ewsmith@uokmax.UUCP (Edward Warren Smith) (04/23/89)
The Laser 128's VDG is an enhanced version of the Apple //c's, including
16 screen border colors, interlacing for smooth text (no raster lines), and
quadruple high resolution (QHR 560 x 384) graphics.
Usually when both my head and the monitor are still, the raster lines
disappear, leaving square-pixeled text with all vertical lines unbroken.
It looks so good with a high-persistence phosphor monitor I want to implement
a circuit on my Apple //e which creates a second NTSC field for anything the
Apple's generator puts out.
Have any commercial or hobby modifications been published to improve the
display by creating the second field by delaying alternate screen redraws?
I have figured (perhaps erroneously) the VDG output must be delayed by some
32 milliseconds after the VBI to allow this. I am also aware the Apple //e's
IOU pops out a VBI signal.
I figure this way: 1 / ( 2(NTSC vertical rate)(raster lines per field) )
= time between two raster lines / 2
Figured: 1 / (2 * 59.86 * 262) = 31.88 milliseconds delay
For Laser 128 owners, you can see what I mean by going into the monitor
and accessing these soft switches:
$C023: CX C = Low-Res color X = Don't Care
$C024: XX Turn on two-field display.
$C025: XX Turn off two-field
$C026: XX Turn on QHR and two-field
$C027: XX Turn off QHR (ignore two-field status).
I see why Apple Computer didn't do this as the two-field operation's
success depends on the monitor, low-persistence monitors will jitter,
others will not be affected by a small delay (and not change the display).
The two-field system is unreliable in the field. But for my Laser 128
and //c Monitor, it noticably improves the text's readability.
Any sources for a VBI-signal triggered 32 ms delay? I may grab a T-Flip
flop and two SAD-1024s and try this myself when I get a chance to dig
into the Apple's innards.
Ed
UUCP: ewsmith@uokmax
CIS: 72401,3314
FIDO: Later
APLE: By May 10 89