The Key to Video Cable Performance
Virtually all TV broadcasters in the U.S. have gone digital. Even Hollywood has converted from film to digital images. While some of these cameras record data on hard drives or SSD (solid-state memory chips), at some point this data needs to be transmitted from point A to point B. If you want to carry the hard drive or SSD from place-to-place, that's fine (a.k.a., 'sneaker net') but it's time-consuming and inefficient. Why not put that signal on a cable and move it from place to place? That's what most broadcasters do. While it's possible to do this with fiber, a more streamlined and economical option is copper—most commonly coax cable.
The photo to the right is Belden 1694A, the world's most popular video cable. It's a good compromise between size, distance, and cost. It is easy to put on connectors, especially the one-piece compression connectors that Belden now offers. (Our current record is two connectors in 33 seconds.) And these connectors, and the 1694A cable, can carry virtually any video signal you might wish to use including analog, 4x3 digital (SD-SDI), high definition (HD-SDI) and even 3gig (1080p/50 or 1080p/60). The only thing you have to consider is how far the cable can go. Here's a chart showing the recommended distance for 1694A and all of our other digital cables.
The serial digital interconnect standards are designed to operate where the signal loss at 1/2 the clock frequency does not exceed the approximate loss value listed below.
The maximum length values shown are based on typical attenuation values for the cables listed and the following criteria:
Maximum length = 30 dB loss at 1/2 the clock frequency: SMTPE 259M, PAL, Widescreen
Maximum length = 20 dB loss at 1/2 the clock frequency: SMTPE 292M and SMPTE 424M
The bit error rate (BER) can vary dramatically as the calculated distances are approached. BER is dependent on receiver design and the losses of the actual coax used.
Note that 1694A shows a distance of 250 ft. at 3G (1080p/60). Some designers and installers say that is not far enough. Well, the numbers in that table are generated by a formula that is in the SMPTE digital standards (SMPTE 292M, 424M etc.). These are not measured distances. A good cable, when driven by good chips, can go much farther. For instance, we have shown at many tradeshows, a Grass Valley 'Trinix' video router, running a 3G (1080p/60) signal 544 ft. on Belden 1694A, a lot farther than 250 ft.
We just suggest that, if you pass that 250 ft. distance, you test the cable to assure performance. Bit errors, eye patterns, anything that shows you that the signal is working. The ultimate way to test any cable (or anything else in the line: cable, connectors, patch cords, patch panels, adaptors, bulkheads, feedthroughs etc.) is to test for Return Loss. We do it in the lab all the time. We do it in the factory on every foot of digital cable that we make. The graph below shows the Return Loss of our one-piece compression HD connectors. The blue line above it is the guaranteed maximum of our digital cables. These all go out to 4.5 GHz to cover 3gig 1080p/60 applications. In the next blog, we'll go into Impedance and Return Loss in detail.
The data above represents the typical performance you can expect from the Belden line of HD Brilliance BNC Connectors. Again, this is not the maximum reading of a single connector, but rather the combined average of over sixty repeat tests.
About the Author
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Steve Lampen, RCDD, worked for Belden for 25 years with his latest role being the multimedia technology manager and product line manager for entertainment products. In 2010, he was named “Educator of the Year” by the National Systems Contractors Association (NSCA). In 2011, he was named “Educator of the Year” by SBE. His book, The Audio-Video Cable Installer’s Pocket Guide, was published by McGraw-Hill.