NTSC video, unlike standard composite, is based on half-lines, not full
lines of video. Standard Macintosh video starts at the top LEFT of the
screen. The Macintosh then increments down the screen one line at a time,
ending the scan at the bottom right. The NTSC scan starts at the top
and MIDDLE of the screen, counts down every other line, and then ends
only halfway across the screen on the bottom, again in the MIDDLE of
the screen.
Now, the problem is that the current Macintosh LC and LC II video
architecture is based on routines with the basic unit as one full line
across. In order to do NTSC, the Macintosh LC II video architecture would
have to understand half-lines.
The Macintosh Display Card 4*8 and 8*24 use a completely programmable
timing logic, which can do NTSC timing quite easily. However, to implement
this logic on the Macintosh LC II logic board would require 3 to 5 thousand
more gates on video ASIC, which are just not available in the current
package. Additional video circuitry would be required.