I’m still confused about the Xbox output resolution and overscan. So the full NTSC raster is 858 x 525, and visible picture is 710.85 x 486. The Xbox output resolution is 720 x 480. Does this mean that if you had a CRT that showed the full raster without any overscan, there would be a thicker border than normal around the Xbox image right? If so, why does Microsoft recommend an 85% overscan when the image is already smaller than the full raster? Or is that just reference to 85% overscan of the full raster, not the 720 x 480 Xbox image? Or is 720 x 480 image blanked automatically so it fills about the same space as the 710 x 486 NTSC visible area, or does the Xbox output image have this blanking information in it?
Edit: So the impression I’m getting is that there is a minimum blanking interval forced by the CRT, but the source signal can add additional blanking.
In analog television systems the vertical blanking interval can be used for datacasting (to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals…
This suggests that any kind of data sent outside the regular data interval of 710.85 samples, even if it’s formatted as video information, will not be projected by the CRT during the blanking period. However,
On many consoles there is an extended blanking period, as the console opts to paint graphics on fewer lines than the television would natively allow, permitting its output to be surrounded by a border. On some very early machines such as the Atari 2600, the programmer is in full control of video output and therefore may select their own blanking period, allowing arbitrarily few painted lines.
This then suggests that there is the ability to add extra blanking to the signal at any point. So in a game like Pong for instance, you could have any area that is known to never have active pixels be defined as a blank rather than as actual data, which I assume would lower the data processing need for the GPU.
So to answer my own question, the Xbox kind of does and doesn’t supply the blanking interval, and the 85% overscan is in reference to the 720 x 480 picture, though this was more of a safe number for worst case scenario as I believe CRTs usually overscanned no more than 5%. The Xbox sends data in a way that conforms to the minimum blanking interval set by the CRT, meaning there are valleys of non-data between each scanline (I think…). If the Xbox tried to supply data for a longer period on each scanline, it would simply be ignored by the CRT. The interesting thing is that this actually does happen for 720×480 signals from the Xbox; the extra 9 samples outside of the 710.85 (711) active picture area couldn’t naturally be seen on a CRT, even without overscan, unless you were to calibrate the CRT’s blanking interval to show this data. And some Xbox games actually do output video data over these 9 extra pixels, like Ninja Gaiden, while most games don’t. But the impression I’m getting is that most Xbox games that were authored “normally” are only using 704 x 480 pixels for active information since this matches the aspect ratio of the 711 x 486 matrix (“1.46”, which when converted to real space with square pixels is 4:3 = 1.33), just to insure that if overscan is disabled that the picture will still look perfectly 4:3 on the screen. Also fun fact, the top and bottom 3 lines would be blank when playing Xbox on a CRT with overscan disabled due to the actual area being 486 lines tall while the Xbox output signal is 480 lines tall.
Info on format conversion: http://xpt.sourceforge.net/techdocs/media/video/dvd/dvd04-DVDAuthoringSpecwise/ar01s02.html