[Coco] 384 or 450 scanlines ?

[Theodore (Alex) Evans]

On 01/09/2014 03:18 PM, Mathieu Bouchard wrote:
>
> If you switch between graphics pages at every vblank, can you get double
> the number of scanlines, as in 2*192=384 or 2*225=450 ?
>
> And then, how do the RGB output differ from NTSC ? What I remember is
> that CGA monitors show a frame in two different passes of 100 scanlines,
> whereas VGA monitors (and NTSC) do it in two normally identical passes
> of 200 scanlines (doublescan). Or am I saying something wrong ?

CGA uses the same scan rates as NTSC.  As the TV folks should confirm, 
an NTSC field consists of 262.5 lines at 15.75kHz.  The CoCo does not 
actually confirm to the standard as each field has a whole number of 
lines making interlacing by just switching images every field not really 
work right.  Some older 1024x768 VGA monitors and cards used interlaced 
video, but basic VGA (640x480) is non-interlaced and all modern SVGA 
displays are non-interlaced.