认为不可能在同轴线缆上实施12G-SDI？请再想一想！

Starting Down the Path Toward 12G-SDI

Several years ago, we created an RG-6 video cable (1694A) 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 double the bandwidth of high definition (HD), which reduced distance capabilities of 1694A down to 78m.

Although it has changed over the years, the magic distance for video cables today is 100m (328 feet). I’ve always wondered where that number came from. Isn’t that the distance limitation of data cables like Category 5e, 6 or 6A? How does that apply to video cable?

But then it occurred to me: Most broadcast and video installations use data cables. In fact, some professionals say that these applications will eventually consist solely of data cables. Right now, many installations have a hybrid design with both data and coax cables, so maybe it makes sense that coax cable follow the same rule.

That’s when we decided to create the first cable designed specifically to carry signals up to 100m for 3G-SDI, 1794A – and we did so about five years ago. This was a slightly larger cable than 1694A. Today, however HD pretty much rules the video world, so the souped-up Type 7 cable we created didn’t end up being a hot seller. Even back then, we knew that the next step for video cables would involve 4K.

But would 12G-SDI signal transmission over coax cable ever be possible? Most people said no.

An Important Note About the Digital Cliff

Now is a good time to point out that the distances quoted in this blog aren’t based on field testing. These are calculated values based on a formula that first appeared in SMPTE ST-259M to avoid what’s known as the “digital cliff.”

The farther you go on a cable, the closer you get to the digital cliff. The picture looks fine, so you don’t really know how close you’re getting. But when you install a cable that’s just a little too long, you get no picture. The receiving chip doesn’t see enough signal (or noise and reflections mess up the signal), so you get nothing. The difference in length from a perfect picture to no picture could be just a few feet.

If you know the clock frequency, or data rate, of the application, you can determine how far along the cable you can safely go and still maintain a picture. In any digital data system, the actual data cannot go past a frequency of half the clock. This is called the Nyquist limit.  Originally, the formula for SD-SDI was -30 dB (attenuation) at ½ 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 change to HD, SMPTE standard ST 292 was written with a more conservative formula of -20 dB at ½ the clock frequency. That means that you can’t go as far. This safe distance was very conservative.

How Far Does It Go?

As time went by, and HD cable performance continued to improve, we consistently received this type of feedback from customers: “I went twice as far as your distance chart indicates!” Some customers even said they could go three times as far.

Not only was the cable improving, but so were the connectors and chips sending and receiving the signals. When it looked like 4K would eventually become standard, Belden lobbied to the SMPTE standards group to change the distance formula for these applications. We proposed a new formula and got our wish: -40 dB at ½ the clock frequency. This means that, for a 12 GHz cable, attenuation must be no more than -40 dB at 6 GHz (½ the 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 chart below, this is why the numbers sometimes don’t make sense. The formulas (shown at the top of each column) keep changing. This is just an excerpt of our complete Recommended Transmission Distance Chart. It shows our first 4K cable, 4794R. Download the entire chart here.

There are quite a few new things to notice on this chart. The first is the column for SMPTE ST-425, which covers quad-link 12 GHz 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 cable performance, which already exists. But, with the new formula (-40 dB at ½ the clock frequency), they go farther than they did in the previous SMPTE ST-424 standard, even though they are the same cables you’ve always been using.

If the cables under the -30 dB and -20 dB formulas can go two or three times the distance shown, where’s 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 in the line (patch panels, patch cords and connectors, adaptors, bulkheads/feed-through, etc.)

A New Cable for 12G-SDI Signal Transmission

We recently released a new cable for 4K/UHDTV (12G-SDI): Belden 4794R. This coax cable is the first designed specifically for 4K single-link UHD video cable in the broadcast market for 12G-SDI signal transmission.

Belden’s 4794R coax for 4K/UHDTV (12G-SDI) offers superior performance, easier installation and reduced weight and space compared to quad or dual link. Learn more, or request a sample here. We’re now working on different sizes of 4K cables, and will be introducing each of them as time goes by. And stay tuned for future blogs discussing 12G-SDI signal transmission and our new coax cables!