What's a "separation view"?
And once you get a difference track where the channels were *not* identical,
what then do you do with the difference track?
BTW, for anyone that's interested, this is how we get terrestrial FM-stereo
that's compatible with FM-mono: The FCC standard requires broadcasters to
take a normal stereophonic track, and then (1) sum the tracks to mono, and
(2) subtract the tracks**, as in this thread, and then broadcast both the
sum and the difference signals simultaneously, the summed track on the
standard monophonic FM frequency, and the difference track on a separate
subcarrier frequency around 38KHz above the standard frequency. A
monophonic FM receiver never even notices the difference signal, delivering
normal FM-mono sound, and a stereophonic receiver uses electronic 'math' to
process the summed track and the difference track back into the original
left and right stereo channels. The system works okay, but is not perfect,
and is inferior in sound quality to FM mono due to an increase in noise and
a reduction in high-frequency audio capabilities.
This is all moot, however, since I think we're going to digital terrestrial
broadcast some time in the near future anyway.
** Subtraction is identical to the technique described earlier: split the
stereophonic track into two mono tracks, invert the signal on one track, and
then mix the split tracks back into one mono track. This works
mathematically in the same sense that adding (-10) to any number is the same
thing as subtracting 10 from that number.
In digital sampling theory, 'mixing' becomes an identical concept to
mathematical 'addition', where each digital sample in one track is simply
summed with the corresponding digital sample in the other track. It's truly
as simple as 2+2=4, which is why certain things in audio recording are
extraordinarily easy when done digitally. Reducing or amplifying a signal,
for another example, is as simple as multiplying each sample by, say 0.8 or
by 1.8, respectively, and saving the result. That is one reason why a very
fast CPU can apply Audacity effects so much faster - it's just millions of
simple - or not-so-simple - computations.
----- Original Message -----
From: "Sami Jumppanen" <sami.jumppanen@...>
To: "Discussion list for Audacity users"
Sent: Thursday, March 01, 2007 10:25 PM
Subject: Re: [Audacity-users] Detect identical left & right channel?
| On 28/02/07, Rich <blueshar@...> wrote:
| > Wait a minute. Since the quick mix is undo-able, it doesn't make sense
| > me that anyone would need the original tracks, plus the mix.
| There is no channel separation view, so we need to create an extra
| track to display the separation.
| > Isn't the mix
| > just a silent flat line utilized to prove to yourself that the left and
| > right channels of a given track were identical (i.e., monophonic
| No and yes. It is what ever it happens to be. According to the subject
| of this thread, yes, but I didn't want to start another thread...
| maybe I should have.
| I want to see what the difference is. The stereo recording could be
| from tape, which has mono signal (which never is really 100% mono due
| to noise, drop-outs etc)...
| > This thread was a response to a specific question (see email subject).
| Seeing. Thanks.
| Sami "Some-E" Jumppanen
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