Re: GS ROM 1 vs ROM 3 VGC differences

[Ed Eastman <noone@nowhere.net>]

Rubywand wrote:
> Unless it's a proprietary development thing, you could put the info up on your AppleLinc site.

I'm not ready to broadcast it to the world, it doesn't work yet!

> In a more general vein, what does a scan doubler do? Like, do you just send two correct width H syncs at the end of each line?

A 'scan doubler' compresses the data that makes up the (for example) 
NTSC scan line to fit into the time frame of a VGA scan line.  (About 
half the time.)  It may also broadcast the compressed scan line twice in 
succession to ensure proper timing and fill out the 'extra' scan lines 
supported on the VGA screen.

IIgs:  AAAAaaaa

       BBBBbbbb

       CCCCcccc

Same signal on VGA:
       A A  A  A
       a a a a
       B B  B B
       b b b b
       C  C C C
       c c c c

With a scan doubler on VGA:
       AAAAaaaa
       AAAAaaaa
       BBBBbbbb
       BBBBbbbb
       CCCCcccc
       CCCCcccc

Thankx,
Ed