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

Michael T. Dean mtdean at thirdcontact.com
Wed May 31 05:53:29 UTC 2006

On 05/31/06 00:37, Brad Templeton wrote:

>Something for the HDTV experts to answer.

Sorry, not an HDTV expert, but I'll try some answers, anyway...

>   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?
>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?

Basically, you can choose any 2 out of 3 of:
    1) full resolution
    2) 1:1 pixel mapping
    3) no overscan
You can /not/ have all three.

>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?

50 years of tradition unhampered by progress...  ;)

>  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?

At least in the US, all TV's have overscan regardless of type--DLP, LCD, 
Plasma, LCD projection, ...  Monitors often don't (although dual-purpose 
monitor/TV's may).


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