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Echo cancelling – Bobs tip

Subject: Echo cancelling – Bobs tip

Posted by: farss

Date: 2/28/2005 12:25:16 PM


First I’ve got to thank somebody from a long time ago over on the audio forum, don’t remember who it was, probably Red or Pipe.
Someone just knocked on my door with an audio disaster. Only had on camera mic to shoot speeches in a large office lobby, sure you get the idea, all glass and marble and the camera a good way from the speakers and a very bad PA system. Very serious echo and resonance.
I’d tried to fix this sort of thing before with very limited success but I had another go and I got this trick to work pretty well, still nothing even remotely like what it would have been with a mic on the lecturn but a big improvement.
I split the audio track into separate clips for each speaker, and opened in SF as take. First I normalised it as the levels were WAY down, then some Eq to kill anything not needed and add some boost at 100Hz and dips at the room resonance. Then used wave hammer at Smooth Compression preset. Saved it back to Vegas, then opened that take in SF as a take again and applied Wave Hammer at Medium Compression preset. Saved that back to Vegas and dragged that take into a new audio track. Then switched on Invert Phase.
Now at this point all you get is rubbish. Slide the track gain down on the highly compressed track, I found a sweet spot at around -12dB but it varied from speaker to speaker.
No doubt with a lot more fine tuning you could get a better result, I’m surprised that I was able to make a quite noticeable improvement without too much tweaking.

Here’s an old trick (from and old dog) dating back to analog recording days that usually gives some improvement (you need a good wave editor like SoundForge):

1. Select All, then Copy the audio.
2. Paste to a new track, invert the waveform, and apply moderate compression.
3. Reduce the new track volume so that a preselected “quiet” area is about 50% of the level of its corresponding area in the original.
4. Paste Mix the new track into the old. Renormalize if necessary.

There should be a noticeable improvement in clarity and echo reduction because you have applied negative feedback to the areas where the echo is most objectionable. Too high a compression or too high level of the feedback track will give a “pumping” effect, however.

Subject: RE: Echo cancelling – Bobs tip

Reply by: plasmavideo

Date: 5/18/2005 11:50:56 PM


Whoever came up with this idea originally just saved a video tape for me. In all of my 30+ audio editing years I have never come across this tip, nor thought of it myself.

I had a video track recorded in the atrium of a local hotel, and the echo was unreal. I used this trick in Vegas and was amazed how clean I could get it by playing around with the compression release time and threshold adjustments on the alternate track. The release time adjustment was the biggest help in overcoming the acoustics delay and still preserving the proper dynamics in the main audio.

Thanks so much for posting this tip!

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