16.033 Including the left and right borders, how many dots are actually sent per line in 320 and 640 modes on the GS?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
16.033 Including the left and right borders, how many dots are actually sent per line in 320 and 640 modes on the GS?
Including the left and right borders, plus the horizontal retrace, the
video sends out more than 320 or 640 "dots" before the screen begins scanning
the next line. Here's a quick summary of the Apple II video that's been used
throughout the whole series (assuming 60 Hz video here). It's a slightly
modified NTSC signal, though the differences are small enough for most monitors
to accomodate.
Everything in the Apple II is derived from a 14.31818 MHz master clock.
This runs both the video and the processor/memory subsystems. The cycle time is
therefore 69.8 ns. For 640-mode (or 80 cols, or double-hires) the dots are sent
out at simply this rate. In 320-mode (40 cols or single-hires) the dots are
sent out at half that rate, 7.15909 MHz (one pixel every 139.6 ns). The NTSC
standard calls for 227.5 cycles of color reference (3.579545 MHz) per
horizontal line, for a horizontal scan rate of 15.7
34 KHz (3.579545/.2275). The Apple II rounds this up to 228 cycles of 3M, so
the horizontal scan rate is 15.699 KHz (3.579545/.228). This is well within the
tolerance of most monitors. So therefore in 320-mode 228*2 = 456 "dots". In
640-mode there are 912 "dots". I put dots in quotes because, obviously, only
320 or 640 of them are actually seen as part of the screen.
When the GS is outputting a composite video signal, each
horizontal line must contain a horizontal sync pulse and color
burst, and there can't be a border during this time, or else the
TV won't register a sync. On the RGB, the GS puts the border
color on. So, the answer to your question is 456-320 or 136
pixels in 320-mode and 912-640 or 272 pixels in 640-mode. This
includes both borders and the horizontal sync. Of course, a good
proportion of this time is not visible; this depends on the
overscan settings of the particular monitor.
By: Eric Jacobs
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