Organ Grinder wrote: ↑Tue Oct 04, 2022 7:13 pm
Well now I'm confused! I though you had live and earth (or ground)
It's signal and screen, but yes, two connections.
However, video (and digital audio and RF) are all high frequency signals that require the cable to have specific characteristics if the signal is to get to the other end intact.
One of those characteristics is called 'characteristic impedance' and its why the cable is identified as 75 Ohms or whatever. The connectors must have the same characteristic impedance too.
The characteristic impedance is determined by the dimensions of the cable and plug/socket – the spacing between the signal core and screen – and the insulating materials between and around them.
That's why BNC connectors have a specific insulation sleeve and screen terminating ferrule, and why the cut cable dimensions are critical. They are fiddly to assemble with the proper tools, and a nightmare without.
Crudely joining two dissimilar cables will create an impedance mismatch, degrading the cable's ability to convey the signal.
RCA-phono plugs/sockets aren't actually 75 Ohm connectors, but they're close and cheap enough for non-professional applications. That's why they're used on consumer video and digital audio equipment in place of the standard professional 75 Ohm BNCs.
And that's why the best solution for interconnecting video gear using BNCs with RCA-phonos us to use standard BNC cables with phono adapters as I linked above. That way the characteristic impedance is maintained and everything works.