Dreamcast:Video Cables

The Dreamcast natively supports 240p, 480i, and 480p resolutions. .

VGA
The VGA output of the Dreamcast is 480p, but it is not the "typical" 480p DTV resolution(720 x 480) of televisions. The VGA output of the Dreamcast is 480p VESA resolution (640 x 480) within a 480p DTV signal.

This means that the correct sample rate of the Dreamcast 480p signal is 858 pixels per line, the same sample rate of any 480p DTV signal. If the signal is interpreted as VESA only 800 pixels per line will be sampled, and at this sample rate 43 pixels columns will be lost within the active picture area. Also, if sampled at 800 pixels per line neighboring columns get blurry since the 43 columns are evenly distributed across the screen.