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Elements of Picture Quality
Picture quality is an intangible subject for the average consumer, but if you put two similar video images with significant quality differences, your eyes will instinctively guide you to the better picture. So how can we define picture quality to some extent and how can your source material and equipment be ultimately responsible for the TV images you watch day in and day out?
In general, the most important aspect of picture quality is contrast ratio, followed by color saturation and accuracy. Then resolution comes into consideration, but it is the biggest technological advancement to come out from the video industry in recent years that is targeted at general consumers. For example if you have been watching CRT TV in the last few years, it is likely that you have been enjoying decent contrast ratio and color in your movies.
I will start by breaking down and discussing the sub-elements that make up color, contrast ratio, resolution and any other aspects which may have a direct impact on PQ. I hope that this will help you to decide what is important to you before making the leap into the High Definition world. I will restrict the discussion mainly to plasma and LCD technologies.
This defined as the ratio of the luminosity of the brightest and the darkest color the system can produce. A HDTV with a high contrast ratio is a desirable aspect in any system. Poor contrast ratio causes loss of detail (it’s there but you just can’t see it when you’re supposed to) and washed out pictures. Colours look unexciting and the images look dull.
Thankfully, the industry is marvelous at constantly pushing the technologies to improve this aspect, so much so that this is not a significant issue with consumers nowadays. The best contrast ratios are available to industrial, commercial and high end pro consumers, entailing a price premium, but the technology is trickling down to consumers.
A CRT or plasma display has a theoretical infinite contrast ratio because its pixels do not emit light when not activated, giving 0 luminance for blacks. However, in real life measurements this is not possible (especially with different standards) but they still produce very good contrast ratios. LCDs on the other hand suffer from a technological limitation that prevents it from achieving the deepest blacks. The constant source of backlighting behind the liquid crystal matrix means some light always gets through. Despite this setback, modern LCDs still enjoy very good contrast ratios, thanks to its excellent brightness.
There are generally two types of contrast ratios related to video. Manufacturers tend to report the higher dynamic (sequential) contrast ratios of their displays but this seldom indicate their real world performance. Dynamic contrast ratio depend significantly on how and when the measurements are made and in our experience, is generally lower than the quoted specifications. Static (simultaneous) contrast ratios are better indicators of performance. They measure the luminance proportion of white to black in the same image and not at separate times. Unfortunately, measurements can be difficult and depend heavily on external factors. For our purposes, it is adequate to use dynamic contrast ratios as long as the display has been calibrated properly.
CRTs and plasmas are acknowledged to have the darkest blacks for general consumer displays. This is important because there is nothing more annoying than having to see grayish blacks (or any other tint of blacks) when you expect absolute, true black in an image. This is especially prominent in dim light conditions when our eyes are especially sensitive to dark images. HDTVs with poor black levels have dark scenes that look uncomfortably bright. LCDs still lose out to the Plasmas in this respect although recent advancements have narrowed the gap.
You can change the brightness controls to set black levels, preferably by using a calibration disc. You want to make sure you don’t set it too low because, apart from having deep blacks, it is also important to maintain black shadow detail. Setting the brightness too low (crushing blacks) can eliminate this detail making your dark scene look completely flat. Doing the opposite will wash out colours at the low end.
Black level retention describes another problem with consumer grade displays in which the black level keeps floating up or down depending on the brightness of the overall image. This is a common problem with CRT but is much less with current flat panels.
High white levels is a very good feature of modern LCD TVs. I feel myself drawn to this single important feature in LCD TVs when watching nature shows, where images become vivid and lively. Also, daytime viewing is more pleasant in displays capable of high white levels. Plasmas and CRTs do not have the same brightness and need a lower ambient light. You should refrain from pushing the contrast control to its limit as bright highlights or details can be lost. (plus reducing your TVs lifespan)
Together with contrast ratio, colour is one of the most abused aspects of picture quality on the showroom floor (where is it normally very bright). To maximize a display’s ability to draw crowds, manufacturers do one or all of the following things, crush whites by blasting the contrast, crush blacks by turning down brightness (hence maximizing contrast ratio), setting a bluish colour temperature, and increasing the colour saturation to artificial levels. What's worse is that some consumers get their HDTVs a few weeks later and leave it at the manufacturer’s settings or do very little changes. Out of the box, two of the most common colour inaccuracies are a bluish colour temperature (which causes poor flesh tones but brightens the image) and a red push in the TVs colour decoding. You’ll see red push in video when the reds are more saturated compared to the other primary colours and flesh tones looking ruddy.
Colour temperature, saturation and accuracy are important characteristics in the final production of images on your display. The standard for colour temperature is 6500K, corresponding to daylight illumination. This is the most natural colour temperature and is used film production; whites above this tend to have a bluish tint and below this, a yellow, reddist tint. You can think of colour temperature as the canvas for a painting. If the white has a predominant bluish or reddish tint, other colours painted on to the canvas will be `contaminated' with that tint, causing inaacurate colours.
Colour saturation can usually be adjusted to a resonable level by eyeballing or using colour patterns that come with calibrations discs or TV channels.
Colour accuracy will depend primarily on how your TV `decodes' the image data. Using non-standard decoding to enhance colours tend to create images that differ from the original film. This is a complex area, but it is enough to know that the colour encoding system used in film production should match the decoders at your TV for accurate reproduction. Unfortunately, most colour decoders at your TV cannot be easily adjusted.
Other factors that can affect your colour reproduction include greyscale and the display characteristic (gamma) of your TV. If all this is important to you, you can enter the realm of amateur calibration or hire a specialist to do this.
The best HD resolution out there can provide 5 times the pixel amount compared to old Standard Definition. It's not 5 times clearer, but there is a night and day difference. Imagine actually seeing grass and dirt on the football field, or making out someone standing in the distance that you couldn’t before. Vanessa Williams (star of Ugly Betty) even went on live to say that she hated HD because it showed her facial imperfections. Why is everyone raving about Planet Earth when there have been countless similar nature documentaries? This is the biggest reason why the video industry have been getting everyone to jump on the bandwagon and with good reason to.
Unfortunately, there is a lot confusion in the consumer market now, especially when there are two `types’ of HD material ( I am referring to 720 vs 1080) and the big boys taking sides on the BluRay vs HD DVD format war. Your only hope is that with a little information, you can avoid being the casualty. So to get the most detail out there, go for 1080 capable TVs . Just make sure you understand the Why 1080p and optimum viewing distance articles to point out certain instances where you may not need higher resolution TVs. For Home Theatre PC fans, make sure you get 1:1 pixel mapping feature (Dot-by-Dot) on your TV for crisp and detailed images.
Perceived sharpness of a picture is related to resolution and the useful concept of acutance which describes the edge contrast in photography. The edge contrast is simply the brightness difference with respect to space. Most flat panels today have the option of artificially `sharpening’ the video image while keeping the same resolution. This creates a crispier edge along the image border, giving the impression of increased picture quality. This generally works on SD material as its resolution is so low compared HD sources. But oversharpening or edge enhancement, creates unsightly halo artifacts around images. To fix this, you can calibrate your TVs sharpness by utilizing some test patterns found in popular calibration discs.
High resolution source like HD movies tend to give sharper pictures compared to SD format. This is especially the case when 720 or 1080 encoded movies are displayed at their respective resolutions on HDTV with no scaling.
When LCDs first came out, motion lag was a particular nuisance due to its technological limitations. Moving images will cast a shadow, making fast action scenes unwatchable. Well, I am pleased to report with the overdrive technology and backlighting improvements now employed in many LCDs, this has become less of a problem. There are some individuals who are still seeing this motion artifact, but I’m glad I'm not one of them. LCD companies have done remarkably well in overcoming their limitations and making their TVs a genuine contender in the HDTV arena.
Another bane of LCD TVs. Newer models have been improving the viewing angles on LCD TVs but I don't think its anywhere near a plasma TV. Companies which quote 176 degree viewing angles neglect to mention the significant drop in contrast and picture quality as you move off centre. I haven’t found one single LCD TV that has solved this but when I do, Ill let you know.
Video Processing Artifacts
In a typical video system, where MPEG decoding, scaling and deinterlacing is employed, a number artifacts can appear on the image which may degrade picture quality. There are not typically seen by the average Joe bloke, but once you see it over and over again, it can be distracting.
An example of artifacts you can see:
- Line twitter
- Mosquito noise
- Chroma Bug
- Digital noise
Ignorance is bliss.
Well that’s it for now. I hope you can use some of the information here as a rough guide to assessing PQ for your next HDTV purchase. Happy hunting!!!
|Neville Chappell says:||03/21/2007 - 16:34|
|Colin Tang says:||03/30/2007 - 06:18|
What TV is this? Is this on HD or SD? There problem could lie in dynamic colour processing being switched on. If the green field actually LOOKS BROWN I suspect you may have a defective set.