[SOLVED] Define underscan only for 720p

[p4trykx]

edit
Look for the partial solution in the last post. It's kind of workaround but it works on Sony TVs

My TV Sony KDL-46v5500 (few years old) does not support fully 720p input but works fine with 1080. The problem is that with 720p I get an overscan. Depending on video source size XBMC switches between those two resolutions.
So when I'm in the main menu or play full HD movies it displays at 1080 just fine without any overscan or black stripes at the edges.
But when displaying something less than 720p the resolution is reduced and portions of screen are cut.
The solution would be to just define somewhere the overscan just for 720p but AFAIK it's not possible to do it on config.txt without affecting 1080p.

I have another TV which works fine with both resolutions so I suspect it's a Sony bug. On LG I get (native) shown both on 720p and 1080p but on Sony only at 1080p.

From Sony

Code:

xbian@xbian:~# tvservice  -m CEA
Group CEA has 16 modes:
           mode 1: 640x480 @ 60Hz, progressive
           mode 2: 720x480 @ 60Hz, progressive
           mode 3: 720x480 @ 60Hz, progressive
           mode 4: 1280x720 @ 60Hz, progressive
           mode 5: 1920x1080 @ 60Hz, interlaced
           mode 6: 720x480 @ 60Hz, interlaced
           mode 7: 720x480 @ 60Hz, interlaced
  (native) mode 16: 1920x1080 @ 60Hz, progressive
           mode 17: 720x576 @ 50Hz, progressive
           mode 18: 720x576 @ 50Hz, progressive
           mode 19: 1280x720 @ 50Hz, progressive
           mode 20: 1920x1080 @ 50Hz, interlaced
           mode 21: 720x576 @ 50Hz, interlaced
           mode 22: 720x576 @ 50Hz, interlaced
           mode 31: 1920x1080 @ 50Hz, progressive
           mode 32: 1920x1080 @ 24Hz, progressive

From LG

Code:

xbian@xbian1:~# tvservice -m CEA
Group CEA has 15 modes:
           mode 1: 640x480 @ 60Hz, progressive
           mode 2: 720x480 @ 60Hz, progressive
           mode 3: 720x480 @ 60Hz, progressive
  (native) mode 4: 1280x720 @ 60Hz, progressive
           mode 5: 1920x1080 @ 60Hz, interlaced
  (native) mode 16: 1920x1080 @ 60Hz, progressive
           mode 17: 720x576 @ 50Hz, progressive
           mode 18: 720x576 @ 50Hz, progressive
           mode 19: 1280x720 @ 50Hz, progressive
           mode 20: 1920x1080 @ 50Hz, interlaced
           mode 21: 720x576 @ 50Hz, interlaced
           mode 31: 1920x1080 @ 50Hz, progressive
           mode 32: 1920x1080 @ 24Hz, progressive
           mode 33: 1920x1080 @ 25Hz, progressive
           mode 34: 1920x1080 @ 30Hz, progressive