posted 12 July 2001 05:38            

downsamp_iir_filter.zip

This uses an adaptive filter that implements a Least Mean Square (LMS) algorithm to update the weights assigned to sine and cosine oscillators tuned to the desired cancellation frequency. This pair of oscillators is mixed to produce an antisound that exactly cancels the signal component at this same frequency.

The sample shows how it can remove the fundamental from a square wave oscillator. As you change the amplitude of the oscillator, the cancellation oscillator automatically adjusts to track the change.

Click on the ANC button to disable the cancellation and hear the full signal spectrum.

This little trick is not flexible to the point of automatically tracking the frequency of the signal fundamental. It is an example of the kind of adaptive filter needed when you already know the frequency of a narrowband noise source -- e.g., a motor, or a fan, or some other oscillatory noise source.

Typically the input signal comes from a sensing microphone, and the cancellation oscillator is fed to a loudspeaker system to cancel the ambient oscillation noise.