Crackle in audio - removal tips?

[Jody Arnott]

Hey all,

I'm editing a wedding video for a friend at the moment. The footage is from a 10 year old tape, and the audio isn't what you'd call clean. There's a pretty consistent crackle/clicking on the tracks.

Fixing up bad audio isn't really my specialty. Could anyone recommend some software/techniques to help me out? Using Avid MC for the editing.

Cheers! :)

[Greg Miller]

Please post a sample so we can hear specifically what you're talking about. Then perhaps someone can make a specific suggestion.

[Jody Arnott]

Here's a sample (attached). Ignore the knocks, it's just people walking around. It's the buzzing I'm interested in removing.

Thanks again :)

[Battle Vaughan]

This is pretty easy to remove with the noise removal tool in almost any audio editing program ---the excellent freeware Audacity is one such (Audacity: Free Audio Editor and Recorder). You sample the noise you want to remove in the first step; then select the clip or portion you want to treat, re-open the noise removal tool, and apply. There are various settings you can experiment with to get the amount of removal that you need without affecting the speech or whatever you want to keep to too great a degree.

In these cases, the success depends on the ratio of the level of the noise to the good part, and on the frequency range that you are removing --- whether or not it affects the good part. It takes some tweaking to get a good result in most cases.

Attached files are waveform before nr, and wavefrom from Audacity after 24dB noise removal from a sample taken from the middle of the image.

Audacity also has a dedicated "click and pop" removal filter. Sony Sound Forge, Adobe Audition, etc. have similar features.

[Greg Miller]

I don't hear anything that I would call "crackle" on this track, nothing that is qute "clicking" either.

You have a significant amount of hum at 150 Hz (perhaps power line or TV sync harmonics). That can be removed by a notch filter.

There is a lot of HF noise up around 15,500 - 15,750 Hz, probably TV sync noise, that can be removed by a low pass filter.

That leaves some very low level buzz, consisting of several peaks between 1,000 and 4,000 Hz. It's hard to pinpoint the exact frequencies because the peaks are somewhat masked by all the room noise. (It's too bad I don't have a section of tape recorded on this same machine, but with the mic gain all the way down.)

Rather than try to tune notch filters to remove all the buzz frequencies, you can use a wide-band subtractive filter. That will also lower the level of the room noise. (If this were my own track I might spend more time on it, trying to tune notch filters. Subtractive is quicker.)

However, a subtractive filter can also have some detrimental effect on the desired audio (dialog?) but since you didn't provide any of that in your sample, we can't really tell how much.

If you listen to a file that has been processed entirely with a broad-band subtractive filter, it may sound unusually quiet, because the filter has removed too much of the natural room tone. Also, you will hear "swooshing" artifacts surrounding any loud sound in the file. (I've seen people describe this as an "underwater" sound.) This can make the voice sound artificial and "robotic" if you're not careful.

I prefer to identify the worst noise sources, and remove them individually, first. Then I use the broad-band subtractive filter very gently, as a last resort. The attached file has been processed with the three filters described above, using CoolEdit Pro 2.1, which was the predecessor to Adobe Audition.

If you provide a somewhat longer file that also contains dialog (it could be mono, to reduce file size), you can hear how the filtering does or does not degrade the voice quality.