Originally Posted by pete ramberg
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Blanking simply blocks out a portion of each edge of the LCD chip, keeping extraneous light from reaching the screen. For example, if you use a 16:9 LCD chip to display a 2.35:1 panoramic image, the top and bottom of the chip is not used, but might have some extraneous light leaking out. The blanking feature basically blocks that light or image from reaching the screen - makes the dark grey bars more black!
I could be misunderstanding your explanation, but I think this explanation is incorrect.
Blanking just sets those pixels to black -- They'll still get the same amount of light through the LCD panel as if the image was black. There is no physical barrier moved in front of the LCD panel to block that extraneous light going through the "black" pixels. If a different aspect ratio image is shown, there is generally no need for or benefit from using blanking (assuming it already has black "bars" or a color to match the content).
There are at least two reasons for blanking options (there could be others as well). Source video could have garbage colors/errors along one or more edges (mostly gone with HDTV stations, but I've seen on some low-budget local stations when they play old analog-converted content and don't bother to overscan to correct their source). In this case one would want to block that portion of the image.
On the other hand, specifically in respect to projectors, one may have a full image in the source but want to display on a smaller or different aspect ratio rectangular screen or wall. In this case one may want to block different amounts from multiple sides so that the image is not being also projected on another wall or outside of the display area.
Here's an article on why there used to be a wider need for overscan
, with a more elegant description of what I was referring to as "garbage" along the edge on some TV stations:
"Overscan also serves another, lesser-known purpose. Since the outside area isn’t going to be viewed anyway (in most cases), it’s used to house important data for analog-to-digital converters. Analog has no way to attach additional information to the picture like digital does (metadata), so this data is tucked neatly into things like blinking pixels or scan lines—think of it as Morse code for TVs. While the majority of everything is completely digital from end-to-end now, there are still some analog-to-digital conversions going on. That’s the problem with old technology that was so widely adopted and use for so long: it’s almost impossible to get rid of it completely."