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HDMI Corner: Overcome the Blame Game to Troubleshoot Problems

With an intense reliance on digital instructions, learn how to work through issues when HDMI cables go down.


"We can produce awesome results and conveniences. But when it all goes down, what then?" -- Boccaccio
Jeff Boccaccio · May 14, 2015

I’m sure many of you fondly recall the analog days when seldom did we have to fear that after the job was completed, the system would fail right then or over time. Yes, that was pre-HDMI, but could it also be that when working in a world that relies heavily on digital instructions the odds soar that problems can pop up at any time?

For the most part it has to do with code, code that millions of people are writing and doing so with their own personal flair. Yes there are rules, but many can be interpreted differently. Consider problems that occur due to automation software, audio processors, lighting and more. Firmware updates have become your morning coffee reading. Even our expensive test equipment can go into the toilet due to the intense reliance on digital instructions. On the positive side, we can produce awesome results and conveniences.

But when it all goes down, what then?

Often when an integrator has to get support for a particular hardware a “blame game” can ensue where the problem is blamed on another piece of equipment in the system, or is told that maybe the stars just aren’t aligning correctly. Or even worse, a manufacturer just washes its hands of the problem.

About three months ago we received a call from one of our DPL Labs members stating that products they had been selling for the past three years were suddenly not working properly with some of the latest switching devices, video processors and even A/V receivers. These are cables! And, since they are DPL rated these products are re-tested every three months — what could cause a cable to fail after a perfect track record for three years?

Working with each company we were able to acquire the products to emulate the situation. A ton of work went into this, requiring addition test equipment, test fixtures, and modified methodology to examine and test distorted waveforms and amplitudes that never had to be examined before.

The real downside is that it calls into question not only these particular cables but just about all cables built with active components or passive cables made using some newly discovered advancements in copper and construction to improve their reliably, not hurt it. You have to consider every combination when testing the factorial of various configurations and products in an effort to pinpoint the problem(s).

As an example, five DUTs (devices under test) yield 120 combinations (1 x 2 x 3 x 4 x 5). We had seven DUTs, taking that factorial to 5,040 setup combinations! We ultimately discovered the problems. It went pretty deep, and really had nothing to do with the design of either product. It was the output HDMI processor (chip) producing incorrect TMDS voltage thresholds, illustrated in the graphic. Besides the data, data rate, data packets, info frames, etc., there is also a critical DC (direct current) component that makes this all work.

Shown, the single ended differential pair in one video channel has a nominal high voltage AVcc and a low voltage of (AVcc-Vswing) subtracting one from the other. However, since they make up a differential pair the net signal has a signal twice the amplitude of the single, or 2xVswing, both having their own DC component. It was these DC numbers that caused the failures, potentially impacting a host of products including cables.

At DPL we test, verify, measure, observe and troubleshoot, and make suggestions. But now what? This is where accountability comes in, and I’m happy to report each company responded with the necessary support and resources to help rectify something like this. It just goes to show, there are manufacturers watching out for you and taking charge of each situation.

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  About the Author

Jeff Boccaccio is president of DPL Labs. Jeff can be reached at info@dpllabs.com. Have a suggestion or a topic you want to read more about? Email Jeff at jeff@dpllabs.com

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