Quote:
Originally Posted by idrougge
What I was wondering was how the 8 MHz clock mapped to the picture generation. The Amiga's 7.xx clock maps directly to the PAL/NTSC colour clock.
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http://alive.atari.org/alive9/ovrscn1.php
"A picture is displayed 50 times a second in 50Hz mode (or 60 times in 60Hz). The
ST generates 313 lines per picture at 50Hz (263 at 60Hz). In theory, it should
generate 312.5 lines, (obtained by successively displaying 312 and 313 lines).
It therefore assumes the monitor can tolerate displaying a picture at 49,92Hz
instead of 50Hz. This also explains why the ST cannot have a real interlace mode
such as that of the Amiga. Each line takes a constant time to be transmitted:
otherwise the picture would be distorted. This time is calculated using the
following formula:
1
-------------------------------------------------------------
(Number of pictures per second * number of lines per picture)
For "Number of pictures per second"=50 and "Number of lines per picture"=312.5,
we get 64 microseconds. But some will wonder how come there are 313 lines, if I
only see 200 on the screen? The other lines are used by the upper and lower
borders, and by the vertical synchronization signal which tells the monitor
where the top of the screen starts. Overscan is the process of converting the
lines used to make up the borders into lines able to display a picture loaded
from memory. I will call "useable screen" that part of the screen not occupied
by borders."