Beginning with the 12G-SDI (II)

Most transmission and video facilities use data cables. In fact, some professionals say that these applications will eventually only consist of data cables.

Steve Lampen *

We 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 the 12G-SDI signal transmission. The 4794R coaxial from Belden for 4K / UHDTV (12G-SDI) offers superior performance, easier installation and reduced weight and space compared to quad or dual links.

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At first glance it is difficult to see what makes this cable different. It still looks like a coaxial cable. But what happens under the cable jacket is what really makes the difference. There are five factors that establish this new 4K coaxial cable design as a separate case:
 
1 Copper conductors lined with silver

It is easy to compare Belden's new 4K coaxial cable design with other coaxial cables. Simply compare them side by side. Remove the jacket, the foil, the mesh and the dielectric, and examine the driver. The central conductor looks different, it's not even copper. Instead, it's silver copper. Silver is the best driver; its use reduces the resistance of all signals and is especially effective for those high frequency signals that travel the surface. Due to the "skin effect", it is important to make sure that the surface of the driver is as perfect as possible. On all our digital video cables, the center conductor surface has a mirror finish. In this 4K cable, it is a silver layer with a mirror finish.
 
2 A mixture of plastic and air bubbles

In the coaxial cable, the dielectric is made of a combination of plastic and air bubbles (nitrogen, actually - much more accurate than regular air). Known as "foam polyethylene injected with nitrogen gas", the amount of nitrogen impacts the speed of the signal traveling along the cable, called the Propagation Rate (Vp). It may seem that adding air would result in a soft plastic (and could if you do not know how to do the gas injection). But the result in the Belden factories is really a hard material. This keeps the center conductor in the center of the dielectric, essentially to maintain the impedance of the cable (for the video coax, the impedance is 75 ohms). A soft and soft material would mean that the manufacturer does not know how to make "Gas injected / foamed" very well. This hard material is called "high density hard cell nitrogen with injected foam". If you are comparing cables, simply tighten the dielectric of each cable with your fingers.
 
3 New type of polyethylene
Another notable change in Belden's 4K coaxial cable design is a new type of polyethylene (the most common plastic in the world). There are hundreds of varieties of polyethylene, all offer different levels of performance and price. The polyethylene used in these new cables offers significantly lower losses than the previous types. As far as we know, this is the first time that this type of polyethylene has been used in a video cable.
 
4 Armor plate attached to the core
Together, the central conductor and the dielectric that surrounds it make up the "core" of the cable. A sheet protector is placed on the core. In the past, this first protector was left loose, wrapped around the plastic core. But the new 4K coaxial cable design joins the foil to the core. With one-piece BNC connectors, the loose sheet may become wrinkled as the connector pushes it, otherwise it is done correctly, which makes the connection bad. With the sheet attached, "wrinkling" is not possible. In addition, the bound sheet stabilizes this layer so that it can not move. And this also stabilizes the cable impedance, which is determined by the distance from the center conductor to this layer of sheet. The BNC slides perfectly, making a quick connector even faster.
 
The new 4K coaxial cable from Belden has only two things in common with its predecessors. First, both versions are the same size. This was achieved by maintaining the same Vp at 83%. Maintain the same dimensions as the previous version allows to use existing connectors with this new cable. Second, the cables have the same PVC jacket. The legend of the print is different, of course, since the new cable is marked "12 GHz 4K UHD." But that's where the similarity ends!
 
Cable mesh performance
After the first sheet, comes a braided layer with a coverage of 95% - the largest possible coverage in a single mesh. Improving braiding performance is one of the most difficult aspects of creating a new 4K coaxial design. If you look at the cable impedance (return loss), the old designs have a lot of peaks at different frequencies. These are caused by the dimensions of the braid, the relationship between the individual conductors, how the braids are woven together, the angle at which they cross each other (braid angle) and many other factors. These peaks were often found at very high frequencies. Most HD cables stop at 3 GHz (or 4,5 GHz for 1080p / 50-60). If the peaks were higher than those frequencies, then they did not matter. There was little signal above, so we ignored them. But now, at 6 GHz or 12 GHz for 4K - and finally 24 GHz or more for 8K - we have signals where the peaks are. We can not ignore them anymore.

The simplest way to deal with these peaks is to simply move them up in frequency. The problem: when we revisit this for 8K, those peaks will be there, bigger than ever, and much harder to deal with. With these new designs, we have been able to test the mesh machines and mark them for the most suitable type of cable - some for 6 GHz, others for 12 GHz. Even these braiders, which are quite old machines, are entering a new era of precision and quality.
 
Below is the performance resulting from the return loss of one of Belden's first 4K cables. The green trace shows a typical cable return loss at 12 GHz. The red line on the green sampling result is a guarantee of return loss. No cable will be worse than those values ​​in red. The blue line represents the limits suggested by SMPTE for the return loss. There is a peak in the 9,5 GHz approx, which is -20 dB return loss (99% of the signal passes and the 1% is reflected). The rest of the cable is between -30 dB and -40 dB of return loss.

The advantages of the "Tri-Shield"
Now, let's go back to the design of the new 4K coaxial cable. On the cable braid of which we have just spoken there is another sheet (foil). This aluminum foil is called "tri-shield". Like the other foil, it is also stuck. In this case, it is attached to the liner. When we strip the cable of this jacket, the first sheet comes out with it. Most installers did not even realize that I was there. But this gives a high shield efficiency - better than 120 dB of noise rejection. This is especially effective for long-distance cables. For smaller cables, we could return to a single sheet, attached to the core and a mesh on it. That will make it a little more flexible, and people expect flexibility from small cables. In addition, small cables do not go that far, and are often used in rack-to-rack cabling, so the possibility of noise and interference is reduced again. A tri-armoring might not be as effective for these cables.

At 12 GHz, our new cable design has improved 8 dB attenuation compared to the previous design. This improved attenuation is due to the silver-coated copper core conductor and the improved polyethylene dielectric, which we mentioned earlier.

If you have questions or comments, please contact me at steve.lampen@belden.com or with our partners in Latin America (juan.cortes@belden.com)
 
* Steve Lampen joined Belden at 1989, being currently product manager for broadcast. Before Belden, he worked as an engineer in radio, film and television. It has an FCC license, SBE and BICSI certifications. At 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
 

Quoting products and technical services for Latin America

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