FM> This is probably totally naive, but why couldn't you do the normal
 FM> common mode rejection, flip it 180 degrees then add it to the sum of the
 FM> original inputs?
 RM> 
 RM> Line 1: A+B. Line 2: A+C.
 RM> 
 RM> Line 1 - line 2: B-C. Flipped: C-B.
 RM> 
 RM> Flipped added to sum: A+B + A+C + C-B = 2A+2C.
 RM> 
 RM> Flipped added to average (half sum):
 RM> 0.5(A+B + A+C) + C-B = A + 1.5C - 0.5B.

  One plus one equals two = naive

... HINT:  why doesn't line 2 have some B and Line 1 some C?

  If you can do that, you're already there.

Regards,
Bob
___ Blue Wave/QWK v2.12
@EOT:

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