Echo cancelling - Bobs tip

farss wrote on 2/28/2005, 4:25 AM
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.
Bob.

Comments

B.Verlik wrote on 3/1/2005, 12:24 PM
Cool tip. I hope I remember where I saw it, when the time comes that I need to read it. (I say that, because even if I file this somewhere, I wont remember where to find it by that time.)
musicvid10 wrote on 3/1/2005, 11:25 PM
Here's the original post, I think the basic technique could be refined with some twiddling. We used it back in the 70's to make usable (?) demos from bar band tapes. I was informed that some of our military guys had picked up the idea in another forum and were quite excited about it. Best advice is use it sparingly.

Quote:
---------------------------------------------------------------
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.
---------------------------------------------------------------
farss wrote on 3/2/2005, 1:10 AM
Thanks for that although I did it with Vegas which would seem a much easier choice that SF which doesn't support multiple tracks. In Vegas you can tweak the settings while you monitor.
Bob.
plasmavideo wrote on 5/18/2005, 3:50 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!
musicvid10 wrote on 5/18/2005, 8:44 PM
Thanks for the tweak.

For the record, I learned this from the guys at American Recording Studios in Denver about 1973, when location recording was still considered hip.
OK, I'm dating myself, again . . .
Spot|DSE wrote on 5/18/2005, 8:46 PM
It's an oldie but a goodie. Jeffrey Fisher put it in his Instant Sound Forge book, but he uses the Mix function in Forge to accomplish it. Doing it in Vegas is much faster.
farss wrote on 5/19/2005, 3:08 AM
I can't even begin to imagine the skill and hard work that it took to get this to work with R2R decks.
Bob.
Grazie wrote on 5/19/2005, 3:26 AM


Yup! - Bob? Let it go . . let it go . . I just about don't remember people biting on a bullet for surgery! I do remember Milk being delivered in glass bottles by a ruddy faced cheerful milkman! Mind you, I remember when a GALLON petrol cost 11p! HUH!

Let it got . . ..


Grazie


plasmavideo wrote on 5/19/2005, 7:47 AM
Subject: RE: Echo cancelling - Bobs tip
QUOTE:

Reply by: musicvid
Date: 5/18/2005 11:44:06 PM

Thanks for the tweak.

For the record, I learned this from the guys at American Recording Studios in Denver about 1973, when location recording was still considered hip.
OK, I'm dating myself, again . . .

QUOTE

How long have you been dating yourself? Do your friends know? . . . nevermind - a poor atempt at humor after 3 hours sleep!

Denver in '73 - that's when KIMN was really smokin'

Dating myself, I had just left a job programming a radio station in '73 and got out of the biz for a short time.
musicvid10 wrote on 8/5/2005, 9:08 AM
**I can't even begin to imagine the skill and hard work that it took to get this to work with R2R decks. **

Bob,
Visualize this: A 24 track mastering console running some really wide tape with the dubbing heads located downstream from the playback heads.The dubbing tech is on headphones with his eyes glued to a dual trace scope chasing the phase by tweaking a large knob on an analog phase shifter. Even the teeniest capstan slip would mess up the whole project. They spent more time cleaning the transport and aligning the heads than actually recording, which could get a little frustrating for us session musicians who were waiting for a go/no-go on the next take. Mixing was truly an art form in those days and I marvel at the magic that was created on the Beatles and Pink Floyd albums in the early seventies.
Coursedesign wrote on 8/5/2005, 9:16 AM
I marvel at the magic that was created on the Beatles and Pink Floyd albums in the early seventies./i>