Video Display Glitches
10 troubleshooting tips.
It's important to have a modicum of test instruments available to you, so you don't wind up throwing darts at an invisible target to solve your problems. A video test pattern generator is very useful, allowing you to quickly and completely sweep your wiring and display components for performance.
With a good test pattern generator, you can set up the display in your workshop or studio and sweep it for the most common picture formats and scan rates in advance. Make sure you check for autosync capabilities with a fine text pattern; this sort of pattern has lots of high-frequency information that drives pixel clocks crazy and will also reveal if the display's internal bandwidth is clipped.
I use two different test pattern generators that help me analyze pretty much any display that crosses my doorstep. AccuPel's HDG2000 is a nice HD pattern generator that supports both 720p and 1080i output through five BNC jacks in either the YPbPr or RGBHV (switchable) formats. A new version, the HDG3000, is now available with DVI connectors.
Extron's VTG300 is another useful gadget. It's actually a combination audio and video test generator, and the video functions include eight different test patterns (with that fine text field I mentioned earlier), selectable YPbPr and RGB output, and a wide selection of SD video, HD video, PC, plasma, D-ILA, and other oddball test formats.
A color bar pattern is also helpful at all resolutions, particularly if you're showing SD and HD video. HD displays should also be swept with a luminance multiburst pattern, which will reveal bandwidth and clock problems in addition to aspect ratio or picture geometry difficulties.
For baseband video, there's no substitute for a waveform analyzer and vectorscope. The former measures luminance signal amplitude and will absolutely show you if you have the correct levels of video set from black to white, while the latter will reveal any problems with chrominance amplitude and phase.
Combination waveform analyzer and vectorscope devices are available as software programs, in addition to dedicated chassis. These instruments are available for both SDTV and HDTV systems, which use slightly different RGB color gamuts.
Cabling problems are not as easy to test, although you can locate obvious cable shorts or opens quickly with nothing more than a volt-ohmmeter (VOM). That same piece of equipment will help you catch potential ground loops by measuring voltage differentials between the cases and ground points of different equipment. Even a few volts is enough to produce a hum bar.
For other more exotic problems, you'll need to get your hands on a time-domain reflectometer; a gadget that can pinpoint a specific cable fault such as a squashed dielectric, broken shield, or other defects that will change cable impedance drastically. They're not cheap, though — it may be more practical to just replace the suspect cable.
If you're pumping wireless video through a facility or modulating video at higher radio frequencies through your system, you'll be well advised to pick up a RF signal analyzer or spectrum analyzer. Both devices can show you the shape and amplitude of any RF signal within their frequency range, and can also help spot cable faults or reveal if you have a particularly lossy piece of coax.
A spectrum analyzer can also show you sources of interference that may be plaguing your installation. Because the analyzer plots signals using amplitude vs. frequency, it's a simple matter to hone in on the offending signal or signals most of the time. I use a Sencore SA1501 for the bulk of my work; it can operate off AC power or its internal battery.
You may even find that your own equipment is emitting RF interference. Last year, a Toshiba plasma TV in an Oregon home was emitting considerable RF energy on the aircraft distress frequency (121.5 MHz), which led to a mass air and land search for a supposedly downed aircraft before the problem was discovered and fixed.
When time permits, it's also a smart idea to “ring out” every piece of equipment you'll be wiring up —whether it's a permanent or temporary installation. Successful troubleshooting is really a process of elimination; the more items you can rule out quickly, the faster you'll reach a solution. In fact, it's not a bad idea to do a dry run of a permanent installation or temporary rig in your facility. This need be nothing more than making up long runs of interconnecting cables and providing a clean, isolated, loop-free source of AC to connect all of the pieces together. If and when you do hit a snag, you can quickly eliminate all of your pre-tested gear as the source of the problem(s).
When I was in the rental & staging business, I used several different subcontractors for equipment such as video projection, image scaling, switching, cameras, SDTV production and switching equipment, VTRs, and other assorted video gear. I learned early on — often the hard way — which of these contractors took the time to troubleshoot and maintain their equipment on a regular basis. You can't just expect things to work off the shelf each and every time. I hope that by demonstrating some of the more common video display problems in the AV industry, I've helped you avoid some late hours in the future, and possibly years off your life trying to fix these glitches while the clock is ticking.
Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at firstname.lastname@example.org.