Dark Details in Plasma Displays

I recently received an email complaining about an LG Plasma display. My correspondent was very dissatisfied with the way dark levels in movies changed their brightness levels. This is taken from my response.

I had actually reviewed the same model of Plasma display (MT-42PZ15) some time previously, but had not noticed any unusual problems in this regard.

We ought to look at problems related to Plasma displays in general. No display technology is perfect. It’s just that each different technology comes with its own different set of imperfections. In the case of plasma displays, the most significant complaints concerns the reproduction of subtle variations of near-black. This is put well in a PDF on display technology locatedĀ here (~230KB), (p.18):

The biggest obstacle that plasma panels have to overcome is their inability to achieve a smooth ramp from full white to dark black. Low shades of grey are particularly troublesome, a noticeable posterised effect often being present during the display of movies or other video programming with dark scenes. In technical terms, this problem is due to insufficient quantisation, or digital sampling of brightness levels. It’s an indication that the display of black remains an issue with PDPs.

To the right are two small B&W pictures to demonstrate what ‘posterization’ is. The top one is unmodified while for the bottom one I reduced the number of grey scales to eight, using Photoshop’s posterize function. Now imagine a dark scene which has a brightness level right on the boundary between two of those posterized dark grey levels. With the very subtlest of changes in the source, it could flick between the two levels. Alternatively, LG’s processing circuitry may be not quite stable, causing ‘black’ levels on the boundary between two of the posterized levels to flick between the two.

Assuming everything is working properly, what can be done to reduce the problem.

First, see if the panel sports what LG calls its ‘Digital Eye’ feature or a similar feature. This appears on a number of conventional TVs and detects the prevailing light conditions. If switched on, it adjusts the picture setting automatically with the intention of optimising the brightness, contrast and colour levels for the best picture. (How a picture looks depends very much upon the environmental light.) If this feature is fitted, consult the manual and find out how to turn it off. I have found such features to have unpredictable effects that are more irritating than useful.

Second, it may be necessary to trade-off dark detail for stable dark areas. Use the picture adjustments here. The two of interest are ‘Contrast’ and ‘Brightness’. In virtually all TVs these operate opposite to the way that you would expect. ‘Brightness’ controls the black levels, ‘Contrast’ controls the brightness levels. Weird, but true.

Put on a DVD with dark scenes at the edge of the frame. Put your screen into 4:3 mode so that there are unused sections on the left and right of the screen. Pause the DVD. Turn down the ‘brightness’ level until the darkest areas of the scene match the darkness of the unused areas of screen on the sides. Does the problem persist? Try turning it down a little more.

Also, try turning down the ‘Contrast’ control. Often this is set far too high. Let’s say that there are 16 shades of grey spread evenly over the luminance spectrum from black to white. If ‘contrast’ is raised too high, then the three brightest shades may become indistinguishable (in other words, the image is ‘clipped’ at the white end). The relevance here is that instead of having 16 visible shades, you only have 14 visible shades. So there is a greater variation between each shade across the brightness spectrum. This will make each step at the dark end more obvious.

You can see how I have replicated this in the picture at the right. Notice how the light grey areas have all become white, while the dark grey areas are now much more sharply differentiated from each other.

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