Most transmission and video facilities use data cables. In fact, some professionals say that these applications will eventually consist solely of data cables.
Steve Lampen*
4K is a term commonly used to describe the resolution of the video screen which is about 4000 pixels. That's about four times the resolution of HD at its highest version: 1080p. The 4K broadcast version, called UHD (ultra-high definition), has a resolution of 3840 pixels by 2160 lines. DCI (Digital Cinema Initiative), the 4K version of Hollywood, has a resolution of 4096 pixels by 2160 lines. Both have a clock speed of about 12 GHz, hence the 12G-SDI.
Several years ago, we created an RG-6 (1694A) video cable that carried HD over 370 feet (113m). But when 3G-SDI hit the scene, that video signal — also called 1080p/60 or 1080p/50 — was twice the high-definition (HD) bandwidth, which reduced the 1694A's distance capabilities to 78m.
Although it has changed over the years, the magic distance for video cables today is 100m (328ft). I've always wondered where that number came from. Isn't that limiting the distance of data cables like Category 5e, 6 or 6A? How does this apply to the video cable? But then it occurred to me: Most transmission and video facilities use data cables. In fact, some professionals say that these applications will eventually consist solely of data cables. Right now, many facilities have a hybrid design with data and coaxial cables, so maybe it makes sense for coaxial cable to follow the same rule.
That's when we decided to create the first cable specifically designed to carry 3G-SDI signals up to 100m:1794A – and we did it about five years ago. It was a slightly larger cable than 1694A. Today, HD almost rules the video world, yet the RG 7 cable we created didn't end up being a super seller. Even then we knew that the next step for video cables would involve 4K.
But would 12G-SDI signal transmission by coaxial cable be possible? Most people said no.
An important note about the digital cliff
Now is a good time to point out that the distances cited in this blog are not based on field tests. These are calculated values based on a formula that first appeared in SMPTE ST-259M to avoid what is known as the "digital cliff". The farther we go on a cable, the closer you get to the digital cliff. The image looks good, so we don't really know how close we are to stopping receiving it. But when you install a cable that is too long, you no longer get any picture. The receiving chip doesn't see enough signal (or noise and reflections mess up the signal), so it gets nothing. The difference in length from a perfect image to no image could be only a few feet.
If you know the clock frequency, or data rate, of the app, we can determine how far the signal can go safely and maintain an image along the cable. In any digital data system, the actual data cannot exceed a frequency of half the clock. This is called the Nyquist limit. Originally, the formula for SD-SDI was -30 dB (attenuation) at 1/2 of the clock frequency. Using this formula, digital signals could easily be sent hundreds, even thousands of feet before reaching the -30dB distance. Then, with the switch to HD, SMPTE ST 292 was written with a more conservative formula of -20 dB at 1/2 clock frequency. That means you can't go that far. This safety distance was very conservative.
How far does it go? Over time, the performance of the HD cable continued to improve, and we consistently received this kind of feedback from customers: "I went twice as far as their distance graph" Some customers even said they could go three times further. Not only did the cable improve, but also the connectors and chips that sent and received the signals.
When it looked like 4K would eventually become standard, Belden pressured the SMPTE standards group to change the distance formula for these applications. We proposed a new formula and got our wish: -40 dB at 1/2 of the clock frequency. This means that, for a 12 GHz cable, the attenuation should not be greater than -40 dB at 6 GHz (1/2 clock frequency of 12 GHz).
For the 6 GHz version of 4K, the formula would be -40 dB at 3 GHz. When comparing the data in the distance table below, this is why it seems that the numbers sometimes don't make sense. Formulas (shown at the top of each column) keep changing. This is just an excerpt from our Table of Recommended Transmission Distances. And it shows our first 4K cable, 4794R.
There are quite a few new things to emphasize in this chart. The first is the column for SMPTE ST-425, which covers 12 GHz quad-link for UHDTV1. This was the original 12 GHz delivery system, which split 12 Gbps into four cables. In that case, each cable carries 3 Gbps/3 GHz for a cable that already exists. But, with the new formula (-40 dB at 1/2 of the clock frequency), they go further than in the previous SMPTE ST-424 standard, even though they are the same cables that have always been used.
If the cables under the formulas -30 dB and -20 dB can go two or three times the distance shown, where is the cliff for the 12G-SDI cable - especially in the SMPTE ST 2082-1 column? The actual location of the cliff is influenced by the quality of the installation, the connectors chosen, the equipment, the chips, the connectors and everything else on the line (connection panels, connection cables and connectors, adapters, feed-through, etc.)
A new cable for 12G-SDI signal transmission
We have recently launched a new cable for 4K/UHDTV (12G-SDI): Belden 4794R. This coaxial cable is the first designed specifically for the 4K single link UHD video cable in the broadcast market for 12G-SDI signal transmission. Belden's coaxial 4794R for 4K/UHDTV (12G-SDI) offers superior performance, easier installation, and reduced weight and space compared to quad or double links.
If you have questions or comments, please contact me at [email protected] or our colleagues in Latin America ([email protected])
*Steve Lampen joined Belden in 1989, currently a product manager for broadcast. Prior to Belden, he worked as an engineer in radio, film and television. It has FCC license, SBE and BICSI certifications. In 2011 he was named "Best Educator" by the Society of Broadcast Engineers. His book "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill. Follow him on his blog www.belden.com/blog
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