Pro AV Logo Originally published as a Consultant's Connection
column in Pro AV Magazine
  June 2003

Video over Twisted Pair, Categorically Speaking

Video-over-twisted-pair technology has developed to the point where high resolution signals can be transported effectively up to 1000 feet without any intermediate electronics required.

By Tim Cape, CTS-D

In the late ‘90s, as LCD projectors took over from CRTs and RGBHV content became more prevalent than composite NTSC, the mass of cabling required for training, videoconference and presentation rooms became more complex, varied and expensive. Cable management became a significant part of pro AV system design in an attempt to control the sometimes massive cable bundles of various types going into racks and from point to point. Pro AV professionals would look with envy at our Data/Telecom brethren and wish that we only had to deal with two or three cable and connector types all running under a standard structured cabling system. Many of us also admired their ability to install beautifully combed cable bundles. But hey, what else did they have to do?

As we watched the fiber and Category 5, 5e and 6 cabling going in, digital audio and video were becoming more prevalent, too. Some of it was going onto this structured cabling in the form of compressed audio and video over Ethernet. Over time, low bandwidth audio and video streams have given way to CD-quality audio and MPEG-2 video delivered over the LAN. Audio and control have almost made the complete transition to LAN, at least technologically, if not in everyday practice. But the preponderance of video on the LAN is Real Networks, Windows Media, QuickTime or H.261. Though useful, this is still compressed, low-quality video compared to direct baseband connections and it isn’t acceptable for local video transport from source to projector.

And then there is high resolution RGBHV to deal with. RGBHV images and video can be transported over the LAN using application-sharing and web-based collaboration. Again, this is useful technology, but it is not as well understood as it should be, and there are frame rate limitations in most cases that can’t match a direct video connection to a projector.

Filling the Gap

For now, we have to stay off the LAN for a lot of pro AV video delivery, at least until bandwidth, standards and IT managers evolve. LAN protocols and bandwidth need to improve, MPEG-4 needs to be a real standard and IT managers need to become more comfortable with audio and video content living on their networks. So what do we do in the meantime?

One thing to do is to rely on the old coax standbys. But if we want to save some conduit (and in some cases, some money), we can take advantage of the efforts of several of the video gadget manufacturers to transport video and audio over Category 5 and 6 cabling. This seems like the answer to a dream come true. We can use all that beautiful cabling and simplify our lives? Well, maybe. Transporting NTSC video over Cat 5/6 cabling is not such a problem, but high resolution RGBHV isn’t so easy.

When these products first came out, they were still lacking in end-to-end signal quality, especially at higher resolutions, and we were still in a Cat 5 world. Today, there are still issues being overcome and the progress has been counterintuitive. The primary obstacle is that Cat 6 cabling doesn’t attempt to maintain equal conductor lengths between pairs even though the overall cable bandwidth for data has increased. In fact, it’s even more divergent by design since the twist ratio is varied between conductor pairs to reduce crosstalk. Even so, current systems (and some being introduced at infoComm) are doing a much better job at transporting high-res video that a few years ago and are acceptable for a large number of applications.

This is a good thing since we now have more options for getting high- and low-res video from point to point without the use of esoteric cabling. This is particularly useful on jobs where the cable installation is happening a long time before the system goes in to allow for flexibility later, or where there is limited conduit space that won’t allow for enough RGBHV cables but can accommodate smaller cables to the right places. The big question is, can the original idea of utilizing a new or existing structured cabling scheme for both low- and high-res video distribution actually work?

A Skewed View

Video-over-twisted-pair technology has developed to the point where high-resolution signals can be transported effectively up to 1000 feet without any intermediate electronics required. But instead of the original hope of using twisted pair cabling that is put in place for data, we are finding that there are some limitations for high-res video transport that make that vision a little dimmer.

One thing to keep in mind is that this is a transitional technology. At some point in the next five or 10 years, we won’t have a need for analog conversion delivery options. All of our signals—high res, low res, audio, video and control—will be on the LAN utilizing the standard structured cabling meant for data/telecom networks. This will eliminate the need for 1,000-foot point-to-point runs of “Cat 5-like” cables.

Our work as AV consultants and systems designers is, in the long term, more about infrastructure than about audio and video. That is, it’s about designing a system that performs the necessary functions for the current need in a user-friendly way that also allows for flexibility in the conversion to, or addition of, new components, functions and technologies over the longest possible time. From this point of view, it really is about infrastructure.

I would love to be able to just add some structured cabling runs to the data/telecom installation and make use of it for both high- and low-res video distribution in a new building. But for now we need to be careful about how we go about this type of universal solution. Because of the skew problems created by varying conductor lengths in Category 5e and 6 cables, the compensating technology needs to be more complex (and expensive) and we need to be more aware of the pitfalls. That is why various manufacturers are offering or recommending “skew-free” cables that actually have more in common with Cat 5 cables than Cat 6, and aren’t rated for data. What this means is that for the next couple of years at least we may be putting in cable that won’t be as long-lasting as a more multi-purpose cabling system.

I’m not suggesting that these conversion solutions are to be avoided. Quite the contrary. These conversion and transport devices for low- and high-resolution video and audio are very handy and definitely fill a gap in the leap from coax to LAN video transportation. However, it’s important to be acutely aware of when we can use a data/telecom structured cabling system and when we need to treat these solutions simply as a dedicated RGBHV cable alternative. If we do share a structured cabling systems with the network guys, there are still more hazards that await us in the telecom room where Ethernet switches and twisted pair video switchers need to live together without being cross-patched, and power over Ethernet lurks on the wires for the unwary AV Tech. It’s a jungle out there, categorically speaking.



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