Re: How does NTSC VIC-II produce 65 cycles per line

> On 12 Jan 2023, at 13:49, Bill Degnan <billdegnan_at_gmail.com> wrote:
> 
> 
> > On 12 Jan 2023, at 12:21, Michiel Boland <michiel_at_boland.org> wrote:
> > 
> > Hi. The PAL VIC-II chip has 9 bits for the X coordinate. That can accommodate 504 pixels (63 cycles) per line. I would assume that the NTSC VIC-II also has only 9 bits for X coordinate. Then how does that chip manage to produce 65 cycles per line? Since 520 pixels would require 10 bits.
> 
> FWIW - a number of cycles per line (in every version) do not output any pixels
> 
> I know that when one changes from / to NTSC they have to change both the crystal and the VIC-II chip, so maybe understanding the relationship between these two might answer your question?  This is a guess, I am not an electrical engineer. :-)
> Bill 

The crystal change is needed because the frequencies (like chroma subcarrier) used by PAL and NTSC differ. Those are not synthesised but derived from the crystal frequency. Generally you can change only VIC chip and leave crystal in place and you'll still have output but there'll be no colour.

What I was referring to is that the number of pixels reflects neither 63 nor 65 cycles per line. There are hblank cycles, which do not put any pixels out and therefore there is no direct relation between number of cycles per line and the number of pixels produces / bit needed to handle them