Noise Gate/Expander

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Description

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.

 

Graphical Depiction

 

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

 

Source Code

Noise_Gate.txt

Noise_Gate.csd

(.csd files can be viewed with Notepad or any text editor)

 

Example Audio Clips

Original unprocessed signal

Original

Noise gated signal

Noise Gate

 

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.

 

References

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.