Getting Rid of Overscan and Centering the Image

If the image displayed on your display device is not properly centered or has overscan, you may be able to correct it by following these procedures. The important thing to understand is that the visible part of the image is only a portion of the overall image you send to the screen. Since HDTVs can be very, very finicky about accepting a signal, it's best to get a modeline they can accept (see above) and then tweak it to get the image you want.

Let's say, in my case, my Olevia LT27HV would sync to a 1280x720 signal, but with the image centered too far to the right and to far up the screen, leaving a black bar on the left and at the bottom.

The orignal modeline that the TV was syncing to:

     # 1280x720 @ 60Hz (1280x720-60) vsync: 60.39Hz, hsync: 45.72kHz, clk: 74.25
     # This is the only true native mode for the LT27HV display.
     Modeline "1280x720-60" 74.25 1280 1312 1592 1624 720 735 742 757 \
       +hsync +vsync

This, however, left me with a black strip of about 1" on the right and on the bottom.

So, to move the image to the left, all we do is move the horizontal sync bar to the right. The horizontal sync bar is given by the 4th and 5th parameters in the modeline (1312 and 1592 above). That bar always stays fixed, so if we 'move' the bar to the left the picture moves to the right. Let's try adding 10 pixels to it:

     Modeline "1280x720-60" 74.25 1280 1322 1602 1624 720 735 742 757 \
       +hsync +vsync

The vertical sync bar is given by the 8th and 9th numbers (735 and 742 above), and works the same way: increase the numbers and the image moves up, decrease the numbers and the image moves down.

Now that the top left is positioned correctly, let's say we needed to fix some overscan. To do that, we'd tell the video card to put the image into a smaller portion of the overall frame it sends to the TV. We'd do that by reducing the numbers in the 3rd and 7th columns (1280 and 720 above). Now they read 1280x720 but if that's too big for the TV to display correctly, a bit of trial and error might produce a modeline where the image is centered correctly, and has no overscan:

     Modeline "1280x720-60" 74.25 1274 1322 1602 1624 716 730 737 757 \
       +hsync +vsync

For an excellent graphical and technical explanation, see:

     http://www.epanorama.net/documents/vga2rgb/timings.html