Re: Auto-correlation


<Don Pearce> wrote in message news:49477b34.503246718@xxxxxxxxxxxx
On Mon, 15 Dec 2008 17:55:43 GMT, Carey Carlan <gulfjoe@xxxxxxxxxxx>
wrote:

I just received a CD of a cassette of my great aunt recorded in the
1960's.
I'm still working on getting the original cassette. No guarantees.

It's a stereo recording of a mono signal. I want to capture only the
information common to both channels and discard any differences as noise.

I should be able to do that with an M-S converter, discarding the side
channel entirely.

Is there a better way?

You've already had a lot of good advice about picking the best channel
and using that, but there is a tool you could try before doing that.
Adobe Audition and its predecessor CoolEdit have a tool called Centre
Channel Extractor. What it does is exactly what you say above. It
examines the stereo signal, amplifies whatever is identical in both
channels and suppresses whatever doesn't correlate - which would be
most of the noise.

It may turn your -15dB noise into -25 or -30dB, which will make a huge
difference when it comes to digital noise removal.

d

I wonder how it works. When you add two random noises together, the power of
the noise goes up 3 dB. When you add two correlated signals together, the
power goes up 6dB. Therefore, the most improvement in S/N that can be had by
summing is 3dB.

I realize that intelligent strategies exist, but, like the various
restoration programs, they tend to do damage as well. Because of the tape
wiggle, it's hard to see how the program could have accurate enough
information to work on.

I can see how the product you mention could be useful if the primary
problem was dropout.

Bob Morein
(310) 237-6511





.

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