Xref: utzoo rec.audio:8714 sci.electronics:4145
Path: utzoo!utgpu!water!watmath!clyde!att!whuts!homxb!homxc!marty
From: marty@homxc.UUCP (M.B.BRILLIANT)
Newsgroups: rec.audio,sci.electronics
Subject: Re: Extracting Mono from Stereo
Message-ID: <3902@homxc.UUCP>
Date: 22 Oct 88 22:42:40 GMT
References: <343@ivucsb.UUCP>
Organization: AT&T Bell Laboratories, Holmdel
Lines: 58


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.

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.

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.

If that works, then you could probably synthesize it in a digital
signal processor without actually using the anechoic chamber.  But I
don't think it would work.  You don't really have a single wave coming
from the middle, you have two separate waves, one from each speaker,
and the directional mike will reject both.

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!

						Marty
M. B. Brilliant		houdi!marty1, homxc!marty

Disclaimer: Opinions stated herein are mine unless and until my employer
	    explicitly claims them; then I lose all rights to them.

> you get R+L+M.  I want just M.  Subtracting the two channels gives you
> R-L.  Subtracting the difference from the summation gives 2L+M.  I want
> just M.
> 
> I need this to provide "centering" when listening to video tapes.  I want
> to remove the mono from the stereo and put that through a speaker sitting
> on the TV.  This way, no matter where you sit, you will always hear
> conversation between on-screen people from the center.
> 
> _Your help would be greatly appreciated.  I'll summarize replies.
> 
> Your smart-aleck answers are not appreciated.  I won't summarize these. :-)
> 
>  
>    /| Todd Day (805)968-9352 |\            "Hurt me, punish me.
> +-+ | The Audio Club at UCSB | +-+      Make me listen to mid-fi!"
> +-+ | 926 B   Camino Del Sur | +-+   {}!pyramid!comdesign!ivucsb!todd
>    \| Isla Vista, CA   93117 |/              todd@ivucsb.UUCP