Re: P21 video (Unidentified subject!)

[Adrian Godwin <agodwin@xxxxxxxxxxx>]

lowry@src.honeywell.com (Dave Lowry) wrote :

> MISC Readers-
> 
> I'm not a video expert, but isn't the problem with P21's colors mostly
> "chroma crawl"?  That's dots that appear to crawl up and down vertical
> edges.
> 
> The luminance and chrominance signals are supposed to be mixed together
> into one signal using some kind of rotating vector scheme.  My guess is that
> Chuck is just adding them linearly.
> 
> -Dave
> 

I'm not a video expert either, but I think the requirement is not on the
mixing method but the synchronisation : the chroma burst (and subsequent
chroma information) is supposed to be locked to the line sync and hence
the luminance. 

There's an 8-field rotating sequence of phase shifts for PAL and a
4-field rotating sequence for NTSC, but a lack of any fixed
relationship between colour subcarrier and line sync will result in a
beating effect visible as "chroma crawl".  I don't know why there's
a fractional relationship between the two frequencies (and hence the
4/8 field sequences) - probably to visually average out other errors.

A cheap way of getting around this is to tweak the subcarrier frequency
until the beat frequency is zero, but this isn't "real" PAL or NTSC
and tends to drift, eventually. The Amiga used a system clock (and
hence all other clocks) related to the subcarrier in order, I think,
to avoid just this problem - as a result, the NTSC Amiga ran at a
slightly different clock speed to the PAL Amiga. 

-adrian