Wednesday, August 26, 2009

Dynamic Contrast of LCD TV with LED BLU

Dynamic Contrast of LCD TV, LCD-screens comprises a backlight unit which is permanently emitting light and an LCD-panel in front of it which modulates transmission of light with respect to intensity and chromaticity. In order to increase the contrast of such LCD-screens the backlight can be (globally) dimmed when the image to be displayed is dark (i.e. not comprising high intensity image data) while the image data is numerically corrected and adapted to the reduced backlight intensity.

In such a way the dark regions in dark images can be improved and the contrast between subsequent frames can be substantially increased. Also the contrast within one frame can be expanded intentionally depending on the histogram of the image (some sporadic highlights in an image may be cut or suppressed). There is quite some digital signal processing required for implementation of the dynamic contrast control technique in a way that is pleasing to the human visual system (e.g no flicker effects must be induced).

The contrast within individual frames (simultaneous contrast) can be increased when the backlight can be locally dimmed. This can be achieved with backlight units that are realized with arrays of LEDs. High-dynamic-range (HDR) LCDs are using that technique in order to realize (static) contrast values in the range of CR > 100,000.

When we go to the electronic shop, there is no surprise to see that they have more than 1,000,000:1 and even 2,000,000:1 Contrast in some of the spec sheet of the latest LED Back lit TV, so in the common paper specification comparison, Does a 300,000:1 Contrast is 100 times better than the 3,000:1, of course, the answer is yes. When we compare then precisely, we can see that there is difference of say some 10 times by our naked eye comparison.

Similarly, this phenomenon ressembly the human ear sensitivity. It is useful and accurate to express perceived loudness in the logarithmic decibel (dB) scale; a change of 1 dB, which corresponds to a 25.9% change in power level, is considered to be the smallest change in sound power level perceivable by the average human ear under idealized test conditions. An increase/decrease of 3 dB corresponds to a doubling/halving of power and distance of average perceivability. An '84 dB' source "speaker" would require a 400-watt amplifier to produce the same audio energy as a '90 dB' source being driven by a 100-watt amplifier.

The CCFL TV is actually hard to have the same amount of Light intensity of LED. So the LED has a high bright side as intensity is higher and the LED can have 2D local dimming as f they are Direct back light that can turn off the back light such that absolute darkness can be made possible, so the paper specification will be many times better.

In fact we can see the difference of the picture, however if you have no intention to scrutinize the TV picture quality, you may not easily see the big difference, also especially in Electronics Store of Hong Kong, they are used to put illumination lights as many as they can. As They believe that a brighter shop will attract more visitors and customers This is a Common Environmental Problem as on the other hand, they turn the air con to a very low temperature such as to offset the immense heat generated by the Lights! Bad Practice!

Since the shops ambient light is so bright that is a draw back for the visitor to distinguish the dynamic contrast of two different TV when they try to choose, so the best is to make a darker demo room for display the TVs such that we can really can see and find out the big difference in contrast.


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