Re: Explanation of the Woz's shift-registers based video output

["BLuRry" <brendan.robert@gmail.com>]

Here's a useful article that might have some clues for you:
http://rich12345.tripod.com/aiivideo/softalk.html

Note that the Apple video circuitry was originally designed against the
NTSC video signal.  I don't know enough about the differences between
PAL and NTSC to give you a full answer but this might at least be a
good start.

Here's some info about NTSC: http://www.ntsc-tv.com/ntsc-main-01.htm --
the Color Decoding part might be useful to understanding how the color
information works in the video signal.

Quote from Bob Bishop's "Have an Apple Split" article:

A standard television picture consists of 525 interlaced scan lines
that start in the upper left-hand corner of the screen and end in the
bottom-right. "Interlaced" means that the image on the screen is
created by first scanning all the even lines of the picture and then
going back and filling in all the odd lines. In each scan line, the
electron beam starts at the left side of the display and travels across
the screen to the right, displaying one line of the image as it moves.
When the beam finishes at the right end of each scan line, it shuts off
for a few microseconds while it repositions itself at the beginning of
the next line to be scanned. This shut-off period is referred to as
horizontal blanking (HBL). When the beam reaches the bottom of the
screen and finishes scanning the last line, it again shuts off, this
time for a few milliseconds while it repositions itself at the top of
the display for the next frame of the image. This second turn-off
period is referred to as vertical blanking (VBL). The total time
required to complete one interlaced frame (including VBLs) is
one-thirtieth of a second.

When the Apple computer was first being designed, it was discovered
that a computer-generated interlaced video signal produced a display
that had an objectionable flicker. So interlacing was not used, even
though this meant that half of the potential vertical resolution would
be lost. This lost spatial resolution allowed Apple to achieve
increased temoral resolution; instead of generating only thirty video
frames per second, the Apple generates sixty, because the Apple uses
the interlace to display a new 192-line frame.

A second design consideration in the Apple II was the decision not to
overscan the screen. "Overscanning" means that the image extends
slightly beyond the four edges of the screen so that the picture fills
the entire viewing area. Instead, so that information in the corners
won't be lost, the Apple leaves a small dark border of unused screen in
all of its display modes. This further reduces the number of visible
scan lines in each frame, so that only 192 lines remain in the display.

The Apple transfers its video data to the screen at a rate of one byte
per machine cycle. Each scan line requires a total of sixty-five cycles
from HBL to HBL. Since the Apple produces a forty-column display (forty
byes per line), we can deduce that twenty-five cycles of each scan line
are spent in a turned-off state and forty cycles are spent turned on.