CompuServe Messages

>>Video displays 525 lines/sec regardless of whether your framebuffer displays 486 or only 480.<< Sorry but you are incorrect on this. 525 is the total number of horizontal scan lines per frame on the video raster. This 525 (262.5 for each field) includes the vertical blanking where no picture is present. The actual maximum number of scan lines used for picture display on the raster is 504, on a targa+ it is further reduced to 486. The maximum allowed in the specs is 21 scan lines for vertical blanking which reduces the picture area to a maximum of 504 lines. The specs refer to blanking, not picture display area. Most devices further crop the picture area on the vertical by an additional 18 lines split between the top and bottom of the picture ( this is not regulated). Using the specification the maximum vertical resolution of NTSC video could only be 504 scan lines as the blanking would not count. Due to the way a TV picture is drawn on the screen it would be impossible to cram more pixels into a scan line than one so the maximum vertical resolution in pixels would be 504 pixels. Computer devices use 480 pixels as one form of vertical resolution. It is a simple matter to convert to one pixel per scan line than try to resample the vertical information in the picture to 504/486 pixels per scan line, therefore approximately 480 is used which includes a 7.5 IRE frame border. Depending on your frame of reference this border may not be part of your resolution calculation as it does not contain any picture information. As for overscan, the amount is controlled by the receiver TV and is not regulated but if 10% it would be displaying 525-(10% * 525) or 472.5 lines completely hiding a black border of a 480 pixel display. The 504 figure is only theoretical as "resolution" is an arbitrary figure and the generally accepted value is luminance at 343 lines or about 70% ot numerical theory. This is due to display screen capability and other factors such as electron beam angle deflection refraction at screen edges etc. The horizontal resolution is a completely different issue since it involves a continuous trace and can be divided as much as the bandwidth of the video signal allows.