Digital Displays FAQ - General Questions...

In it’s most simple definition contrast ratio is the amount of scales of colour a display can produce between full on black and full on white. It is represented as a ratio of black:white. So a display capable of showing absolutely zero light would have an infinite contrast ratio. This is of course VERY difficult as even projected light bouncing back from cinema room walls will decrease the contrast of an image.

Contrast Ratio is measured in a couple of ways popular to display manufacturers.

Full On / Full Off is one the more rogue manufacturers will prefer as it will give the highest figure. The measurer first measures light output showing a full bright white screen, then does the same showing a deep full black screen. Of course in between this two measurements what's to say the manufacturer is not making adjustments to the projector to achieve a brighter white at test 1, and a darker black at test 2....

Ansi Contrast is our preferred measurement style. It is done with a checkerboard pattern of equal sized black and white squares. This means the projector must try to achieve it's best black (no light) while at the same time trying to achieve it's brightest white (full white). This is a far more realistic test as it avoids the ability to use artifically high/low settings which in the real world would give you a truly ridiculous looking picture.

Dynamic Contrast Ratio is one of our pet peaves. Common in LCD technology (flat screens and projected). The display will judge the overall brightness of the signal and dynamically alter such factors as gamma, brightness, IRIS etc to achieve the highest contrast range for just that portion of an image. This can mean for the viewer an obvious shift in black level, a change in colour accuracy depending on the brightness of the image and other such artefacts. Better technologies such as plasma and DLP do not need these cheats to achieve a high contrast image.

This is something inherent in all digital displays. When the video signal is received by the display it must first de-interlace the picture (turn it into progressive scan), and secondly scale the picture (stretch or shrink the incoming signal to fit the native resolution of the display). This process takes a certain amount of time, which varies from screen. Since during this time (usually around 20 milliseconds) the audio signal is being fed directly to the speakers, the sound is technically played before the picture is. 20ms is an incredibly short amount of time though and many people do not notice.

However, the symptom is quite common on certain channels broadcast via terrestrial, satellite, or cable television. This arises when the broadcast itself is sent out of sync in the first place (see Sky Sports News for some of the worst!) and as such can increase the delay time to around 80 milliseconds, which is more noticeable – including on large screen normal TVs.

For those of you who have introduced a dedicated video processor into your system, this may exaggerate the problem further. Many AV amps now include audio delay functions to combat this, and we have found an analogue audio effects box that can also be used.

A “Dead Pixel” can occur on practically any display (plasma, LCD, DLP), but is only usually heard of on displays based on LCD technology. In the context of home cinema this means projectors that use LCD chips, and LCD televisions and monitors.

Due to the technological constraints of building an LCD chip, manufacturers can only guarantee that approximately 99.9% of pixels will be functional. The actual number of allowable failures varies from manufacturer to manufacturer and from product to product.

A pixel can be stuck open (brite white dot), stuck closed (no output) or stuck on red, green or blue. Many manufacturers place a different weighting on different types of pixel defect e.g. a projector with 2 blue pixels and a black one may still be within specification and as such not-faulty, whereas that same projector with just two white pixels may be outside of specification and therefore warrants a replacement.

Defective pixels cannot be repaired (although strangely enough some have been known to pop back into life!!!)

Sky TV (and indeed Freeview and Digital Cable) is sent as a compressed picture, digitally to your set-top box. The compression that is used is more than that used on DVD discs and as a result the picture will suffer what is known as MPEG artefacts. Basically the picture has been compressed really small and blowing it back up again doesn't get exactly the picture it was in the first place!!

To a degree the scaling/deinterlacing/progressive scan in the display will improve on this – better displays will make a better job of it. You can also opt for a dedicated video processor to improve the picture further.