~4Dgifts/toolbox/src/exampleCode/audio/reverb README
Simulated reverberation for real-time audio input and output.
Written by Gints Klimanis
Silicon Graphics Computer Systems
1994
reverb usage: -help
-verbose
-level 0..1
-noMonoProcess each input processed
-reverbTime seconds
-noPrime
-type {comb, allpass,
schroeder1, schroeder2,
chamberlin,
moorer, moorer2,
er7, er19
moorer2er7, moorer2er19}
all of the documentation contained herein is
lifted directly from the reverb.c source file.
The algorithms generate room ambience rather than a realistic room
response. For proper operation, ensure the input and output sampling
rates are equal and neither changes over time.
The most advanced algorithm in this code is moorer2er19, which
consumes roughly 50% of a 100 MHz R4000.
For the pronounced effect, try:
> reverb -type moorer2er19 -reverbTime 10
For reverberation times > 30 seconds, the decay seems to fizzle out
quite abruptly. The cause is likely caused by single-precision
floating point artifacts in the computation of the low-pass filter
nested in each comb filter.
-noMonoProcess specifies that a reverberator is computed for
each input channel. Otherwise, all inputs are
summed into a single reverberator. Algorithm
types 'comb' and 'allpass' are processed
with a feedback gain of opposite polarity in
each of two output channels.
-reverbTime default is 8 seconds. Applies only to comb,
allpass, schroeder2, moorer, moorer2, moorer2er7,
moorer2er19 algorithms. Others have decay time
as specified in design.
-noPrime prevents reciculated delay buffer lengths from
quantization to prime value. The "priming"
seems to reduce ringing due to transients such
as snare drum strike and low-level whining in
the presence of input noise, so this parameter
is provided for comparison.
-type: defaults is moorer2
comb, allpass comb and all-pass Interpolated Infinite Impulse
Response (IIIR) filters.
schroeder1 quint series all-pass filter
schroeder2 quad parallel comb into dual series all-pass
filters
chamberlin quint series all-pass filter
moorer Moorer's hex parallel comb filters into single
all-pass filter
moorer2 hex parallel filtered-comb into single all-pass
filters
er7, er19 early reflections patterns w/7 and 19 taps,
respectively
moorer2er7, moorer2er19 early reflections + hex parallel
filtered-comb into single all-pass filters.
Early reflections fed into comb network.
Scatter in 19 tap ER pattern yields algorithm
with significantly less ring excited by
transients such as snare drum strike.
To my ears, moorer2 rings less to transients than moorer2er7,
moorer2er19. However, moorer2er7 and moorer2er19 have smoother
decay than moorer2. Not sure if ringing is caused by an error
in my implementation or by a comb-filter quality imposed by the
early reflection pattern. Consider a diffusor other than the
early reflection pattern.
The following references contain detailed discussions of the
non-proprietary implementations found in the code in reverb.c:
+ Schroeder, M.R. and Logan, B.F., "'Colorless' Artificial Reverberation."
Journal of the Audio Engineering Society v. 9, n. 3: pp. 192-197, 1962
+ Schroeder, M.R. , "Natural Sounding Artificial Reverberation." Journal
of the Audio Engineering Society v. 10, n. 3: pp. 219-223, 1962
+ Moorer, J.A., "About This Reverberation Business", Foundations of
Computer Music, Roads and Strawn, ed.: pp. 605-639, 1985
+ Chamberlin, Hal, Musical Applications of Microprocessors, 2nd Ed.,
Hayden Books, A Division of Howard Sams & Co., Indianapolis, Indiana,
pp. 508-512, 1985
Manfred Schroeder's conditions for artificial reverberators
(lifted from paper):
1) The frequency response must be flat when measured with narrow bands
of noise, with the bandwidths corresponding to that of the transients
in the sound to be reverberated. This condition is, of course,
fulfilled by reverberators which have a flat response even for
sinusoidal excitation.
2) The normal modes of the reverberator must overlap and cover the entire
audio frequency range.
3) The reverberation times of the individual modes must be equal or
nearly equal so that different frequency components of the sound will
decay with equal rates.
4) The echo density a short interval after shock excitation must be high
enough so that individual echos are not resolved by the ear.
5) The echo response must be free from periodicities (flutter echos)
6) The amplitude-frequency response must not exhibit any apparent
periodicities. Periodic or comblike frequency responses produce an
unpleasant hollow, reedy or metallic sound quality and give the
impression that the sound is transmitted through a hollow tube or
barrel.
Source
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