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From: todd@ivucsb.UUCP (Todd Day)
Newsgroups: rec.audio,sci.electronics
Subject: Re: Extracting Mono from Stereo
Message-ID: <353@ivucsb.UUCP>
Date: 26 Oct 88 13:56:58 GMT
Article-I.D.: ivucsb.353
References: <343@ivucsb.UUCP> <3902@homxc.UUCP>
Reply-To: todd@ivucsb.UUCP (Todd Day)
Organization: The Audio Club at UCSB, Isla Vista, California
Lines: 60

In article <3902@homxc.UUCP> marty@homxc.UUCP (M.B.BRILLIANT) writes:
_
_In article <343@ivucsb.UUCP> Todd Day explains his previous query:
_> ....
_> Let's say the stuff in the left channel is L+0.5M (stuff that is not in
_> the center + half of the stuff that is in the center (mono)).  Right
_> channel is R+0.5M.  Now, I want just M....
_
_The man wants to solve two equations for 3 unknowns:
_	L + M/2 = known
_	R + M/2 = also_known
_L, R, and M unknown, solve for M.  Can't in general be done.

Well, then, how come your ears solve that equation everytime you listen
to stereo?

_In the original acoustic space you have more than two sources of sound.
_You have wavefronts coming from many directions, and you can tell the
_difference between a tone coming from point A and a tone of the same
_pitch from point B.

Is this generally true?  If I put a tone in the left channel at
value 3 and the SAME tone in the right channel at value 5, and
the tones are in phase, where does it sound like it's coming
from?  I think it will sound like it is coming from right of
center, as the two tones are in phase.  This is what I assume
for my "center channel extractor", i.e., that you have the same
tone at the same phase and amplitude coming from both speakers
and this tone will fool your ears into thinking that there is
sound coming from a center channel.

_But now you have only two channels, not a three-dimensional wave
_medium.  Let's fantasize.  Put two stereo speakers in an anechoic
_chamber, and when there is an M sound they create a synthetic wavefront
_coming from the space between them.  Now set up a directional mike so
_that it only picks up sound from the space between the speakers, not
_from either speaker alone.

There is not really any sound coming from the center.  Your ears
*think* there is.  Stereo is just a charade to fool your ears into
thinking there is a full soundstage between your speakers, when in
reality, sound is just coming from two sets of discrete speakers
on either end of the sound stage.

_Two equations in three unknowns, then.  Can't do it.  But I've been
_made to eat my words before in this newsgroup, so come on, help this
_guy out, prove me wrong!

I think it can be done.  After all, your ears do it for you everyday.
Remember, after the "center channel" gets mixed with the stereo, it
does not really get lost... your ears can pick it out and localize it
as coming from the center of the stereo stage.  I just want to find
some mathematical easy way of mimicking the ears.  I haven't found
an "easy way" yet!

 
   /| 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