Re: IIgs video modes...

[dempson@actrix.gen.nz (David Empson)]

Michael J. Mahon <mjmahon@aol.com> wrote:

> David Empson wrote:
> > bieling@terra.es <bieling@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.

My primary interest with respect to the composite output is in the
luminance (monochrome portion of the video output), which will be pixel
based and must be generated at a higher horizontal dot clock rate in SHR
mode.

The RGB data going into the modulator must supply individual RGB pixels
at the same rate. The chrominance which is converted into the colour
burst will be at a lower resolution than the luminance.

The RGB video output will also have higher frequency individual pixels
in SHR mode.

-- 
David Empson
dempson@actrix.gen.nz