How to remove noise from AGC-leveled audio file?
Old 30th January 2013
  #1
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How to remove noise from AGC-leveled audio file?

I have a client who wants a video file cleaned up. The file was recorded on a video cam that has AGC - so the audio stays level but the background noise is going up down throughout the file.

I've tried using various noise reduction tools (Sound Soap, Izoptope, etc.) but with the noise floor rising and falling it's not working too well.

I don't have the advanced Izotope with the "level tracking" feature but I'm not sure that would work any better since the audio level is constant - it's the noise floor that goes up and down (and I don't have $1500 to find out).

I'm beginning to think my only choice is to automate the threshold of the noise reduction manually through the whole track. Getting this to sound right (without artifacts) could be a real challenge.

Any other techniques or tool suggestions?
Old 30th January 2013
  #2
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Give us a short sample to look at. Sometimes it's possible to automatically level the noise using a sidechain compression based on a bandlimited noise signal (with an added bonus of undoing the work of AGC).
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Old 30th January 2013
  #3
I know this might be a pain depending if it's a lengthy segment, and just how much/often the level is changing, but you can try several (3?) levels of Broadband noise reduction, and edit between those where appropriate. Not the fastest/simplest nor the most elegant, but might work depending on the material.

The only other (slightly crazy) idea crossing my mind at the moment is trying to key in on the noise while avoiding the content into a sidechained compressor bringing it back to a steady noisefloor, sort of try to undo the AGC, de-noise, and then apply any dynamics needed to resulting file, but the odds of this solving more than it fixes, or avoiding tons of compression artifacts are probably 0.

Are they focusing on fidelity or audibility? Is this music or speech? Broadband, focused etc. Maybe post a sample?
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Old 30th January 2013
  #4
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Thanks guys for the suggestions.

I don't have access to post a sample right now but I can tell you that it's a combination of music and dialog - a musical play that was recorded. The noise floor sometimes rises up quite high during the lower level passages and it becomes distracting.

I'm going to try both of your suggestions - the keyed sidechain comp and some manual split points set at different thresholds.


Riccardo - perhaps this post is better suited for the "Post Production" forum? If so, please move it. Thank you.
Old 30th January 2013
  #5
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Adam Dempsey's Avatar
 

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Originally Posted by Rob Murray View Post
Are they focusing on fidelity or audibility?
That alone can be a very important distinction to make and will dictate the approach.
Old 30th January 2013
  #6
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Quote:
Originally Posted by Adam Dempsey View Post
That alone can be a very important distinction to make and will dictate the approach.
I suppose it's a matter of "fidelity". The issue is not that the audio is lost in the noise it's that the noise is distracting to the audio.

During musical passages the noise floor is virtually inaudible whereas during dialog passages the gain (and noise) comes way up. Some compressor literature refers to this as the "breathing" effect.

Is there any way to reduce this type of noise?
Old 31st January 2013
  #7
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Quote:
Originally Posted by Nonlinear View Post
I suppose it's a matter of "fidelity". The issue is not that the audio is lost in the noise it's that the noise is distracting to the audio.

During musical passages the noise floor is virtually inaudible whereas during dialog passages the gain (and noise) comes way up. Some compressor literature refers to this as the "breathing" effect.

Is there any way to reduce this type of noise?
Sounds like perhaps an expander/single ended noise reduction, on just the dialogue sections, might work. In the analogue domain and for cassette mastering I used to use a Symetrix 511A extensively for such things, for gentle expansion combined with its sliding HF filter. Used judiciously it worked really well, quite smooth yet fast enough for dealing with speech.
Old 31st January 2013
  #8
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Quote:
Originally Posted by Adam Dempsey View Post
Sounds like perhaps an expander/single ended noise reduction, on just the dialogue sections, might work. In the analogue domain and for cassette mastering I used to use a Symetrix 511A extensively for such things, for gentle expansion combined with its sliding HF filter. Used judiciously it worked really well, quite smooth yet fast enough for dealing with speech.
OK, thank you - will give it a try!
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