Because of UTP’s economy over coax, many system designers are turning to UTP transmission equipment to distribute component RGB video. Shown here are Belden’s Brilliance VideoTwist UTP cables.
In the first article of this series on UTP cable, I discussed using it for video. But one aspect we didn't talk about was multiple video streams. After all, UTP normally comes four-pairs to a cable. Does that mean you can run four video streams simultaneously? Well, yes, sort of.
What about crosstalk between pairs? In the original article of this series, last September, we mentioned that the crosstalk at analog audio frequencies was vanishingly low, even -95dB on a Category 5 patch cable, and unreadable on Cat 6. But video is not audio. What's the crosstalk for video frequencies?
This is where the specifications for UTP cable come in handy. For frequencies greater than 1MHz, there are a slew of parameters to check out. They are part of the standard for all cables (EIA/TIA568-B.2). And if the measurements don't fall on the precise frequency you want, such as the 4.2MHz bandwidth of analog video, you can easily extrapolate. So what is the pair-to-pair crosstalk of UTP at 4.2MHz?
Well, that's only half a question, because there are many kinds of crosstalk in UTP — near-end crosstalk (NEXT); far-end crosstalk (FEXT); and power-sum (PS) versions of NEXT and FEXT, where three pairs are energized and one is read. So, PSNEXT would be a good choice to compare, because we're talking about running four videos down the four pairs.
PSNEXT for Cat 5 and 5e at 4.2MHz is -53dB. But what number do you want? Is -53dB good video crosstalk or bad? I've talked to a lot of video engineers about this, and occasionally I see a reference on this subject. Most want to see -60dB crosstalk.
If -60dB PSNEXT is good, then what is -53dB PSNEXT? Not so good? It depends on the kind of video quality you're sending on the cable. If this is an analog surveillance camera (or four surveillance cameras on one cable), then -53dB is probably the most noiseless surveillance picture quality you've ever seen!
If you're shipping analog video around your home, -53dB is better than anything you ever got off air, and maybe even off cable. If you're talking about analog video in a professional installation, then -53dB just doesn't cut it. In which case, you switch to Cat 6 cable, which has a PSNEXT of -63dB at 4.2MHz.
And there's a second factor to consider in a multiple-pair cable. What if you use the four pairs to deliver four parts of a single signal? After all, that's what they do with Gigabit Ethernet. The signal is divided among the four pairs and combined at the other end. In this case, the timing is critical. This is measured as delay skew or just skew. In the data world, the four parts of the signal must arrive within 45 nanoseconds (nsec) of each other.
In the video world, we too have divided signals. They're called “component” signals, such as RGB. They too have to be recombined, and the timing of the signals is critical. In RGB, the signals have to be within 40nsec of each other. It's interesting that these two numbers — 45nsec for data and 40 nsec for RGB — are so close!
Table 1. The relationship between cable length, cable type and skew Click here to see an enlarged diagram.
Table 1 shows how the cable length affects skew. Does Table 1 mean that you can go thousands of feet running RGB? Absolutely not. The attenuation of these cables is the same as it has always been, based mostly on the 24 AWG of the pairs. (Cat 6 can be 23 AWG, even 22 AWG).
What the numbers above tell you is that as your RGB signal is fading away, it is perfectly in time. Few installers would run these cables farther than 250ft, so they might want tighter skew for better resolution, such as a projector in a large auditorium. Table 1 shows that, to a great extent, lower and lower skew does not give you much, because you can't go far enough to use it.
And the ultimate expression of this quest for low skew is the “no-skew” cables that are currently available. While it is impossible to build a cable with no skew, there are four-pair cables with typical skew of 0.5nsec/100m and a maximum of 2.2nsec/100m. The distance then to 40nsec, as shown in Table 1, is ridiculous. Can you go a mile on this cable? No. But, as the signals fade into the sunset, boy, are they in time!
The real problem with no-skew cables is that in order to arrive at these amazing numbers, manufacturers have had to use four pairs with identical twisting (“lay length”). Unfortunately, this results in terrible crosstalk. It won't even pass Cat 3 requirements. (That's now standard telephone cable.) So no-skew is not really data cable at all. It is useful for RGB or VGA applications (or related systems), and that's it!
It is no surprise then that some manufacturers have gone in the other direction. Why not make a cable that will meet Cat 5e or 6, but with ultra-low skew? And, indeed, there are cables that meet Cat 5e with a maximum skew of 9nsec/100m, and Cat 6 cables with a maximum skew of 10nsec/100m. They're also listed in Table 1. You can use them as data cable and as RGB/VGA cables. The photo on page 64 shows each of these cables. The flat blue cable is Cat 6/low skew, the green one is Cat 5e/low skew, and the purple one is identical pair no-skew.
One obvious failing of UTP is that you don't have enough pairs to do RGBHV. You can do combined sync, RGBS, on four pairs. And there are some baluns that put one of the sync signals on green (remember sync on green), so you can run five signals on four pairs.
Four-pair UTP is cheap — even Cat 6 cable is inexpensive — when compared with RGB-bundled coaxes. If your equipment is set up to run twisted pairs and RJ-45 connectors, the cost savings can be attractive. But, if you are adapting coax-based equipment to run on UTP, you have to add the cost of the baluns at the source and destination. And there's the reliability factor of more equipment and more connectors to fail than just cable alone.
Many designers and installers want to run not just RGB but Video Graphics Array (VGA). This may be run like RGB, but it is not RGB. For one thing, it is a progressive scan signal. For another, it comes in different resolutions (i.e. bandwidths and sweep frequencies). Table 2 shows some of the common frequencies of VGA and its cousins.
Table 2. Common frequencies of VGA and its cousins. Click here to see an enlarged diagram.
Table 2 immediately shows a problem. Cat 5 and 5e are specified in the standard only to 100MHz. A few manufacturers test beyond this spec, but many do not. So if you put SXGA or something with an even high-sweep frequency on Cat 5 or 5e, what will happen?
Well, that's the point: You don't know. It could work just fine. Or, the results could be horrible. You just don't know. You really have to see if the cable you're using is tested to the appropriate frequency for your application. Or, you could just move to Cat 6, which is specified out to 250MHz.
Of course, there are many formats beyond those in Table 2. The simple rule is that the cable you use must be tested and verified out to the frequency you need. This is one of those “gotcha” situations when using existing older Cat cables for non-data applications.
If you have comments or suggestions, or wish to continue this discussion offline, just send an email to firstname.lastname@example.org.
Steve Lampen has worked for Belden for 14 years and is currently multimedia technology manager. He holds an FCC Lifetime General License, is an SBE Certified Radio Broadcast Engineer and a BICSI Registered Communication Distribution Designer. His latest book, “The Audio-Video Cable Installer's Pocket Guide,” is published by McGraw-Hill.
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