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