For the life of me, I can't quite figure out how the NES renders it image on an interlaced screen. The PPU sends out 240 line frames 60 times per second to an NTSC monitor. The NTSC standard draws 262.5 lines 60 times per second, odd lines first, even lines next, and so on. It is designed to show a 525 line image at 30 frames per second. Of those lines, 480 are visible while some of the excess are used for sychronization. The NES every frame uses 22 lines for this, and sends 240 lines 60 times per second. But the NES can draw all of its graphics on the odd lines. So what does it do to the even lines?
On a TV, the NES appears to cover the whole of the visible screen. In fact, on most average CRT screens the NES image even displays outside of the visible screen. The NES cannot simply only draw to the odd lines. While scanlines are relatively approximated on a TV monitor because the phosphor dots do not line up horizontally, all the scanlines are used. Yet the picture should be sandwitched in the middle of the screen. (Still talking NTSC here.) Since the ratio of a NES frame is 1.06:1, the screen stretches the horizontal pixels. It does not have the resolution to split up the images to combine them into one full frame like other devices that can display 480i. Is there some form of line doubling going on here like on a VGA monitor?
So what is going on here? How does the NES show a full screen on an NTSC device? I think its essentially widening the beams, using scanlines as wide as possible so that the odd and even fields of a NES frame take up the full screen. Am I right?