Update:
ATTENTION The cable pairs in CAT6 cable are not wired this way !!!! This circuit works for small distances but on longer runs channel 2 talks into channel 3 and vice versa.
Didn’t redo my boards, did just cut and rewire wrong traces.

Cable pairs are (1,2) / (3,6) / (4,5) / (7,8)

This is nothing new and has been done in the security CCTV world for years, but I’m still surprised it was so easy.
Thursday we have to make a multicamera capture of the premiere of the Fribourg Opera – in Fribourg’s brand new theatre – which is huge – and we just don’t have enough cables to get all the signals out of the venue into a quiet place where I can do the mixing.

But they do have kilometers of CAT6 cabling, patchbays everywhere, and even some Extron BNC-RJ45 adaptors to send video throughout the building. But why use one adaptor and one cable per camera – there’s 8 wires in a ethernet cable. It should be possible to send 4 symmetric video signals, each on a twisted pair..
Using an inverting opamp to make a symmetric signal doesn’t sound too terrifying, and a differential amplifier on the other side to get an asymmetric out of it again, neither.
So last night after returning from the theater I fired up EAGLE and sketched a 4 channel BNC video to RJ45 converter, and surprise: It seems to work really well.
Dead simple. Just beware that instead of the LM833 I’m using MAX4392 opamps which are much better suited for video (pinout is the same, I simply can’t find the symbol for it in EAGLE)

I tested one channel today at the venue through around 100m of CAT6 and the picture was perfect. Can’t wait to see all four cameras tomorrow..

The transmitter:

Two channels shown in closeup

The receiver

EAGLE files

Very quickly hacked together – should have done other things today and it has to work tomorrow:
4BNCToRJ45_20120124.zip

Pictures

I’m really in a hurry, this has to work tomorrow… no time to build a case…

Transmitter