After I posted the "Intro to VoIP Security panel podcast, I received a nice note from a listener suggesting that I try a high-pass or notch filter to remove the buzzing that in the podcast.  The note prompted me to investigate further because I, too, had been rather annoyed to have this sound in the recording, especially because I had been recording directly from the conference room mixer!  I heard it while there at the Internet Telephony conference, and tried tweaking the mixer a bit to see if I could drop out the buzz, but found no way to do so and had to conclude that it was originating somewhere in the audio equipment they were using.  So I did really want to get rid of it... but then when I came back wanted to get at least one of the panels out and didn't have time to track down the problem.  But after the comment and before I did the next panel I searched...

...and found in the Audacity wiki - "Eliminating a Continuous High-Pitched Whistle-Like Noise from a Recording", a great little tutorial that helped me figure out what I needed to do.  It involves a few steps, basically:

1. Select part of the recording that is the closest to silence that you can get - where you only hear the buzz.
2. Go to the "Analyze" menu and choose "Plot Spectrum".
3. Identify one of the peaks.
4. Go to the "Effects" menu, choose "Nyquist Prompt" and enter "(notch2 s freq value)" (ex. "(notch2 s 1019 25)")
5. Play the sound and listen for the buzz.
6. If the sound is still there, go back to step #2 and identify another frequency and repeat the process. (writing down the frequencies you are using as you go along)
7. When you have eliminated as much of the buzz as you can, select another segment of sound (preferably several seconds) that includes human voices and repeat step #4 for each of the frequencies you wrote down (unfortunately, per another forum post, there appears to currently be no way to set up a notch filter for several frequencies).
8. Listen to the resulting segment to ensure that it still sounds okay (hopefully sans buzz).
9. Select the entire audio file and apply the notch filters to the entire selection.
10. Listen to your clean(er) audio file.

That's basically what I did... although in thinking about it I might have had a step between 8 and 9 where I used "Undo" to remove the notch filters on the small human voice segment before applying the filters across the entire file.  For my own record, here's the sequence I did:

(notch s 1019 25)
(notch s 2046 25)
(notch s 21649 25)
(notch s 21650 25)
(notch s 21652 25)

That all seemed to do the trick. There still a bit of a low hum, but I also tried a high-pass filter that would basically wipe out everything under a certain threshold, but using numbers down like 100Hz I didn't discern any real difference - and I was reluctant to go too much higher and start impacting voices.

Anyway, you can hear the difference on the "VoIP Security Best Practices" panel.  Still a small buzz... but at least the high-pitched one is gone.  (And suggestions on killing the low buzz are always welcome. :-)

Tags: audacity, audio, podcasting, post-production