Belden 1872A MediaTwist Cat 6 cable.
Last month, we covered unshielded twisted pairs (UTP) to carry audio. The highest-quality UTP is computer cable, premise/data cable. It's called Cat 5, Cat 5e or Cat 6. Cat 5 has been around for more than 10 years, so it may be familiar to you.
While Cat 5 cable is still in use, if you try to buy some, you'll have a pretty hard time. The reason is that Cat 5 cable is no longer part of the EIA/TIA 568 standard. You can still buy Cat 3 (for telephone applications) but the other cables you will see are Cat 5e and Cat 6 cables. Of course, there are billions of feet of Cat 5 already installed, so you might run into it there. It's just hard to buy these days.
In last month's article, we covered the proper procedures to use UTP cable for analog and digital audio, basically putting a single channel on each pair. In the case of AES digital, you have an option of one or two channels per pair. This means a piece of Cat 5e or Cat 6 cable could be an eight-channel digital audio snake cable. Pretty cool!
We also talked about running audio in a networked format, as part of a 100 Base-T Ethernet signal. Then, you can use much of the cheap IT equipment used to run networked data to run your audio or video. However, there's one problem with networks: latency.
What's the hold up?
It takes time to turn audio or video into a data signal. It also takes time to send those packets of data down the cable. If one of the packets has an error, standard Ethernet protocol resends those bad packets, which takes even more time. (Many audio and video networking schemes do not allow resending packets, meaning that the packets must be perfect right out of the gate and remain perfect all the way to the destination device.)
Then, the packets are reassembled, and the audio, video, control signal or whatever you put in at the beginning comes out the other end. All of this takes time. If you're recording the signal, or sending it across the country, a few milliseconds means nothing. However, if you are monitoring your voice audio on a pair of headphones while talking on-air, just a few milliseconds of delay will cause you great confusion, and you can end up sounding like you're intoxicated. So what's causing the delay?
It's really down to number crunching. If you use professional (and expensive) equipment, you may get sufficient speed to minimize the encoding delays. Will the extra cost be worth the expense? Your call.
Don't blame the cable
Don't blame the cable for the delay. Most Category cables have a speed (velocity of propagation) of 70 percent the speed of light. If the signal is audio, you couldn't hear that delay, even if the signal went 1000mi!
Unfortunately, even minute signal delays can be a problem for video signals. After all, the typical analog video signal goes up to 4.2MHz, and that requires a lot more bandwidth than analog audio. This means the number crunching, encoding and latency can get pretty intense. Of course, monitoring a picture that's delayed a few milliseconds is not the same as monitoring audio live on headphones; therefore, latency is less of an issue.
Don't get too satisfied because if you're shipping audio with video, then delay is again a serious problem. If both the audio and video signals are processed separately, the audio is encoded much quicker than is the video. This often creates the notorious lip-sync problem that was so common in the early DTV days. Again, these types of delay are not caused by anything in the cable — they are software, processing and number crunching problems.
One of the problems with video is that it comes in such a wide variety of signal qualities. We start with surveillance video, where UTP is already a major contender. Many surveillance cameras have RJ-45 connectors on them instead of the standard BNC or F connector. The reasons are quite obvious and practical.
First, UTP comes in four pairs, so one cable can carry video on one pair, controls on another pair and double up the remaining pairs for power. Everything you need is in one cable, whereas before it took three separate cables.
There are cameras where the video signal is converted into 100 Base-T data, and the camera essentially becomes a node in a network. In the industrial world, the hot thing with cameras in factories is 1000 Base-T (Gig-E) networked video cameras.
Cat 5e and Cat 6 cable were invented to carry 100 Base-T, which runs at up to 100Mb/s. They were also created to have sufficient bandwith to support 1000 Base-T, which is 1Gb/s. And, yes, there are cameras with incredibly fine resolution that need just that kind of bandwidth. This includes industrial and HD cameras, which are used for looking at things like the insides of ICs or super-fine surface-mount assemblies.
You can imagine if you're in a factory looking at ICs traveling down an assembly line at 1Gb/s, latency can be a serious problem. By the time your camera sees a problem and the fault is recognized, the chip is way down the line. So, number crunching is serious!
Cost vs raw performance
Cat 6 UTP can be easily converted for video or audio applications with baluns. Photo courtesy Belden CDT.
In the broadcast world of 601/SDI and SDI/HD, we're getting to the hairy edge of UTP. Even Cat 6 is tested only to a bandwidth of 250MHz. To carry 1GHz of bandwidth, the signal is split between all four pairs. This means that any timing variations between the cable pairs is crucial to eventually recombining the signals. The timing variations between cable pairs is called delay skew or just skew.
Let's assume we have a broadcast SDI video signal, with a data rate of 270Mb/s (for component SDI) and a bandwidth of 135MHz (Manchester coding). This signal can easily be carried on a single pair of Cat 6. Note that the 100MHz bandwidth of Cat 5 or Cat 5e no longer has sufficient bandwith to carry this signal. Besides, the baluns to convert to balance line are expensive, $80 a piece or more, and you need two, one at each end.
So, can we transport high-quality signals around a BOC or NOC on UTP cable? Is UTP better than coax?
Absolutely not. If you compare raw performance, coax is better in almost every way. When it comes to impedance stability, return loss, attenuation and maximum usable distance, coax is superior.
So why the rush to UTP? Because it's cheap, readily available and easy to install. A couple of well-trained cavemen could easily install it. In addition, UTP cable has natural noise rejection because it is a balanced line. Coax is unbalanced, which makes it susceptible to common mode noise. UTP also has four pairs, so you can combine lots of things onto one cable, making complex installations easier.
Even with the benefits of coax, UTP is approaching coax performance. Some bonded-pair construction UTP cables have a typical impedance tolerance of ±7Ω. That sounds pretty good until you compare it with a precision video coax that is typically specified at ±1.5Ω or less. But, if you compare a twisted pair with even high-quality AES pairs, they're 110Ωs ±20 percent (22Ω variation). Suddenly ±7Ω looks pretty good.
The choice between UTP and coax is simplicity and cost versus raw performance. So why would anyone buy any of those expensive baluns to run SDI? Well, what if you have to run a camera from the GM's office, but, of course, there's no coax going in there? If he has Cat 6 for his laptop, you just need a couple of baluns, and you're in business.
At one state college auditorium, engineers installed three runs of digital coax and a few runs of Cat 6 to run DMX lighting and other computer equipment. Naturally, as soon as they were finished, some VIP showed up, and they needed to support six cameras. So what did our clever engineer do? He purchased some of those expensive baluns and used the UTP cable he already had in place. Halfway through the shoot, he suddenly realized that he couldn't tell which cameras were on coax and which cameras were on twisted pairs.
There is a reason for the similar quality. These are digital signals. With digital signals, as long as all the bits get to the destination, then the picture is perfect. It doesn't matter if they got there on coax, twisted pairs or a pair of Dixie cups with a string. If all the bits get there in the right order, at the right time and with enough signal strength, you can't tell the difference. Perfect is perfect.
In the next article of this series, we'll revisit the world of UTP with a look at the emerging 10Gb/s copper cables, their applications for HD video and the parallel video uses for Cat 5e and Cat 6, such as RGB and VGA.
Steve Lampen is multimedia technology manager for Belden. 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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