Re: Apple ][gs VGA Card

[nathan@pride.ugcs.caltech.edu (Nathan Mates)]

In article <gPR9Tc3w165w@ersys.edmonton.ab.ca>,
Herbert Fung <herbertf@ersys.edmonton.ab.ca> wrote:
>>Interlaced = the image is sent in two parts (theoretically more but I've
>>not seen it implemented).  The video signal represents half the screen
>>each time.  The hardware outputs the signal for alternate scan lines
>>(btw a scanline = one horizontal row of pixels) all the way down the
>>screen, and then the next cycle provides the ones in between which were
>>skipped last time.  This makes the screen update cycle half of the
>>original and can occasionally show picture abberations like flicker
>>(uncommon) or aliasing.

>How does this help display higher resolution modes?

   Under one mode, everything is sent out twice: once for the even
rows, and then the same thing on the odd rows. This makes sure
everything appears in larger pixels (even wonder why IBM 320x200 has
such big ugly blocky pixels?). But, if you were to send a different
picture on the even and odd rows, then you can double the vertical
resolution without too  much effort. Thus, the difference between
640x200 and 640x400 is that in the x200 mode, each row is displayed
twice, but only once in x400. 

Nathan Mates