www.piclist.com/techref/index.htm?key=inverte+wave

Basic principal is to produce the same wave 180 degrees out of phase. Thus

the sound waves cancel in energy. http://www.headwize.com/ has some stuff on

Active Noise Reduction. In particular the article titled "Adaptive Active

Noise Control for Headphones Using the TMS320C30 DSP" is quite good. In this

article they describe the basic theory of Active Noise Reduction examine the

current (at the time) systems which are generally passive (non-adaptive)

algorithms and present an algorithm for Adaptive Noise Reduction. This is

basically a passive noise reduction system that looks at how much noise got

through (error) and uses this to adapt the algorithm. There system also

allows for selective Noise reduction (i.e. eliminate the jackhammer but keep

your coworker telling you you're about to be hit by a truck) but I don't

know that the algorithms for this are discussed in detail.

This article provides the DSP algorithms used by the system.

Tom.

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See also: www.piclist.com/techref/index.htm?key=inverte+wave