[Coco] 384 or 450 scanlines ?

[Gene Heskett]

On Monday 13 January 2014 14:47:33 Arthur Flexser did opine:

> I guess what's puzzling me is this:  as I now understand it, every
> frame is ordinarily repeated twice in succession identically, with the
> second repetition landing smack on top of the first, with a frame
> occurring every 60th of a second and using only every other scan line.
>  Wouldn't it have looked better if the second repetition was one scan
> line offset from the first instead of on top of it?  Would the 6947Y
> VDG accomplish that?
> 
> Art

No, and to my knowledge it can't.  The Y version could only do both lower 
case and upper case, which has nothing to do with this.  To all, generating 
an interlaced signal for the sync's involves a huge number of binary 
divider stages, every one of with is working with odd numbers made even by 
doubling the parent clocking.  TV sync generators of the day were often 
done in ttl logic, needing quite a few chips (10 or more) to get the job 
done.  Thats chip real estate that did not exist when even the coco3 was 
being designed.

Such chips have been designed in the later '90's, but even then they needed 
lots of external supervision to get their ducks all waddling in a row.
 
> On Mon, Jan 13, 2014 at 1:48 PM, Nick Marentes <nickma at optusnet.com.au> 
wrote:
> > Arthur Flexser <flexser at ...> writes:
> >> 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?
> > 
> > And I thought that the interlacing was fixed in the TV/monitor of an
> > interlaced set. That a monitor always provides 525 lines interlaced
> > (NTSC).
> > 
> > That's why I thought the CoCo had to provide either 2 fields or 1
> > field and a blank field to satisfy the interlaced function of the
> > monitor.
> > 
> > I guess it makes sense that the early video game systems and TV's
> > would just create a single 262 line display that is repeated. I
> > didn't know that the TV/monitor could stop creating the interlaced
> > field.
> > 
> > Nick
> > 
> > 
> > 
> > 
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> 
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Cheers, Gene
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