RE: stereo to mono

[Hoover Alan <HooverA@tce.com>]

Alex (and Aden)-

When you invert one channel and then sum the two, as you did, you remove 
all
signals where L=R, which is mono.  This is commonly known as the (L+R)
signal.  What you are left with is commonly known as the difference signal,
or (L-R).  A stereo signal consists of (L+R) and (L-R).  If you add the 
two,
you are left with only L, if you subtract the two, you are left with only 
R.

You can do as Aden says, first obtain the difference signal and then add
that to the total, and you will be left with the sum, or mono signal.  
There
is an another, perhaps easier way to achieve this.    Just combine L and R
without inverting one channel.  Then, you remove all signals where L= -R.
This is the part of the total signal which produces a sense of 
spaciousness,
and is commonly called the (L-R) or difference signal.  You will remove 
this
whenever you simply sum L and R.  

(Incidentally, if you boost the (L-R) relative to the (L+R), you achieve
"expanded stereo", or "SRS", or Carver "Sonic Holography", or any of a
number of schemes developed since Dynaco (David Hafler) devised "Matrix
Surround Stereo" in the '60's by connecting one speaker across the (+)
output terminals of a stereo amplifier, and placing this speaker in the 
rear
of the room.  It is all the same thing.)  The new Fender acoustic amp does
something like this.  You can do cool things with the difference signal
(L-R), which you can isolate by inverting one channel and then summing the
two.  For instance, you can get rotating speaker effects by either 
amplitude
modulating this signal and/or doing time-delay modulation to it (passing it
through a choruser, phasor, etc.), then combining it back with the sum 
(L+R)
signal. 

If you are familiar with opamps, as I assume you might be since you are
experimenting with these things, you can easily and cheaply make a circuit
using readily-available quad opamps to have both the sum and difference
channels at your disposal to do all kinds of creative processing:  Sum the
two channels, for (L+R), which is mono.  Invert one channel and then sum 
the
two, which is the stereo difference signal.  Now, if you sum and difference
these two signals unmodified, then you end up with the original L and R
channels (stereo).  You can play with the sum and difference signals to
achieve cool effects before recombining.  This is how a lot of movie
soundtrack effects are achieved, also reverb effects, particularly if the
reverb and other time delay functions are used in the difference signal.  

-----Original Message-----
From: Alex [mailto:alex_d@netcomuk.co.uk]
Sent: Saturday, October 10, 1998 7:03 PM
To: Loopers-Delight@annihilist.com
Subject: stereo to mono



I'm using SoundForge and have found that you can remove the mono elements
of a stereo track...... leaving you mostly with stereo effects (inverting
one channel and combing to mono)

I would like to use this in reverse.

Is there any way that you can remove the stereo elements leaving on the
mono parts of a track? I've had no luck so far, and suspect it may be
physically impossible.