[mythtv-users] Stupid overscan question

[chris at cpr.homelinux.net]

On Wed, Nov 09, 2005 at 08:36:09AM -0800, Ben Dash wrote:
> Could someone please explain to me how overscan works?

There's two types of overscan.

The one you set from within MythTV's setup menu is really just "crop 
and zoom".  If you overscan by 2% then the original recording has the 
outer 2% of the image clipped off on each side and the remaining 96% is 
stretched to fit the requested screen size.  This gets rid of any edge 
noise that may have been captured by the recording.  It doesn't move 
the visible output on the display.

The more traditional use of the term applies to the horizontal and 
vertical timing of the signal.  The broadcasters can't do the "crop and 
zoom" for you (as the cropped and zoomed image would pick up the same 
edge noise during transmission) so they send the whole frame and expect 
your TV to only look at the middle 96% of the image.  New LCD-type 
monitors would do that using a digital crop and zoom, but older 
televisions would "overscan", which is to say that a 26-inch TV would 
sweep the electron beam across the CRT as if it was 28 inches, and 
those extra few inches of signal would be behind the bezel in a 
non-phosphor area where you couldn't see it.

> I'm using the nv driver for my xBox and the top of the
> displayed screen is a little narrower than the bottom,
> i.e. the bottom reaches the screen edge and the top is
> inset a little.

Positioning problems are more properly fixed by altering the modeline 
timings to reduce the retrace delay.  If you apply a constant overscan, 
by the time you get the top of the image to the top of the screen you 
will lose far too much off the bottom of the image.  I don't know if 
the XBox driver supports custom modelines or not.