issue07

EMUSIC-L Digest                                      Volume 56, Issue 07

This issue's topics:
	
	Center channel extractor (3 messages)

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Date:         Mon, 13 Sep 1993 11:22:38 CDT
From:         Bob Crispen 
Subject:      Center channel extractor

I seem to recall that there's a way to hook up outputs (e.g., of a
preamp or power amp) and use phase cancellation to produce a simulated
rear speaker ambient sound.

I also recall reading on EMUSIC-L a circuit diagram for Y-ing cables
together which I saved and took home, since plain old Y cables are a
Bad Idea.

So these two thoughts rattling around in my otherwise empty brain
produced a question:  Is there an easy-to-build circuit that will take
a stereo signal and produce:

(a) the same signal less the center signals (i.e., stereo, but with
a hole in the middle) and

(b) the center signal only (i.e., a signal which is present in both
L and R channels in equal amounts)?

And (c) is it possible to control the width of the center hole?  That
is, if a signal is present in both L and R and the signal in L is
at least n% (variable n) of the signal in R, it's extracted.  Note that
(c) isn't an absolute requirement if it makes the whole thing an order
of magnitude more difficult.  If a circuit that does something like (a)
and (b), is easy to build, it might be fun to play with it and use it on
a project only when it sounds OK.

Or am I talking about something that the laws of physics won't let you
do without a great deal of trouble (e.g., DSPs, microprocessors)?

Thanks,
+-------------------------------+--------------------------------------+
| Rev. Bob "Bob" Crispen        |            "IT FOUND ME!"            |
| crispen@foxy.boeing.com       |      Campus Crusade for Cthulhu      |
+-------------------------------+--------------------------------------+

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Date:         Mon, 13 Sep 1993 17:08:36 -0700
From:         Michael O'Hara 
Subject:      Re: Center channel extractor

It is easy to use op-amps to make a "center channel" filter.

There is even a "nulling" ability.

Invert one channel and feed it and the other channel into a
"difference amp".  This will give you R - L, or the "ambient" sound.

You can expand the concept by adding/subtracting the difference signal
from the left or the right channels, this is less useful.

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Date:         Mon, 13 Sep 1993 18:41:31 BST
From:         "Steven D. Bramson" 
Subject:      Re: centre channel extractor

Bob Crispen Writes

---------------------------

Is there an easy-to-build circuit that will take
a stereo signal and produce:

(a) the same signal less the center signals (i.e., stereo, but with
a hole in the middle) and

(b) the center signal only (i.e., a signal which is present in both
L and R channels in equal amounts)?

And (c) is it possible to control the width of the center hole?  That
is, if a signal is present in both L and R and the signal in L is
at least n% (variable n) of the signal in R, it's extracted.  Note that
(c) isn't an absolute requirement if it makes the whole thing an order
of magnitude more difficult.  If a circuit that does something like (a)
and (b), is easy to build, it might be fun to play with it and use it on
a project only when it sounds OK.

-----------------------------------------------------


In the UK there is a science and technology TV magazine programme called
Tomorow's World.  A few years back they had what you might call an instant
Karaoke machine.  A black box electronic gizmo that looked for and removed
part of the audio signal that is common to both left and right channels.
You could pan this left and right, thus achieving (c) above.  The
application they suggested was that in a typical recording of a song, the
instruments are left and right but the lead voice is in the middle.  Using
the gizmo you could knock out the lead singer and sing along yourself.  I
suspect the circuit was far from simple.

 Steven D Bramson    Data Management Group   JET Joint Undertaking
 Abingdon     Oxfordshire              OX14 3EA     United Kingdom
 E-mail sdb@jet.uk   Voice 0235 465 013           Fax 0235 465 399
------------------------------------------------------------------
 JET is Europe's fusion research project
 Disclaimer: Any views expressed are mine and do not represent those of JET

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