Digital Signage

Resolution & Cable Length: An Inverse Relationship

Checklist Item Under Test: 3.2.7: The design will pass the required system bandwidth, taking into account supported video resolutions and estimated cable lengths along the entire signal path.

Reasoning: Manufacturers and clients are clamoring for higher resolutions. Although their effectiveness at typical conference room viewing distances is questionable (I still recommend a preferred resolution of 720p in my EDID plans for readability in most spaces), we certainly need to plan for it. It is important to realize that, in most cases, as video signal bandwidth goes up, allowable cable length goes down. Thus, if a particular cable length passes a 1080p signal perfectly well, it might be too long to pass a 4K signal. Similarly, if an HD-SDI cable passes a 1.5G (1080i) signal, it might not pass a 3G (1080p) signal. It is important to take cable length into account for new installations, as well as system upgrades. It can be a very costly oversight.

The Story: It was a rushed job. The client needed to upgrade the system to use a 4K display over the weekend. The whole video solution was going to be replaced as well, to be able to pass 4K signals. I shouldn’t say the “whole” video solution. We would be reusing the cables and just replacing the switcher and endpoints. And why shouldn’t we? The system was only put in two years ago with shielded Cat6 cabling. Both the cable manufacturer and the video solution manufacturer recommended this type of cable for exactly this type of 4K system.

The location for the upgrade was New York City, where real estate is at a premium. The annual rent on a 50-square-foot rack closet is roughly $4000. (You read that right: A small closet costs commercial NYC tenants about $330/month. Oy!) In order to alleviate that cost, this particular client had one consolidated rack room for the entire floor. The head-end equipment for all the different conference rooms was located in a rack room with 10 other racks to drive the 11 systems on the floor. With this particular HDBaseT solution, all was well at 1080p. However, all was not well at 4K.

We could not get the signal to light up on the display. We brought the source to the display with no problem (source and sink were OK). We qualified the cables between the room and the rack with no problems (tested successfully to “1000BaseT”). The video switcher registered the transmitter and receiver just fine. It should have worked, but it didn’t.

I asked one of the specialists to run the cable qualification test one more time, and she again said, “The cable is good. The cable terminations are good. It passes through 1000BaseT. The cable length is only 85m.” Everyone knows that Catx cable is good to 100m for network data. But, as she said 85m, my heart sank. I forgot to check the cable length limitations for the new resolution.

As it turns out, this particular cable with this particular system could pass 1080p up to 300 feet. However, it could only pass 4K up to 150 feet; 85m is roughly 280 feet. We were out of luck. We didn’t leave them with a nonfunctioning system, of course. We just configured the EDID to a limited number of resolutions excluding 4K, but we did have to have a tense conversation with the client, explaining that their existing copper infrastructure would not pass the signal. It would have to be replaced with different cabling or a fiber solution, both being very costly.

There are many ways to test for cable length. You can trace out the cable and get an estimate if the path can be seen/followed. You can use a TDR to get the length of cable while qualifying it, and many cable qualifiers have TDR capabilities. Some cable manufacturers even print distances on the cable jacket, so you take the difference between the numbers at each end and that is how long the cable is.

The point is to keep cable length in mind when stepping up resolutions. This is true with HDBaseT solutions, as well as HD-SDI solutions. As resolutions go up, the required bandwidth goes up, and there is a good possibility that allowable cable length goes down, sometimes dramatically.

That was a sad day, but a very important lesson was learned and checklists were updated so it wouldn’t happen again. In the analog days, if a cable was too long, the signal was just a little dim or a little fuzzy. It still worked, kinda. In the digital world, there is a cliff effect, where the signal remains perfect until you pass the allowable cable length, and then it just won’t work at all. The signal “falls off the cliff” at that distance. Stay away from the cliff, my friends. Enjoy the view several feet away from that edge, and so will your clients on their 4K displays.

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