[Coco] 384 or 450 scanlines ?

[Nick Marentes]

Theodore (Alex) Evans <alxevans at ...> writes:

> 
> 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 ?


My understanding is that the CoCo creates 60 alternating *fields*... one has
CoCo video image (225 active visible scanlines) and the other is totally
blank. This amounts to 30 *frames* when interlaced (NTSC).

In order to have 450 scanlines, we need to have data for that 2nd blank
field. As it stands, we get the black scanlines.

Merely flipping screens as you mentioned, would not do it because each
flipped page appears on the same active field. There is always going to be
that blank field.

There are 2 methods that can "conjure" a 450 scanlines though.

One is a hardware mod (created by Alan Dekok I believe) that "physically"
allows the video frame to offset by 1 scanline

or

John Kowalsi's trick of fooling the 1986 GIME to do a "physical" offset.

Once you can offset the image so that it is positioned where the black
scanlines normally would be, then you can switch the display page to a
second image to create the 450 scanlines of data.

I think there is still a bit of flicker because it will essentially be
creating a 30 fps "vga" like display but it will do what you asked.

I contemplated using this as the title page for my game but because it only
worked (John's software method) on '86 GIME's, I didn't think it worth the
effort and additional RAM for such a small gain in visual display.

Nick