BVP3 and triax

Here the Sony BVP-3a camera is shown with its dedicated Triax 'back end' attached. This
would be a typical configuration for mobile in-the-field use as part of a multi-camera shoot.
Triax is a clever (though a complex and expensive) system to enable a broadcast or
professional television camera to 'communicate' with its base station, and hence vision
mixer/switcher by means of a single fairly light weight tri-axial cable. Perhaps at first sight one
might wonder why you needed a special adapter to do this seemingly rather simple task,
especially as the 'box of tricks' is so complex (see picture below).

Well naturally, hardly anything is ever as simple as it first seems. Consider a lone cameraman
stuck out on the side of say a football field. Of course we know that the camera's pictures will
have to be fed to an outside broadcast 'scanner', where it will probably be mixed with other
camera images, possibly recorded, or then sent via a microwave (or a satellite) link back to the
main TV station.

Trouble is, that our cameraman will need to be able to send his composite video, component
video (and these days digital video), and possibly the local microphone pick-up as well, over
the long distance back to the truck/scanner without loss of quality. In addition the camera will
need power, probably synchronizing signals, and some or all of its technical parameters may
need to be remotely adjusted (such as iris, RGB gains, white balance, black balance, sub carrier
phase etc). The camera operator will also naturally need to be in real-time two way
communication with his director.

So down our little cable a number of very different signals will need to be travelling both to and
from the camera, all at the same time and with no loss of video (or audio) quality. Simple you
say; just use a multi way cable, but this won't work over more than just a few meters. The video
signal will be quickly degraded and the low DC voltage (12V) loss to the camera would be far
too significant. So the Triax system was developed, and this consists of a base control unit, a
camera adapter (back end in this case) and a simple double screened coaxial cable. Well, the
cable is quite simple, but very tough and quite expensive in itself. These days all digital systems
are used, but in the 1980's a perfectly acceptable mostly analog system did the job. Power was
transmitted to the camera as 250 Volts AC down the two separate screens. In the camera adapter
this was converted to 12V DC by a switched mode regulated power supply. Special precautions
were taken to monitor earth leakage etc to prevent electric shock and nasty things happening
under fault (damaged cable) conditions. The various video and audio signals from the camera
were modulated into various separate frequency modulated radio carriers, and sent down the
centre conductor of the cable. At the same time RF modulated 'talk back' communication, return
programme video feeds, and control instructions, all on different radio frequencies an
modulation modes were travelling in opposite direction back to the camera. The triax camera
adapters sorted all of this lot out, the camera got DC power, synchronizing and control signals,
the cameraman had his programme video feeds and two way communication to his boss, and the
director had 'perfect' video and audio to entertain us with. Simple!

Er, not quite. As you can see from the photograph above, a fair amount of electronics are needed
to perform this rather complicated counjouring trick.

Click on the pictures for further information.