[Coco] 384 or 450 scanlines ?

Arthur Flexser flexser at fiu.edu
Mon Jan 13 13:10:23 EST 2014


Thanks for the explanation, John.  I'm curious, though, about why the
6847 non-interlaced VDG was chosen for the CoCo rather than the 6847Y
interlaced version.  And, would a 6847Y VDG work if you replaced the
6847 in a CoCo with it?

Art

On Mon, Jan 13, 2014 at 9:55 AM, John Kowalski <sockmaster at gmail.com> wrote:
> There seems to be a lot of confusion about the way video is formatted
> on the CoCo.
>
> The video output of the CoCo and pretty much all  pre-1990s home
> computers and game consoles is progressive scan with 262 lines at 60
> FRAMES per second (for NTSC) with 192 to 225 of the lines displaying
> the picture and the rest of the lines being border and vertical
> blanking.
>
> That means, 60 full frames per second.  Each frame contains the entire
> image and the images on-screen can be updated/animated at the full 60
> fps.
>
> ---
>
> Television and computers with interlaced video output 525 lines, 30
> times a second.  This breaks down into two FIELDS of 262.5 lines each.
>  These are output at 60 fields per second, but it takes two fields to
> make a complete 525 line frame, hence it's 30 frames per second, while
> also being 60 fields per second.
>
> A quirk: this means, it *is* possible to display motion at 60 fps even
> in interlaced video... just that each unique moving image only
> individually contains half of the full frame's resolution and only
> objects on the screen that are not moving are displayed at full
> resolution.
>
> ---
>
> The difference between the two...  Non-interlaced / progressive video
> generates 262 lines per frame/field.   Interlaced video generates
> 262.5 lines per field.   It's this half-step - alternating the
> vertical sync timing by half a line every field that causes the
> displayed video to wobble up and down by half a line each alternating
> field.
>
> You can make the CoCo's video output alternate between two different
> frames, but it still won't cause the monitor to 'wobble' / interlace
> the two images.
>
> You can trick the CoCo to generate alternating frames of 262 and 263
> lines, and this may cause some (but not all) monitors to wobble /
> interlace them, but it's still not technically standard interlaced
> video.
>
> What you'd have to do is find some way to cause the vertical sync
> pulse to delay by half a scan line every 2nd frame, which would turn
> those 262 and 263 line frames into 262.5 and 262.5 line fields.  If we
> could do that, then we'd have true interlaced video.
>
> --
> John Kowalski / Sock Master
> http://users.aei.ca/twilight/sock
>
> --
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> Coco at maltedmedia.com
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