[OT] ish ..... Power over Coax for Camera

[nick@xxxxxxxxxxxxxxx]

One of my cameras (the oldest) is fed from a Coaxialable (sic) Power
Supply.  This is badged as "Constant Current 200mA" and has two
BNC
connectors.  One connects to the camera and the other to wherever.
I have yet to get the meter out, but I am assuming 12 volts DC.

My newer cameras are all on a four core cable - two for the signal
and two for the power, so this old one is the odd one out.  I don't
want to replace it because replacing the cable run would be
tortuous.  However, I am conscious that the supplier told me (when
the newer cameras came along) that the coax power supplies can be
quite unreliable.

As I already have a copious supply of 12 volts DC, is it a
relatively simple matter to knock up a circuit to inject some into
the coax leading to the camera (presumably whilst isolating it from
the video feed on to the rest of the system)?  If so, it would seem
sensible to dispense with the separate mains driven PSU.

All thoughts welcome,

Nick