[mythtv-users] Resolution

[James L. Paul]

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On Wednesday 03 December 2003 10:43, audin at okb-1.org wrote:
> > > So the way I understood it, although all NTSC video has 525 lines,
> > > about 480 of them viewable, the analog storage and reproduction of VHS
> > > tape doesn't accurately retain the resolution of all them, and results
> > > in about half. It's as if the interlaced video interpolates itself
> > > unnecessarily. This may be totally wrong. I've never questioned it
> > > though since seems to match my experience.
> >
> > 	I thought about that, and that's actually what I was thinkign when I
> > was searching for the definition of resolution.  I'd be tempted to
> > believe it, but remember that two adjacent lines on the screen are put on
> > the tape 1/30th of a second later (well, 1/29.97th, but who's counting!).
> >  If they were to bleed into each other, they would soften the whole thing
> > up so that you couldn't distinguish individual fields.  Recording from
> > VHS tape at 480 vertical pixels definately shows a clearly interlaced
> > picture.
> >
> > 	I'm not saying that there's not bleed-through between adjacent scans on
> > the tape, but it's not much of a factor.
>
> 	Except that fields are layed down in succession.  All lines making up
> the first field are layed down, then the lines for the second field.  So
> the crosstalk (which is in fact deliberate in VHS EP/SLP recordings, the
> edges of the scans actually overlap) is between alternate lines.  So you
> get both the crappy quality of crosstalk AND the crappy quality of
> interlacing all at once.
>
> :)
>
> 	VHS also has a general problem of only providing 3MHz bandwidth, while
> a full NTSC signal is 6MHz...  I believe this is SVHS's major improvement,
> it provides 5MHz or more.

There you go! That's the easiest way of explaining it, rather than trivia 
about the way VHS stores fields on tape. However it stores the video on tape, 
VHS results in less bandwidth than needed for NTSC. And the term crosstalk is 
when I should have been using instead of bleed.

Thanks!
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