[mythtv-users] Question about overscan (in the TVs)

Brad Templeton brad+myth at templetons.com
Wed May 31 04:37:44 UTC 2006


Something for the HDTV experts to answer.   My 1280x720 DLP TV in theory has in
it a DLP chip with that resolution, which obtains colour by beaming the light through
a colour wheel.

As most of us know, when you feed a signal at 1280x720 into the TV via DVI or VGA or
even component, you typically see a lot of overscan, which is to say that the
edges of the image are not visible, they are behind the mask around the screen if
they are there at all.   So one must adjust mythtv settings to have it do the UI
only in the visible, uncropped area of the screen, and even consider doing this to
the video images.    Overscan, which came from old analog TVs, means that you are
not intended to see the very edges of an image, and in fact on many TVs if you did
see it you would see annoying VBI stuff.

What I want to know is this.  Since the DLP is a digital device, I want one pixel
in my outgoing image to correspond to one DLP pixel.    Since the visible part of
the screen appears to cover only perhaps 1100 x 690 pixels, what is happening?

a) The DLP is going 1:1 with the pixels, but in fact that light is being discarded
or masked?

or 

b) The DLP's resolution is aimed to be very close to the size of the visible
screen, and those pixels from the edges are not be projected at all.  Thus, in
addition, the row of 1100 pixels sent is being stretched so you don't get a 1:1
pixel matchup?


Now my TV has a "compressed" mode on the VGA input that does not overscan nearly
so much.  But it's also clearly blurry, which implies that some hardware or software
could be resampling the input image to make this happen.   This implies answer (a).

Why is there so much overscan?  I understand the need for a little but what I see
is larger than seems reasonable?

On LCD panels and plasmas, you can actually see the pixels and measure their
pitch, which would let you know just how many uncropped pixels are visible on the
screen.  Anybody measured that?


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