Re: Apple ][gs VGA Card

[toddpw@avarice.ugcs.caltech.edu (Todd P. Whitesel)]

herbertf@ersys.edmonton.ab.ca (Herbert Fung) writes:

>I see what you are saying, but I can't understand how we perceive it as
>higher resolution. Perhaps it's just an optical illusion.

Actually it is. The two alternating frames are offset from each other by
half a vertical scan-line, so they mesh together visually and your eye's
persistence of vision puts them together to make a 400 line picture.

This method was invented long ago and is used in broadcast television.
It exploits the scanning operation of the CRT in the vertical direction.
Interlacing is accomplished by delaying the "odd" frame by half a
scan-line time, so it appears half a scan-line lower on the screen than
an "even" frame.

Todd Whitesel
toddpw @ ugcs.caltech.edu