[cma-l] Request for advice on cabling a TV studio!

[Two Lochs Radio]

Hi Dave 

Wiring the rooms with multiple Cat 5 cabling is an excellent and very practical way to provide for flexible connections between rooms. Cat 5 cable is very well suited to handling computer networking (of course), balanced audio, balanced video, digital audio, MIDI, security video and power, USB extension, telephone (inc ISDN and broadband), remote keyboard/screen control and numerous random control signals.

However, note I said 'balanced' audio and video. You are probably well used to using balanced audio signals and they are perfectly suited to Cat 5 over extended distances, using twisted pair of unshielded or lightly shielded cabling. However, standard video signals are 'unbalanced' and designed for single-core cables with heaving coaxial screening, and will not travel successfully in that form on Cat5 cable. To run them over twisted pair cable such as Cat5, you need to use little balancing/unbalancing transformers at either end - known by the shorthand of 'Baluns'.

These used to be very expensive, but quite good performance ones are now widely available at around £50/pair for composite video, and will send a clean signal typically at least 50m on Cat-5. Longer runs may need active amplifiers and 'equalization' to minimize ghosting and loss of definition, but with good equipment and good transformers it's not always needed even for a few hundred metres.

One cat 5 cable has 4 twisted pairs and can carry composite video and two stereo channels down one cable, with a pair spare. But if you are running them all down one cable for much distance it's extra important to get well-matched baluns and use full spec Cat-5 cable to minimize the risks of crosstalk, which would lead to 'video buzz' on the audio. If this is a problem or serious concern, it can be minimized by using shielded twisted pair (Cat 5 STP) for extra shielding and run video and audio on separate Cat5 cables.

This is assuming you are using composite video - where al colour and luminance is combined into a single composite video signal. Things are a bit more complex if your equipment and quality demands the use of 'component RGB' where the red, green and blue (and possibly 'syncs') need to travel on separate pairs. In that case, for starters you need 3 or 4 baluns at each end (multiple baluns are available for the purpose in one box), but you also need to ensure matched timings between the signals, andt his can be more tricky. In particular, you need to stay away from Cat-6//7 cabling as the tecniques these use to boost digital throughput an dminimize crosstalk cause delay differences between different pairs. For high-quality work, expensive line drivers are used which include equalizers and pahse shifts to compensate for cable delays.

If you are getting to that level of work you almost certainly need to call in a prefessional expert, but for standard audio and composite video I would just arm yourself with a pile of decent baluns form areputable supplier and give it a go (or experiment first with really cheap ones from eBay in case you find that's all you need - you'll find ones meant for CCTV cameras for under a tenner a pair!).

There are loads of suppliers of comercial quality baluns, including:
http://www.cyberselect.co.uk/product/438
http://www.nexxia.co.uk/Audio_Connectors/Cat5_audio_video_baluns.htm?gclid=COnU6vie_p8CFZAA4wodgkdNkQ
http://www.lindy.co.uk/cctv-video-balun-cat5e-6-extender-1-x-bnc-to-2-pin-connector-2-pack/32533.html
http://www.blackbox.co.uk/products/display.asp?id=0&doc=ic443a-r2&tx=7&sx=3
http://www.betterbox.co.uk/acatalog/Product_List__VGA_RGB_Component_Video_Baluns_552.html
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