Understanding Video-over-Cat5 cable

 There’s a growing trend in live and installation video circles to move towards the use of Cat5 twisted pair networking cables for long video and VGA runs.

As with many new technologies, it’s easy to get caught out, so read on for a quick primer from an engineer’s perspective…

It seems like magic – run video or VGA signals for 100s of metres with no signal loss at all!! This is the claim of many video-over-Cat5 products, and it’s easy to be skeptical.

Let’s get some basic electronics out of the way first.

Cat5 cable is a multi-core cable using 4 twisted pairs of wires, and was first used in ethernet networking where it uses differential signaling to achieve very good noise and interference immunity over long cable runs.

Differential signalling

The theory is very similar to that used in balanced line microphone connections, i.e. rather than sending a low voltage signal down a long wire leaving it susceptible to interference and noise injection, you send the signal as a small voltage difference between the signal on TWO wires wound tightly together along their length. The noise and interference affect both wires the same, but at the other end you are not looking at the signal, but rather the difference between the signals so if you subtract one from the other, the noise cancels out and you are left with just the signal you wanted.

An additional benefit is that the two signals can be electrically isolated so there is no danger of a ground loop forming.

Differential video

This approach has been applied to audio signals for many years, and then in networking but has recently been applied to long video and VGA signal runs too, where the advantages are similar. It is more of a challenge for video and VGA since the frequencies are higher, but it is now technically possible and can be used very effectively. For composite video a single twisted pair is used, while for S-Video, or VGA multiple pairs are used, and in some cases audio is encoded on the spare pairs as well.

The applications for this are pretty clear:

• Driving video and audio to a remote TV in the foyer or overflow room.
• Avoiding huge clumsy video cable looms that take up space and weight
• routing cables through small gaps or hidden channels to keep everything neat.
• Re-positioning your video control area to a better location without worrying about cable lengths.

…and so on.
Three solutions – only two are real

If you have done some searching on the web for”Cat5″ and “Video” you have probably come across a few different solutions.

There are three basic approaches,  one of which is totally bogus and two of which are worth considering:

•  D.I.Y. using Cat5 cable only. You may come across various home-brew instructions for wiring up a VGA connector to either end of a Cat5 cable, and claiming miraculous results. This approach is totally bogus, as it doesn’t use the differential signaling approach mentioned above, so has no electrical advantage. You are just replacing a matched 75 ohm cable with an unmatched twisted pair, and the results will be terrible. Just walk away from this nonsense.
• Passive ‘Baluns’. This approach uses passive transformers to convert the unbalanced video signal to a balanced signal to drive down the Cat5. Balun stands for ‘Balance/Unbalance’ and these units are effectively bi-directional. The advantages of this approach is that the adapters are cheap, but the disadvantage is that it’s very hard to make transformers that work well at high frequencies, so inevitably you tend to get a roll-off in performance at higher frequencies. This will show up as a loss of detail on edges and the overall effect is to ‘blur’ the signal. In addition in a Balun, because there is no power, there is no opportunity to have any other electronics to ‘pre-compensate’ the signal (see later). These passive baluns are fine for using in non-critical applications, like wiring up a stage confidence monitor, or a TV in the foyer, but you would avoid them for critical applications like your main projector feed.
• Active adapters. This is a more sophisticated approach that uses electronics to convert the signals to balanced pairs, and allows the addition of ‘pre-compensation’ circuits. These circuits can estimate the amount of high frequency losses likely to occur (you select the cable length with switches) and will boost those frequencies artificially BEFORE sending the signal down the cable, so that by the time they get to the other end they are pretty much normal. The end result is that you can get sharper, clearer images using an active adapter than you would get for a passive balun. Of course the active adapters are more expensive and require power supplies at both ends, but for your critical main projector feed, for example, the extra cost is worth it in terms of improved image quality.

So, all together this sounds like a fantastic solution to an age old problem. You can send video for 100s of metres down cheap thin cable and get great results.

So why on earth would you NOT use this approach?  Well, let’s look at the reasons to use and not use this approach…

When to use Video over Cat5?

•  When you have a REALLY long run. Here it’s a no-brainer, since the cost of the adapters will be saved in the cheaper cabling, and you will avoid lots of problems of ground loops, interference, etc. My current rule-of-thumb is that if you are going more than, say, 50m then you should be seriously considering Cat5.
• When you are installing cabling. It is MUCH easier to install Cat5 cable and you may be installing lots of it anyway for computer networking. Adding a few extra runs for some video feeds becomes trivial. Indeed if you have ‘flood-cabled’ your facility you then have the flexibility to move feeds around by just re-patching Cat5 cables. NOTE: Video over Cat5 and Ethernet do NOT mix. You can use the same cables, but you can’t use the same routers, hubs, or adapters.
• When you are only going point-to-point. Most adapters are 1-to-1, and while you CAN get distribution systems, they can be expensive and are proprietary, so you can’t mix and match vendors.

When NOT to use Video over Cat5?

• On short runs. Here the economics don’t make sense and a good VGA cable will probably out-perform all but the most expensive solutions.
• For one-to-many systems. If you are distributing video to various sources, it can get more tricky using Cat5. As mentioned above there ARE distribution systems available, but it can get expensive very quickly.
• When cables will be badly treated. It is possible to get rugged Cat5 cable and ruggedised connectors, but most of the adapters use the standard RJ45 connectors that are quite flimsy and easily broken. If you intend to use Cat5 in a live context I would strongly advise finding a way to use more flexible stranded core cable rather than the normal solid core cable that is not designed to be coiled or flexed. Also there are connectors that use the XLR-type shell with an RJ45 inside, and some time spent making up some adapters would be a good plan.

So…there’s a quick intro to this topic.

I haven’t linked to specific products here, since I don’t like linking to stuff I haven’t tested out, but if you remember the two basic types and choose carefully, there is plenty of scope to make good use of this technology in a VLOBLIVE context.