In the Apple ][ the long cycle is used to maintain a constant
relationship between COLOR REFERENCE and the horizontal scans. I'm
just wondering why a constant phase relationship is needed. If there
was no long cycle, the COLOR REFERENCE would toggle each line, as
intended by the NTSC standard. Some modifications would be necessary
though:
The vertical counter needs to be synchronized to the alternating COLOR
REFERENCE phase. I would feed COLOR REFERENCE to D13-6 (see
Understanding the Apple ][ page 3-14), so that the counter latches the
state of COLOR REFERENCE when preset instead of toggling before every
preset.
The H0 signal fed to A8-14 (page 8-12) needs to be XORed with VA.
In HIRES mode programming needs to be changed to compensate for the
alternating phase relations.
I'd expect better picture quality because straight vertical lines
would not be colored in one color but alternating green/red or blue/
orange. If the TV has a comb filter, the lines would not be colored at
all.
I don't have an old apple ][, I just have //e's, so I can't try it on
my own.
Does anybody know other reasons for the long cycle?
Ferdinand Meyer-Hermann