Noise Gate/Expander
The expander is used for
adjusting the dynamics of an audio signal. The expander works opposite of the
compressor in that levels lower (quieter) than a user-specified threshold are
reduced according to a ratio. A ratio of 3:1 indicates that below the
threshold, an input signal decrease of 1 dB will drop the output signal level
by 3 dB. The expander effect can be used to reduce audible unwanted background
noise. A noise gate is an expander with a ratio of 10:1 or higher. The ratio
can be so high that a signal below the threshold can be attenuated.
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/ratio. If the input signal
level is greater, the gain output is set to 1.
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)
Original unprocessed
signal |
|
Noise gated signal |
The original unprocessed
signal is an oscillator whose gain decreases from full volume to silence linearly,
then increases back to full volume linearly.
When the noise gate is applied, the signal cuts off when its level drops
below the threshold.
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.