Getting only the "common" signal

johnr66 · 12th February 2010, 02:29 AM

I know I can take the difference of the left and right channels (L-R) to produce a quasi rear channel. The sum of L and R to get a center channel or mono sound. The problem is, if there is a signal in only L or R channel it still appears quite strong in the mono (L+R) channel. It is fine and quite normal, but lets say I want to diminish output to the center channel unless the signal is common to both channels, how can I do this? I'm guessing some sort of multiplier, So 2L*0R = 0C, or 2L*2R=4C

This is not a home theater project, but listening to only the L, R, L-R or L*R lets me here instruments or vocals that were covered by the louder vocals or instruments.

godfrey · 12th February 2010, 08:13 AM

I'm pretty sure it's not possible to extract only the common signal.
btw - multiplication will give pure distortion e.g. piano * trumpet = nasty mess

star882 · 12th February 2010, 10:55 PM

Take the FFT and look at what's common to both?

Skip Pack · 12th February 2010, 11:39 PM

This has been something I have thought about doing for
years without getting to a good conceptual answer. The
FFT analysis might work but I'm not sophisticated enough
to judge that possibility.

My basic notion is that you would have vastly improved
image stability, potentially more accurate as well, with
a true center only (common) signal. Two speakers, near
left and right, with the standard stereo signals, and far
left and right channels with the left and right only signals.
I'd love to hear that.

I have never heard nor seen a mathematical description
of the trifield method of synthesizing multiple channels from
a stereo signal. I have heard it done on Meridian systems and
it's impressive. Jim Bongiorno sells a box that creates three
channels, I believe on the same or similar principles. Let's
hope this thread brings some approaches to light along
those lines.

Skip

Steerpike · 13th February 2010, 11:50 AM

It can only be done digitally. Done, I believe, by means of autocorrelation.
The later Dolby Pro-Logic decoders did this to extract the dialogue channel
(I have a Yamaha DSP1000 that does this - that part of it is quite effective, but overall it is not a pleasant sounding piece of machinery)

Once you have the common signal, you can subtract it by analogue means from the matrixed Left channel and matrixed Right channel signals to get a more-left-than-left and more-right-than-right.

Ron E · 09:53 PM

L, R, L+R and L-R are all you can do passively, AFAIK

TheSeekerr · 13th February 2010, 10:42 PM

What if you were to do:

"Common Channel" = L - (L-R)

Doing L-R should leave you with the material which exists only in the left channel. Subtracting that from the left channel should give you only the material which is common to both channels.

I can't see why it wouldn't work, but it seems too obvious to have been overlooked. What am I missing?

Steerpike · 13th February 2010, 10:53 PM

Quote:

Originally Posted by theseekerr

what if you were to do:
"common channel" = l - (l-r)

l - (l-r) = + r

TheSeekerr · 14th February 2010, 01:56 AM

I can see that algebraically, but I'm having trouble with it logically. Shall have to break out Audacity and prove it to myself, I guess....

ragnar59 · 14th February 2010, 05:29 AM

Talking Heads "Once In A Lifetime (stereo difference)"

is this what you're trying to do

12th February 2010, 02:29 AM	#1
johnr66 diyAudio Member Join Date: Mar 2009	Getting only the "common" signal I know I can take the difference of the left and right channels (L-R) to produce a quasi rear channel. The sum of L and R to get a center channel or mono sound. The problem is, if there is a signal in only L or R channel it still appears quite strong in the mono (L+R) channel. It is fine and quite normal, but lets say I want to diminish output to the center channel unless the signal is common to both channels, how can I do this? I'm guessing some sort of multiplier, So 2L0R = 0C, or 2L2R=4C This is not a home theater project, but listening to only the L, R, L-R or L*R lets me here instruments or vocals that were covered by the louder vocals or instruments.

12th February 2010, 10:55 PM	#3
star882 diyAudio Member Join Date: Mar 2007	Take the FFT and look at what's common to both? __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

13th February 2010, 11:50 AM	#5
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	It can only be done digitally. Done, I believe, by means of autocorrelation. The later Dolby Pro-Logic decoders did this to extract the dialogue channel (I have a Yamaha DSP1000 that does this - that part of it is quite effective, but overall it is not a pleasant sounding piece of machinery) Once you have the common signal, you can subtract it by analogue means from the matrixed Left channel and matrixed Right channel signals to get a more-left-than-left and more-right-than-right. __________________ Steerpike's Toybox

13th February 2010, 09:40 PM	#6
Ron E diyAudio Member Join Date: Jun 2002 Location: USA, MN	L, R, L+R and L-R are all you can do passively, AFAIK __________________ Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan Science is a way of thinking much more than it is a body of knowledge. --Carl Sagan Last edited by Ron E; 13th February 2010 at 09:53 PM.

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12th February 2010, 08:13 AM	#2
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	I'm pretty sure it's not possible to extract only the common signal. btw - multiplication will give pure distortion e.g. piano * trumpet = nasty mess

12th February 2010, 11:39 PM	#4
Skip Pack diyAudio Member Join Date: Feb 2004 Location: Northern California	This has been something I have thought about doing for years without getting to a good conceptual answer. The FFT analysis might work but I'm not sophisticated enough to judge that possibility. My basic notion is that you would have vastly improved image stability, potentially more accurate as well, with a true center only (common) signal. Two speakers, near left and right, with the standard stereo signals, and far left and right channels with the left and right only signals. I'd love to hear that. I have never heard nor seen a mathematical description of the trifield method of synthesizing multiple channels from a stereo signal. I have heard it done on Meridian systems and it's impressive. Jim Bongiorno sells a box that creates three channels, I believe on the same or similar principles. Let's hope this thread brings some approaches to light along those lines. Skip

13th February 2010, 10:42 PM	#7
TheSeekerr diyAudio Member Join Date: Jan 2008	What if you were to do: "Common Channel" = L - (L-R) Doing L-R should leave you with the material which exists only in the left channel. Subtracting that from the left channel should give you only the material which is common to both channels. I can't see why it wouldn't work, but it seems too obvious to have been overlooked. What am I missing?

14th February 2010, 01:56 AM	#9
TheSeekerr diyAudio Member Join Date: Jan 2008	I can see that algebraically, but I'm having trouble with it logically. Shall have to break out Audacity and prove it to myself, I guess....

14th February 2010, 05:29 AM	#10
ragnar59 diyAudio Member Join Date: Feb 2010	Talking Heads "Once In A Lifetime (stereo difference)" is this what you're trying to do

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