[Coco] Re: CoCo to SVGA

[John Kowalski]

At 11:28 PM 31/10/2004 -0600, Roger Taylor wrote:
>If Super-MAC was coded to take advantage of some kind of GIME flaw that 
>causes the vertical offset on some monitors to move up or down the screen 
>every so slightly, then one of the fields (flickered frames) could be done 
>this way to cause a real nice interlaced video mode.  This would give twice 
>the vertical height of normal video.  A 640x400 image might be 
>displayable.  Keep in mind that the height of the screen wouldn't change, 
>but you'd get ~better~ vertical resolution.

The 1986 version of the GIME has a bug where you can cause it to skip count
of scan lines.  Triggering this effect once every two frames would cause the
video generator to display 263 scan lines on even frames, and 262 scan lines
on odd frames.
It's not exactly how NTSC interlacing is generated, but it's enough to cause
many monitors to shift the lines down by half a scan line.

And if you wanted, you could trigger the effect more than once per frame.
It may actually be possible to push it so far as to generate oddball video
like 120Hz with 132 scan lines - instead of the usual 60Hz and 263 scanlines.

                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/