NES Graphics – Part 3 • Hacker News Board

Re: NES Graphics – Part 3

#21

Post by jeff_tyrrill » Tue, Dec 22, 2015, 1:50 AM UTC

cbd1984 wrote: post #18

Something else the article gets wrong, and this is a nitpick, is that it implies every scanline is redrawn every single time. This is called progressive video, and CRTs didn't do this. CRTs did something called interlaced video, where every other scanline is redrawn in a given screen update: Evens, then odds, then evens, and so on. This was done to reduce bandwidth requirements and to prevent flicker. (Progressive ve…

Watch a broadcast TV signal on an old CRT: The scanlines appear to roll up (or down, depending on how you are trying to perceive them).

Hook up an NES to an old CRT and look closely at the screen: The scanlines are absolutely still. (This is also the case with the full-screen menus of VCRs, with the white text on blue background.)

The NES most definitely outputs progressive-scan video at 240p, with all 240 lines drawn 60 times per second.

Re: NES Graphics – Part 3

#22

Post by thought_alarm » Tue, Dec 22, 2015, 2:05 AM UTC

cbd1984 wrote: post #18

Something else the article gets wrong, and this is a nitpick, is that it implies every scanline is redrawn every single time. This is called progressive video, and CRTs didn't do this. CRTs did something called interlaced video, where every other scanline is redrawn in a given screen update: Evens, then odds, then evens, and so on. This was done to reduce bandwidth requirements and to prevent flicker. (Progressive ve…

It's interesting how often people make this mistake.

The NES, like virtually all home computing hardware of the 70s and 80s, operates at 60 hz progressive scan of 200 lines, give or take a few. That is, of course, the same bandwidth of the 525 interlaced lines of broadcast video.

In those days, the horizontal and vertical resolution was bound by RAM capacity rather than pure video signal bandwidth.

In either case, the screen is updated at 60 hz.

Re: NES Graphics – Part 3

#23

Post by rasz_pl » Tue, Dec 22, 2015, 7:13 AM UTC

Oh boy, those were the tough times for chip designers. sprite0 and mapper triggering are clever hacks that could have been avoided with <100 transistors (8 bit counter) in the PPU to let you trigger an interrupt on arbitrary raster.