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On Tue, 5 Oct 2004, Eric Ball wrote:
> > VSYNC is used to generate the composite sync output SYN. (how, exactly?
> > is SYN simply VSYNC xor'd with the horizontal blank?)
>
> Almost. VSYNC causes the horizontal sync pulse timing to change from a
> single ~4.7uS pulse per line to two ~27.3uS pulses per line. These are the
> "serration pulses" which cause the vertical retrace to occur. (NTSC timing)
Ack. How is all this derived?
The NTSC colorburst frequency is 3.579545 MHz.
Divide by 3 to get a horizontal counter frequency of 1.1931816... MHz.
228 counts total per line.
160 counts of displayed pixels, followed by 68 counts of blank.
1 line takes 191.08573855 uS.
The blank lasts 56.99 uS.
Where does 4.7 uS come from? Does it happen at the beginning or end of
the blank? Where does 27.3 uS come from?
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