Xref: utzoo rec.audio:8755 sci.electronics:4165
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From: todd@ivucsb.UUCP (Todd Day)
Newsgroups: rec.audio,sci.electronics
Subject: Extracting "Center Channel" from Stereo Image
Summary: I repost original request with less vagueness
Keywords: stereo center channel
Message-ID: <355@ivucsb.UUCP>
Date: 26 Oct 88 14:39:18 GMT
Article-I.D.: ivucsb.355
Organization: The Audio Club at UCSB, Isla Vista, California
Lines: 71

OK, judging from the response via email and replied postings, I
really did not do a good job of stating my problem.  The biggest
problem was me saying "mono" when I meant "center channel".

I will try once more with the following example.

Let's say you are watching Johny Carson.  The Tonight Show is
now broadcast in stereo.  During the monologue, Johny appears
to be talking from the center of the stereo image, Ed chuckles
occasionally in the left channel, and Doc responds to Johny's
jokes from the right channel.

Now, if you are sitting right in the middle of your speakers,
you hear the stereo image as presented above.  Alas, if you
sit to the left of center, it sounds like Johny is sitting
in Ed's lap.

What I propose is that we remove Johny from the "center channel",
if you will, and put his voice in a speaker that sits on top
of the TV.  In this manner, you will remove the dependence of
having to sit in the exact center to hear the center channel
properly.  You will be able to sit right next to the left
speaker, and Johny will still sound like he's talking from
the center (during his monologue, he is the only person
on camera).

OK, I hope I have presented a reasonalble example.  I already
know that this cannot be done by simply taking L-R (which
contains no center channel) and subtract this from L+M+R...

Oh oh, I can see we are going to have a notation crisis.  For
the purposes of this discussion, I'll make the following
assumptions.
The left channel contains half of that which is in the center
channel and all of that which is in the left channel but
not in the center.
Therefore, the left channel will be  L+0.5C.
This means the right channel will be R+0.5C.

Now, one method that will work, but is very calculation intensive
(I am assuming digital domain here, but analog replies are
welcome also :-), is to copy the ears directly.  In other words,
break the frequency spectrum of both channels up into small
pieces and compare the levels of each spectrum chunk.  If a
chunk in the left channel is about the same amplitude as a chunk
in the right channel, then call that chunk part of the "center
channel" and pass it through.  Note the assumptions I am
making here!  I am assuming that the only way the ear hears
sound coming from the center channel is if there is a tone in
the left channel that has the exact same frequency, amplitude,
and phase as a tone in the right channel.  This is not neccessarily
a valid assumption, but will be true almost all of the time
for audio sources that accompany video.

The problem with the above solution is that the quality of the
results is related to the size of the frequency chunks.  I am
afraid that I wouldn't be able to make the chunks small enough
to get a quality result.

So, that is where you come in.  I am simply wondering if anyone
on the net has read any articles or know of any "quick and
dirty ways" to remove the "center channel" AND use it to
drive another speaker.

Thank you for your time and patience with my previous vagueness.

 
   /| Todd Day (805)968-9352 |\              "I go to school, but
+-+ | The Audio Club at UCSB | +-+    I never learn what I want to know."
+-+ | 926 B   Camino Del Sur | +-+     ..!pyramid!comdesign!ivucsb!todd
   \| Isla Vista, CA   93117 |/                todd@ivucsb.UUCP