21.08.07
Getting rid of gremlins
Video problems can crop up at any time with little warning. Here are ten common problems and possible solutions for them.
Ghost images and white outlines
An effect known as ringing, it is caused by a mismatch in the impedance in the cables between source and display. Abnormally high standing waves are formed, and a significant part of the signal is reflected back to the source from the displays input terminals. This results in a second out-of-phase image.
The problem can be traced to incorrectly set terminations on displays, distribution amplifiers and video interfaces such as scalers or scan converters.
Computer and component video images appear pink or green
There’s a three-wire component (YpbPr) signal connected to an RGB input, or vice-versa. This is a particularly common problem with projectors that only provide one 15-pin VGA or a DVI-I input for both computer and video connections.
The projector doesn’t recognise the incoming signal format, or attempts to guess as to the right format. Looking through the menu of the projector should find you a signal format selection option. It could even be on the remote control. It is worth noting that some displays with DVI connectors may have trouble with 720p/60 DVI HDCP sources, but not with 1080i or 480p.
Text or fine graphics are blurred or otherwise unclear
There are two options here. One, the display’s bandwidth is constricted, or two, you need to use better cable! Many electronic displays have significantly larger HD signal bandwidth through their RGB and DVI inputs than through HD component video connections, so this is worth bearing in mind.
Colours are missing or wrong
First check your cables. It’s very easy in an RGB system to mix up the blue and green. In the case of YPbPr, the red and blue cables can also become flipped by mistake. If all you see is a green image then both of the colour difference connections are bad in some way. If the green cable itself is faulty then you won’t see any picture at all because the picture sync is on the green (Y) channel.
Colour brightness is out
It is possible that voltage levels and / or terminations are wrongly selected. In the case of RGB, each channel is 700mV, while in the YPbPr system only the luminance channel (Y) is 700mV. The colour difference channels (Pb and Pr) are ±350mV.
Ground loops
Ground loops are some of the most common video interference problems found in staged events, permanent installations and also trade-shows. The likely cause is mixing and connecting balanced and unbalanced video signal cables.
Ground loops are caused by a difference in voltage over two legs of a grounded power system that are some distance apart and can even be as high as several volts. They can be eliminated by ensuring that common phases are used for all interconnected video, and indeed audio, devices in a system and that a common ground bus is used with the lowest possible resistance and impedance.
They can also be created by RF coaxial cables, which are unbalanced transmission lines. Baluns and isolators are available from a number of sources to prevent loops from forming.
Component signals exhibit crawling edge artefacts
The possibilities here are a ground loop or RF interference entering the system around the frequency of the source signal. It could be directly coupled through a ground loop or other common connection. Alternatively there could be inductively/ capacitively coupled interference due to proximity to the RF source.
The ferrite cores supplied with many flat panel TVs are useful for making low-frequency traps for AV 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 ground also helps.
No signal from a DVI source appears on the display
The most likely explanation is that the display doesn’t support the DVI HDCP (high definition copy protection) standard. Either no signal is apparent or a message on the display appears, telling you that the signal isn’t legal.
HDTV images are not centred
The problem isn’t as common any more, but when it does occur it’s usually caused by a horizontal sync timing and blanking interval problem. Some projectors and monitors don’t recognise tri-level sync and only work correctly with bi-level sync, the standard for analogue composite and component video and RGB displays.
Depending on the manufacturer, set-top receivers may output bi- or tr- level sync through their DVI or component connections. Images maybe shifted or distorted, totally green, or non existent.
Use the RGB output if possible because this is more likely to be configured to the more common bi-level.
Green shift on a display
If you connect a 1080i or 720p signal through an older multisync display via RGB can result in a monochrome green image. The problem occurs mainly when the only input on a display is an RGB-type 15 –pin connection, and it is designed to switch between the YPbPr and RGB colour spaces. It detects the unique HDTV scan rates and uses only the Y channel information, hence the green tint.
Modern displays generally don’t suffer from this issue, but you may have to select the colour space manually or use a signal format converter.
Stop guessing!
So how to diagnose these issues stop these things occurring in the first place?
The first thing is to have suitable test equipment available (see “The AV tool kit” on page 26). Video test pattern generators are exceptionally important because they allow you to check performance in your cables and display components.
A fine text pattern will enable you to check the autosync capabilities of the display. Colour bar patterns work for both SD and HD resolutions. You should use a luminance multiburst pattern on HD displays. This will spot problems with pixel clocks and bandwidth as well as aspect ratio issues.
Cables are harder to test, but obvious shorts can be simply identified with a standard voltmeter. These can also find ground loops by checking the voltage differences between the ground points and cases of equipment. Just a couple of volts will create a hum bar.
In situations where you are using wireless transmission methods for video signals, an RF signal analyser will show the shape and amplitude of any signal within its range. It can also reveal sources of potential interference, and spot coax faults in the case of a very lossy cable.
It’s normally a very good idea to do a dry run of a permanent installation in the workshop. Of course this isn’t always possible but it need not be more complicated than setting up long runs of cable and introducing a loop-free source of AC to connect the equipment together. When a problem occurs you are then able to discount all the previously tested equipment from the investigation.