[sci.electronics] Extracting "Center Channel" from Stereo Image

todd@ivucsb.UUCP (Todd Day) (10/26/88)

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

jgk@ihwpt.ATT.COM (joe klinger) (10/28/88)

In article <355@ivucsb.UUCP>, todd@ivucsb.UUCP (Todd Day) writes:
> 
> 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).
	<stuff deleted>
> Therefore, the left channel will be  L+0.5C.
> This means the right channel will be R+0.5C.


The difference between left, right and all locations in between
in conventional stereo is one of amplitude differences.
At an audio mixing console you simply take an input and
route it to the master stereo bus through either a direct patch,
(either left, right, or both) or more commonly, through a 
"pan" (panorama) pot which does a crossfade of the signal between
the left and right channels.  Signals placed in the center of
the room are represented equally on the left and right channels
but signals off center are represented higher on one side
than the other.

To this point I believe we are in aggrement.  To simply sum
the left and right channels together which would certainly yield
L+R, but would also destroy your stereo imaging as signals located
on the left and right speakers would have a stronger
representation from the center speaker.  

Digital..... I"ve used a Bruel and Kjaer dual channel analyzer
which had the capabality of displaying the cross correlating spectrum
of both channels, that is the signal that both channels have in common.
I belive what they are displaying is the residual of a difference 
between the channels.  

I don't think you have to try to model the ear but if you can, 
write a paper on it, many people would like to know how.  
The mechanism which provides image localization however, is
psychological, aided by physiological transducers (ears).  True 
the ear has a narrowband filter-like composition, but that is
only the nature of "sampling" (aargh... I didn't mean that).

You can probably do something like L, R, C separation digitally,
but it is going to cost you in processing power; bandwidth * 2.4 (Nylquist)
* 16 or so bits of resolution * two A/D channels and one to three D/A channels.
Not considering DSP programming time.  Can you really process two channels
of data on one DSP?  
I'd really like to see an analog solution, it must exist in the form of 
a black box somewhere, perhaps in Bob Carvers lab or over at Aphex or
Omnisonics.

Good Luck

Joe Klinger
att!iexist!jgk

Disclaimer -	These opinions are my own and may or may not be supported
		by my employer.

johng@trwind.UUCP (John Greene) (10/29/88)

I was just thinking about your article and had an idea.  I haven't thought
out the details but it may work.  

If you ran both channels into an op-amp one side in the non-inverting input and
the other in the inverting input you would get everything you *don't* want at 
the output.  You take this signal and a combined L+R signal into the inverting
and non-inverting inputs of another op-amp.  This would cancel out everything 
thing that you do not want leaving only the stuff you desire.  The tricky part 
would be in getting the levels just right but it may turn out that they don't 
have to be that closely matched to get the desired effect.

Hope this helps.

-- 
John E. Greene    "People are just like frankfurters....You have to decide
                   if you're going to be a hot dog or just another wiener" DLR
TRW Information Networks Division 23800 Hawthorne Blvd, Torrance CA 90505
ARPA: johng@trwind.ind.TRW.COM  USENET: ..trwrb!trwind!johng

smadi@rlgvax.UUCP (Smadi Paradise) (11/01/88)

In article <355@ivucsb.UUCP> todd@ivucsb.UUCP (Todd Day) writes:
>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).

Very old (tube) Fisher amps used to have terminals for _three_
speakers, labled ``L'', ``R'' and ``Center''.  The last was connected
(via two power resistors? transformer tap?) to the left and right.
I have never hooked the third speaker up, but it might be doing just
what you had in mind. 

Leaving phase aside for a moment, the ceneter speaker adds k * (l + r)
to both channels. This reduces the stereo separation, and creates
a _better_ stereo image when the speakers are placed too far from each
other. (The inverse is also true: adding -k * (l + r) to both channels
widens the stereo image, and this is done in many boom-boxes.)

I'm afraid that the next lines are not going to be of much help,
but it seems as if your question can be unasked by ``correct'' placement
of the speakers. In general, people tend to place them too far from
each other: the speaker-listener distance should always be larger
than the speaker-speaker. This does not ruin the stereo image:
reasonable speakers tend to create an image which is much wider
than the distance between them!

On

cuongla@skat.usc.edu (Cuong T. La) (11/07/88)

Well I don't remember who originally asked for a simple way to extract the 
center channel from distince left and right channels but here is what I have
came up with:

Okay, lets forget about math for a while and concentrate on what we want.
Suppose , we are watching Jonny Carson again and he is the only one talking
at the moment. Instead of having half of his voice out the right channel and
half out the left, we want him to sound out a third speaker at the center.
At this instant, we probably want the side speakers to mute for clarity.
However, anytime someone else who is not at the center speakes, we want the
center speaker off and the side speakers back on. So , the following
logic applies:
let A equals L +.5M, B equals R + .5M,  and X equal a control signal produced
by A and B to  control A and B. 0 is no signal, ED is ED Mc'man , Other is
the other guy on the opposite side of ED, and Jon is of course Jonny
 				we have:
A     B      X
--------------
0     0      0             any EE majors out there will recognize the
ED    0      0       	   table to be that of an AND gate. An AND
0     Other  0             gate can be represent mathematicly as 
Jon   Jon    Jon	   
					X = AB

So, the answer is to take the channels and MULTIPLY them together.
This creates a control  signal suitable for turning the A + B (which is
MONO) signal on or off at the appropriate time. Of course, we must have a 
circuit of sufficiently fast response so that only a very small time period
is compared by the circuit at any given time. For something to start with,
try digging up some basic digital circuit books and look up simple diode 
implimentation of the AND gate. Then replace the diodes with high value
resistors.
	There you have it folks, either I am half right or half left...
I mean half right or totally wrong about this  so don't FLAME too much if
it turns out to be another smart aleck answer...
(ps, I hope this gets at least past the modem line I am calling on as this
is my very 1st post on this system)

Chris