Home > Gear, Video > SDI is not composite video.

SDI is not composite video.

February 20, 2008 Leave a comment Go to comments

bnc-plug.jpgAs an amateur radio operator, I’ve learned first-hand how the type of cable used to connect the antenna to the radio makes a huge difference to how well things work.

So I am continually surprised when video ops plug whatever cable they have on hand onto whatever BNC jack that needs it. There’s a lot to consider here as our productions move to HD-SDI.

lan_cable_tester_22.jpgFor instance, in local area networking, as well as radio, BNC jacks are used. These jacks are 50Ω of resistance. They expect to see plugs and cables that have 50Ω of resistance. Typical video cables are 79 /79Ω of resistance. Putting the wrong BNC cable on the wrong jacks creates a resistance mismatch and some signal gets bounced back.

Extron has published a detailed overview of how to calculate maximum cable length for SDI video. They cover important aspects like clocking, re-clocking, jitter, wander, and numerous other considerations never-before considered back in the composite days. This goes far beyond “velocity matched” cables for component (to make sure all three colors arrive at the same time).

Utilizing the maximum calculated cable length in a primary distribution run for SDI is NOT a good idea. Suppose you have made the maximum length run. Now, you connect a 10-foot patch cable at the end to include some other device and, suddenly, there is no video image! You have just experienced the “cliff effect.” When the loss parameters of the SDI signal exceed the receiver’s ability to recapture the data, the system completely fails, ungracefully.

sdifig5.jpg

Cable loss affects the amplitude of the SDI signal while jitter affects the zero crossing point of the data edges. The data edges appear to dance back and forth with random uncertainty. There is a jitter budget allowance, but since noise and jitter effects can become generally random, bit error rate can creep up periodically and cause lost data. If the jitter budget is exceeded, data cannot be recovered at all.

If you are looking at building some new production setups involving SDI and HD-SDI, you need to take a few moments and start to build your cable knowledge for the digital millennium.

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