[Coco] 384 or 450 scanlines ?

[Gene Heskett]

On Friday 10 January 2014 07:42:14 Nick Marentes did opine:

> Ciaran Anscomb <cocomalt at ...> writes:
> > I see it as only doing progressive - half resolution, double rate
> > (50Hz or 60Hz) progressive.  The machines never actually *emit* that
> > blank field you describe, the lines for it are just never scanned.
> 
> It would be good to know for sure.
> 
> I thought that a CoCo compatible RGB, composite or TV is an interlace
> video device only. 15Khz interlaced at 50/60 hertz.
> 
> If the CoCo is not generating the blank field and it's progressive,
> surely the frequency would be higher? (and it's not)
> 
> Anyone done some checks with a silly scope?
> 
> Nick
> 
The ntsc coco does 262 lines, progressive, at 60 hz for the ntsc version.  
I don't imagine the pal version is different except of course the vertical 
scan as at 50 hz.

True ntsc is 262.5 lines per vertical scan, which when doubled by the 
interlace is 525 lines at 29.94 full frames/second.  The vertical rate is 
59.94, repeated so that each succeeding field is offset in the vertical 
direction by 1/2 line, so that if everything works as planned, the 
individual fields lines fit between the previous field lines by persistence 
of vision in the human eye, with 2 fields making a full frame.  That 
doubled the apparent flicker rate, and halved the bandwidth needed to 
transmit it. 

Similar concessions were made when designing the pal std.

Cheers, Gene
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