posted 30 March 2003 20:30            

uglychirpremover.kym

I had a recording that has the remnants of a '70s style wah-wah guitar popping through every now and then at 2200 Hz, 2600 Hz, and 2800 Hz. I found these events using the spectrograph display in CoolEdit/Pro. Each time it appears it lasts for only a fraction of a second.

But I began to feel like I was walking around with a small stone in my shoe whenever I listened to this track. Before long, that ugly little chirp was all I could hear...

These chirps were very narrow band, and so using a bandpass filter and a compressor did not seem like such a good idea, especially given the coloration that such a filterbank could produce. All I needed was a gated notching filter at each of these three frequencies whenever the chirp appeared. Leaving a set of notching filters running all the time would likewise produce undesirable coloration. If these chirps could be gated out then most of the time the coloration would be completely neutral.

Hence, this Sound is a series of 3 notch filters with adjustable center frequencies, Q's, and depths. Each of the notch filters is associated with a narrow bandpass filter which feeds a peak detector with adjustable attack and release. These peak detectors then feed threshold detectors. The threshold detectors are incorporated into the notch filter depth parameters to selectively gate out their corresponding chirp components when they appear.

The incoming sound is delayed into the notch filters compared with the peak detectors so that the detector attack has a chance to become established before the notch filters see the sound.

Identical processing is applied to both left and right channels. I found that even though the chirp occurred only on the right channel, unless I treated both channels in the same way, it introduced an undesirable shift in the stereo image.

The result works remarkably well. These frequency components are gated out only briefly, leaving the original coloration intact for most of the music. I cannot detect any change in coloration or stereo field placement during the gating. At most I can barely detect some remnant of the original chirps, but severely attenuated.

Now I don't have to limp around whenever I listen to that track!

- DM

[Handy Guide to Filter Q Settings...

Bandwidth Q
1 octave 1.414
1/2 octave 2.871
1/3 octave 4.318
1/4 octave 5.764
1/5 octave 7.208
1/6 octave 8.651
1/8 octave 11.54
1/10 octave 14.42

]