[mythtv-users] Affect of overscan / zoom on image quality?

Andy Burns fedora at adslpipe.co.uk
Sun Feb 10 08:48:46 UTC 2008

On 09/02/2008 12:57, Christopher Friend wrote:

> Hey all, does anyone know what the affect of using mythtv's inbuilt 
> overscan correction of zoomed aspect ratios is on video quality? I'm 
> currently using a frontend to output to a PAL 4:3 crt tv witha 720*576 
> modeline, 

generally you want as little munging of size/resolution/interlacing as 
possible to give a good picture.

> and watching PAL DVB-T broadcast tv.

PAL doesn't come into it, that is an analogue system, you probably just 
mean 720x576 at 50i (DVD terminology has muddied the water here)

> Because most of the 
> btoadcast stuff is widescreen (1048x576?) 

16:9 isn't broadcast in higher resolution than 4:3, it uses FHA (Full 
height Anamorphic) which means it is broadcast squeezed horizontally, 
then your receiver (mythth in this case) stretches is it back for 
display, that's it if it's displaying on 16:9, but in your case it's 
displaying on 4:3, so it has to either shrink the picture and add black 
bars to the top/bottom (this looses vertical detail), or zoom into the 
central part of the picture and chop off the left/right.

On "decent" channels they use 704x576 or 720x576, the "lower quality" 
channels use 540x768

See http://www.mymuxdata.tk/ for details per channel.

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