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Digital Signal Idio-Synchronicities: Digital video problems. Part 2

Checklist Items Under Test: 6.46: For HDMI signals, using an appropriate HDMI generator, display with HDCP enabled, for the following resolutions and timings, as required in the design (check all that apply):

Inspect each, leaving the signal on for several seconds (make sure no “sparklies” are seen): Appropriate HDMI Generator required.

Reasoning: It is fairly obvious when there is a problem with analog signals. If audio is distorting, it sounds squared. If wireless microphones are interacting with other transmitters, the intermodulation sounds like a feedback squeal. If video signals are too strong or too weak, they appear bloomed or dim, respectively. It’s easy to interpret problems in the analog realm; not so in the digital world. When dealing with digital video, oftentimes the signal passes, it doesn’t pass at all or, in the most annoying cases, video is intermittent. Determining where the problems are can be a daunting task.

The Story: One of our specialists wasn’t going to be able to meet the display technician who was coming to our shop to repair a 60-inch LCD TV. He asked if I could cover for him and walk the technician through the issues we were having with the display. I said to myself, “Self? How hard could this be?” I told our specialist that it wouldn’t be a problem. I’m a 19-year veteran of the industry….How much time could this take? I had no idea what I was walking into.

As per our ISO 9000 Quality Management System, the non-conforming display was tagged with a description of the issues. I saw the tag from across the room, and as I got closer, I saw my first clue that this was not your typical “broken TV problem,” like shipping damage or a non-functioning power supply. The tag read like this:

“Display will not transmit EDID information consistently to the digital video switcher while in standby mode. It can take up to five minutes for a source to display when going through the switcher, if at all. The switcher, cable and receiver were all successfully tested with another display of the same make and model. If the source does not display at all, removing power completely from the display will at least bring back the delayed reaction. Display works fine with a local source and single HDMI cable. Also, switching away from HDMI Input 1 and then back to HDMI Input 1 will produce non-conforming color space rendering of the source.”

Reading this, I immediately (and wrongly) blamed the digital switcher manufacturer. Clearly, there must be something wrong with the switcher or Catx transmitter/receiver being used. Connecting a source to the display with HDMI works well, but when a source goes through the system, that’s when the problems start. But, why would it work on another display of the same make and model with the same system setup? And what about the color space issue? Maybe there’s something funny with the video processing board of the display.

At any rate, the display technician arrived and found nothing wrong with the display itself or the processing board. Off the record, he informed me that this particular model did have several problems with integrated systems. He explained that the manufacturer outsourced the processor to several different factories in several different countries. Some work, some don’t. It’s essentially a game of Russian roulette: Order the display and keep your fingers crossed. Further, the manufacturer, distributors and vendors have no way of knowing which displays have the defective boards so, of course, they typically will not admit that there’s an issue at all: “This is the first time I’m hearing of this…blah, blah, blah.” The commercial AV market represents such a small portion of their sales, I can see their reasoning (albeit with teeth gritted).

After smacking my head and cursing the AVR specialist who left me this mess, as well as the manufacturer whose lack of quality management wasted several of my precious hours, I reminisced about the days of RGBHV. If the voltage was too low, the image was dim; if it was too high, the image bloomed. If a resolution wasn’t supported, it still kind of displayed (although it looked funny). If a termination was poorly done, the image would still be there, but it would have a ghost. No image? No problem, probably a short somewhere. With digital video, sniffing out problems is vastly more difficult. We have to worry about things like EDID, EDID management, digital video transmitters and receivers, category/fiber cable terminations, particular inputs on displays and more.

For the most part (aside from “sparklies”), if the system is good, the video is great. If the system has issues, the video doesn’t show at all. We have no clues as to where to begin. Luckily, with tools such as HDMI analyzers, cable qualifiers, digital video generators and portable digital video monitors, troubleshooting can go pretty quickly. We can confirm that the display and switcher are communicating with the devices using the analyzer (or switcher). We can rule out cabling and termination issues with the qualifiers. We can rule out source issues with a known, good generator. We can rule out display issues with a known, good monitor. However, if the root cause of the problem is that one internal circuit board on the display was manufactured in Indonesia and another was manufactured in China, troubleshooting may take a little longer. “Ain’t nobody got time for that!” (Sweet Brown)

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