Re: IIgs video modes...

["bieling@terra.es" <bieling@terra.es>]

On Feb 27, 10:00 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> David Empson wrote:
> > biel...@terra.es <biel...@terra.es> wrote:
>
> >>On 27 feb, 03:52, "Michael J. Mahon" <mjma...@aol.com> wrote:
>
> >>>I didn't think that SHR was useful on an NTSC monitor--or at least
> >>>not in color.  I thought it was an RGB mode.
>
> >>I didn't know... then it doesn't matter if the pixel clock is 8/16
> >>Mhz..
>
> > SHR mode seems to work fine on the composite (monochrome) monitor which
> > has been connected to my IIgs for at least the last decade (since my RGB
> > monitor gave up).
>
> > The horizontal line is the same width in SHR and other video modes,
> > implying the same time (about 40 microseconds) for the active portion of
> > the row, so it must have a higher frequency dot clock in order to fit
> > 320 or 640 pixels in the same space as 280 or 560 for other modes.
>
> > The standard video modes use a dot clock frequency of 14.31818 MHz or
> > 7.15909 MHz. In order to get the correct number of pixels, SHR mode must
> > use 8/7 times these, i.e. 16.36363 MHz or 8.18181 MHz.
>
> > If I remember right, the IIgs has a master clock frequency of 28.63636
> > MHz. Deriving 8.18181 from that is reasonably OK: a divisor of 3.5, i.e.
> > 7 half cycles. It would be harder to derive 16.36363 MHz, which would
> > require a divisor of 1.75. DPLL, perhaps?
>
> > Not sure how this would work for NTSC. I need to confer with a friend
> > who understands TV more than I do.
>
> The answer is in the schematic--the NTSC color signal is created
> not by the multiple-of-3.58MHz pulse scheme that is the Apple II
> standard, but by a chrominance modulator driven by the RGB signals.
> Therefore, its color rendition is not dependent on its pixel rate.
>
> -michael

How many colors can one of these pixels have ?
I ask, if this is so, in a CRO you'd see a "sinusoidal-like" carrier
added on top of the typical pixels' squarewave (that now have in fact
become the luminance ?).
More or less ?
You mean this differs from the Apple II standard that you've sometimes
called "artifact color"... right ?

Jorge.