <10ms
latency in real-time on 1 core.

~~~
acjohnson55
Yeah, but if you post-process the impulse response, it's difficult to end up
with something else that "looks" as much like a real impulse response. If
that's important to you.

Don't get me wrong, I'm not trying to argue against using convolutional
reverb. I was just throwing out a couple reasons why people still use other
approaches.

------
dx7tnt
I think Abbey Road (and probably other studios of the era) had an ambiphonic
system similar to as described in the paper. They had an array of speakers
built into the walls of their recording room so that could feed back signals
from it, artificially changing the length of the reverb decay of the room.

[https://soundgas.com/pages/grampian-ambiophonic-unit-
type-66...](https://soundgas.com/pages/grampian-ambiophonic-unit-type-666)

[http://www.sydbarrettpinkfloyd.com/2009/08/unforgotten-
hero-...](http://www.sydbarrettpinkfloyd.com/2009/08/unforgotten-hero-peter-
bown-and-piper.html?m=1)

