I read many resources on the internet talking about NTSC, PAL, timing and voltage levels. But I have specific practical question, and I can not find definite answer to it. There're very helpful posts on Ataris, but they also more theoretical than practical.
The situation: the video display processor (VDP) outputs 1368 color clocks per line in NTSC mode, with 1024 in the visible "active screen" area. It has dot clock of 3.579545*6=21.477270 MHz, and thus display of whole line takes 21477270/1368 ~ 63.669 us (15700 Hz).
VDP has graphics modes of 256 pixels per line, and another of 512 pixels per line - displayed within 1024 clock dot periods. I assume that in former mode each pixel is clocked 4 times, and in latter 2 times.
In RGB mode receiver is free to choose its dot clock, performing oversampling or detection of the underlying a dot clock, or just use clock in accordance to its screen resolution.
But for me it is not clear what happens in NTSC mode - in both composite and S-video (with luma and chroma separated).
As I understand, NTSC's "transmission speed" is 3.579545 MHz, thus it will be performing undersampling of the color (and luma) data generated by the VDP.
Simple calculation gives the following:
- Dot clock = 3579545 Hz
- Line display time = 63695.24618 us
- Dots displayed on the line = 228.
Using the similar calculations for 1024 displayable area I get 170.6666667 dots.
Even for 256 pixels per line, with 3.579545 MHz clock, the media will be able to sample first dot of first group of 4, then third dot of the second group, and will miss third group completely. And so on, with each third dot missed completely.
From this I conclude that composite can not achieve 256 dots/pixels, and image will be displayed with color data loss?
Then how old good CRT monitors were able to display very decent 256 horizontal dot picture over composite?