This is an adaptive feedback squeal cancellation system. As long as you tune the Freq parameter within 800 Hz of the actual squeal, this system will lock onto the actual frequency and then generate the anti-sound and mix with the original sound. Cancellation is quite good, being on the order of 40-50 dB. The sound as submitted uses a test oscillator to generate the squeal. You can simply replace that oscillator with your input channel. The oscilloscope display in the VCS shows the extent to which cancellation is happening.

The system performs this feat by cross correlating the input signal with a sine and cosine oscillator both tuned to the same frequency. The output of these I and Q channels is then fed to an ArcTangent? Sound to compute the instantaneous phase angle of the input with respect to the oscillators. This phase angle is differentiated with respect to time -- giving a frequency deviation measurement. This deviation is integrated and then fed back to the oscillators with inverted sign. Hence the oscillators adaptively seek out the frequency of the strongest signal component (the feedback squeal) and then generate its anti-sound for mixing with the original sound.

-- DavidMcClain - 23 Nov 2003