Compressor/Limiter
A compressor is used for
reducing the dynamics of an audio signal. Levels louder than a user-specified
threshold are reduced according to a compression ratio. A ratio of 2:1
indicates that above the threshold, an input signal level increase of 2 dB will
raise the output signal level by 1 dB. A limiter is a compressor with a ratio
of 10:1 or higher. The input signal is delayed long enough to test its level
and compress it if it exceeds the threshold.
The ‘Test Level’ function
box compares the level of the input signal with the threshold value. If the
threshold is greater, the gain output is equal to 1. If the input signal level
is greater, the gain output is set to 1/ratio.
Effect
Formula
y[n] = f(x[n]) *
x[n] |
y[n] = output
signal |
x[n] = input
signal |
f(x[n]) = 1 if
x[n] <= threshold |
f(x[n]) = 1/ratio
if x[n] > threshold |
(.csd files can be viewed with Notepad or any text
editor)
Example
Audio Clips
Original unprocessed
signal |
|
Compressed signal |
|
Limited signal |
Aurally, it is difficult to
tell the difference between these sound clips. Differences in the output levels
can be observed by analyzing the output of the csound compiler. The original
signal peaks at 40957.5, well past the maximum sample value of 32767. The
compressed signal is reduced to 33099.23688, still slightly above the maximum
value. The limited signal is reduced to 29989.523927, below the maximum value.
Original: peak CH 1: 40957.500000 (written: 1.000000) at 131575
Compressed: peak CH 1: 33099.203688 (written: 1.000000) at 132055
Limited: peak CH 1: 29989.523927 (written: 0.915236) at 132055
Lehman, Scott
(1996). Effects Explained. Harmony Central. Retrieved 6/04 from
http://www.harmony-central.com/Effects/effects-explained.html
Mikelson, Hans
(2000). Modeling a multieffects processor in Csound. In Boulanger,
Richard (2000), The Csound book (pp 575-594). Cambridge, MA: MIT Press.
Schindler,
Allan. (1998). Eastman Csound tutorial.
Eastman School of Music. Retrieved 6/04 from
http://www.esm.rochester.edu/onlinedocs/allan.cs/
Vercoe, Barry.
(1992). The public Csound reference manual, version 4.16. MIT
Press. Retrieved 6/04 from http://www.lakewoodsound.com/csound/hypertext/manual.htm
Zolzer, Udo.
(2002). Digital audio effects. West Sussex, England: Baffins Lane.