Virtually all TV broadcasters around the world have gone digital. Even Hollywood has converted from film to digital images. While some of these cameras record the data on hard drives or SSD (solid-state drives), at some point you’re going to send this data, those digital images, from Point A to Point B. If you want to carry the hard drive or SSD from place to place, that’s fine (so-called “sneaker net”) but it is time-consuming and inefficient. Why not put that signal on a cable to move it from place to place? That’s what most broadcasters do. And, while you could do this on fiber, converting from electrons to photons and back again at the other end, the economical and simple way is still over copper cable, 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, 4×3 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, as shown above, are part of the SMPTE standard and are designed to keep the end-user or installer safely away from the digital “cliff” where attenuation and bit errors force chips to stop working. As can be seen, these “standard” distances are very conservative and most installations can go farther than these distances. Shown as “independent testing” distances. The actual cliff is still further than these numbers. These are not measured distances, they are calculated distances from the formula shown below. 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/50) signal 178 meters on Belden 1694A, a lot farther than 82 meters. And this was just the length of the sample we supplied to them. The actual “maximum” is something beyond that! On the chart above, we have also provided the results of independent “real world” test results showing how far the Belden cables can actually go. In this case, these signals are measured, not calculated.
The recommended “standard” length values shown are based on typical attenuation values for the cables listed and the following criteria:
- Recommended length = 30 dB loss at 1/2 the clock frequency: SMTPE 259M, PAL, Widescreen
- Recommended length = 20 dB loss at 1/2 the clock frequency: SMTPE 292M and SMPTE 424M
The actual “maximum distance” on any cable is a combination of factors including the choice of connectors, the chips in the sending and receiving equipment, and many other factors. When passing these recommended “standard” distances, it is suggested that the user or installer look at eye patterns or bit error rates (BER) to assure system performance. Bit errors can vary dramatically as the “cliff” gets nearer.
Note that 1694A shows a recommended “standard” distance of 82 meters 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.). We just suggest that, if you pass that 82 meter 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, feed throughs 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 it is the guaranteed maximum return loss of our digital cables. These all go out to 4.5 GHz to cover 3gig 1080p/50 applications. Click here to read about 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. The YELLOW line is the maximum return loss we guarantee on our cables.
If you have question or comments, maybe ideas for new cables or connectors, stop by our booth at the IBC Exhibition, in Amsterdam on September 11 through September 15. We’ will be exhibiting with Grass Valley in Hall 1 at stand D11. Or just drop me an email firstname.lastname@example.org.