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Video Display Glitches

10 troubleshooting tips.

Every experienced AV integrator knows all too well they can strike anytime, anywhere — video problems that crop up without warning. The scene might be a large hotel ballroom, where your staging company has just finished setting up an elaborate multi-screen video presentation. You've run the audio and video cables, lighting is set, and the rehearsal is about to begin when suddenly you notice it — a bar that's faintly visible, slowly scrolling across your projected images from bottom to top over and over again. Jump to a classroom setting where the instructor's PC desktop images are being shown on a cluster of plasma monitors. Everything looks great, except for those white outlines adjacent to any dark text or graphics. Come to think of it, the text isn't quite as sharp as it could be either, not to mention some colors seem muted or may be missing altogether. Let's say you invite some business associates home to watch the big football game in HDTV in your den, using a portable projector from the office. You wire it up to your cable TV set-top receiver, turn on the projector, and are baffled by the squashed pictures in front of you. Try as you might, you just can't expand these images to their normal shape. What happened?

During my many years in the AV industry, I've come across all kinds of video problems, and have watched as each technical difficulty brought about unnecessary stress and aggravation for one or more people involved (including myself). The keyword here is “unnecessary.” Looking back in retrospect, I now realize the answers were obvious — the technical people troubleshooting the situation just didn't have the luxury of sitting back and mulling over the big picture, thinking about possible causes and coming up with logical solutions. Instead, they did what any reasonable person would do: Panic!

So after many late nights and lots of product swapping, the problems described above as well as countless others were indeed solved, with plenty of notes taken and filed away for future catastrophes. In an effort to save you the headache of remedying many video glitches, let's take a look at 10 common video display problems I've run into over the years as well as their solutions.

1. Ground loops are heard and seen. Ground loops are still the most common video interference problems I see at trade shows, staged events, and even in permanent installations. They can pop up anywhere, and the likely culprit is a decision — intentionally or otherwise — to mix and interconnect balanced and unbalanced signal cables.

Ground loops are caused by a difference in voltage across two legs of a grounded power system that are some distance apart and can even rise to several volts. The easiest way to eliminate them is to make sure that common phases are used for all interconnected video (and audio) devices in a system and that a common ground bus is used with the lowest possible resistance and impedance.

Ground loops can also be easily created by RF coaxial cables, which are by definition unbalanced transmission lines. Baluns and isolators are available from a number of sources to prevent loops from forming.

2. Computer and component video images appear pink or green. You've connected a three-wire component (YPbPr) signal to an RGB input, or vice-versa. This is a common problem with projectors that provide only one 15-pin VGA or a DVI-I input for both computer and video connections.

The projector simply doesn't recognize the incoming signal format, or makes an “educated guess” as to the correct format. A quick hop to the menu should reveal a signal format selection button, or it may even be present on the remote control. Note that some industrial displays with DVI connectors often have trouble with 720p/60 DVI HDCP sources (see next item), but not with 1080i and 480p formats.

3. Text and fine graphics are blurred or otherwise indistinct. Either the display's bandwidth is constricted, or you need to use better cable. Keep in mind that many electronic displays have much greater HD signal bandwidth through their RGB and DVI inputs than through so-called HD component video connections.

4. Colors are missing or incorrect. Check your cables; in an RGB system it's quite easy to reverse or mix up the green and blue connections. In a YPbPr system, the red (Pr) and blue (Pb) cables can also be flipped. If all you see is a green image with a YPbPr source, then both of the color difference connections are bad. If the green cable is bad, you won't see anything as the picture sync is on the green (Y) channel.

I did a seminar at InfoComm a few years back where all that could be seen of my presentation were bluish and greenish colors. As it turned out, the projector had a problem with its red (R) input —something we quickly figured out by driving that input with only one of the three RGB cables.

5. Colors are too bright or too dim. Voltage levels and/or terminations may be incorrectly selected. In the RGB signal format, each channel is 700 mV, while in the YPbPr system only the luminance channel (Y) is 700 mV. The color difference channels (Pb and Pr) are ±350 mV.

6. Ghost images or white outlines are seen. This is known as “ringing” and is caused by an impedance mismatch in the cables between source and display. High standing waves are introduced, and a significant part of the signal is reflected back toward the source from the display's input terminals, resulting in a second, out-of-phase image. This problem can also be traced to incorrectly set terminations on displays, distribution amplifiers, and video interfaces, such as scalers and scan converters.

7. Crawling edge artifacts are present on component signals. This could be due to a ground loop, or it could be RF interference getting into your system at or near the frequency of the source signal. The interference could be directly coupled through a ground loop or other common connection, or inductively/capacitively coupled through proximity to the RF source.

The ferrite chokes supplied with plasma and LCD TVs are handy for creating low-frequency “traps” for AC signals that can get into power and speaker lines. Higher frequency signals will require bypass capacitors and possibly chokes at each signal input. Using cable with increased shield coverage and good grounds also helps.

8. No signal from a DVI source (set-top receiver, DVD player) is seen on a DVI-equipped display. In all probability, the projector or monitor doesn't support the DVI High Definition Copy Protection (HDCP) protocol that's standard on all consumer set-top receivers and DVD players. Either no signal is seen or an onscreen display informs you that the signal is “illegal” or something to that effect.

9. HDTV images don't center correctly on a display. This is usually caused by a horizontal sync timing and blanking interval problem, and was common with the older RCA DTC100 satellite/terrestrial digital TV receivers. One manufacturer (NEC) even went to the trouble of building a table of sync timing information into its display products just to accommodate this set-top receiver.

The problem doesn't seem as widespread these days, although there is a new sync format (tri-level) to deal with. Some projectors and monitors don't recognize tri-level sync and only work correctly with bi-level sync, the standard for analog composite and component video and RGB displays.

Depending on the manufacturer, set-top receivers may output bi-level or tri-level sync through their component or DVI connections. The result may be a horizontally or vertically shifted or distorted image, a green image (see next item), or no image at all. When in doubt, try to use the RGB output, as this connection is nearly always set up for the more commonly used bi-level sync.

10. Green shift on a display. Connecting an HDTV signal (1080i or 720p) through an older multisync display in the RGB cable format may result in a monochrome image with a noticeable greenish color cast. This usually happens with a projector or display that has only a 15-pin RGB type input, and is supposed to automatically switch between the YPbPr and RGB color spaces. The display detects the unique HDTV scan rates (33.8 kHz for 1080i, 44.9 kHz for 720p) and uses only the Y channel information, hence the green tint. Most modern displays don't have this problem, but if yours does, you may need to manually select the color space from the display menu, or use an RGB-to-YPbPr signal format converter to assure the correct space and color weighting (59/30/11 for YPbPr and 100/100/100 for RGB).

How can you prevent some of these problems before they begin? The first thing you should do is stop guessing.



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