Underscan issues with HD...

[dfitz43]

Hi all,

I'm just wondering if any one else has experienced problems with ATSC signals being very slightly underscanned in full screen when doing 1:1 pixel mapping with Sage (I'm currently running 4.1.12) ?

I just got a new plasma and am sending it native 1366 x 768 via DVI from a 6600 (non-GT). The desktop is perfectly displayed to the screen edges.

However, when I run HD signals, there is slight, but noticeable (ok, you have to be up close, but still, I don't want any pixels unutilized, isn't that the point of the native res?) underscanning of the image at the right and bottom of the screen.

It seems to me like the networks aren't actually using the full ATSC spec, i.e., my NBC is sending out something more like 1910 x 1060 or something. Thus when you display this signal with no overscan at all, there's some non-used pixels present.

I should also mention that the amount of underscanning varies depending on the network (Fox has a pretty significant band on the right edge of the picture, ABC appears to be almost entirely filling the screen).

I'm guessing this must be a network issue, since I'm confident that I'm getting 1:1 mapping on my display. I just thought that there wasn't supposed to be any overscan built into the HD spec.

I'm using overlay (since I could never get VMR9 stable with HD material). Could this make any difference?

I know there are overscan controls in Sage, but it's a bit annoying to make it overscan by 2 or 3% and then get menus chopped off a bit. Sounds like this may be the best solution, though.

Anybody else have this problem or insight into it?

cheers,
Dave

[dfitz43]

Ok, I came across this on the web:

SMTPE 274M-2003 standard, which is adopted by the ATSC for 1080 resolutions and which is, in turn, adopted by the FCC, specifies a region they call the 'clean aperture' or the area of the 1920x1080 image that is to be substantially free of edge case errors (sync or edge related). Quoting from the spec:

The clean aperture of the picture defines a region 1888 samples in width by 1062 lines high, symmetrically located in the production aperture. The clean aperture shall be substantially free from transient effects due to blanking and picture processing.

So I guess there is some overscan built into the ATSC spec, good to know.

cheers,
Dave