esp-project 18-help with mod?

[bob_v5]

Hi. First of all im not sure if i have posted this in the right place, same as always
I built the simple surround decoder from the esp site.
http://sound.westhost.com/project18.htm
"A possibility that springs to mind is that we could then subtract the difference information from this output, so that only material that is absolutely common to both channels would be reproduced. "
I have being trying to do this for 12 hours now, and just cant figure it out. I have tried too many differnt things to list in detail, but here is a rough idea.
1-subtract differnce (rear) from sum (centre). didnt work.
2-invert differnce, then add to centre. didnt work.
3-get 2 copies of centre, invert one, add differnce as "noise", then put the whole lot through a differential input opamp. (like whats used to turn balanced to unbalanced)
4-a whole load of variations on the above themes, too stupid and numerous to list.
Any idea of what to try next would be great. I have to admit I dont really know what im doing, but I really tried to figure it out myself! Please help!
(this is where someone says i already did it right, and i was misunderstanding what my (virtual) scope was telling me....doh!)

[bob_v5]

I guess nobody knows then?

[djk]

[bob_v5]

Cheers for the reply, but thats not what I had in mind. First of all, its the line level circuit that I wanted to alter. On the diagram posted here, what I would like to do is have the center signal, without any of the rear signal. This way the center channel would not contain any of the left or right information, to avoid narrowing the soundstage.