[mythtv-users] Resolution

James L. Paul james at mauibay.net
Wed Dec 3 16:08:26 EST 2003


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On Wednesday 03 December 2003 10:30, papenfuss at juneau.me.vt.edu wrote:
> On Wed, 3 Dec 2003,  James L. Paul wrote:
> > > SO, with this in mind, take a few data points.  NTSC VHS has 240 lines.
> > > Formatted at 4:3, this translates into 480 vertical pixels (NTSC
> > > viewable scan lines), and horizonally (240 * 4/3) = 320.  So, to
> > > capture VHS-quality video, you need a capture resolution of 320x480...
> > > 1/2 D1 is close at 352x480.  To capture high-quality broadcast at 330
> > > lines, (330 * 4/3) = 440... SVCD is close at 480x480.  Capturing any
> > > more than that wastes encoding time, space, encoding quality, etc.
> >
> > This is a much better description than mine. As I remember it though
> > there was some effect caused by interlacing, real-world signal
> > loss/distortion and quality of video tape that made VHS generally
> > considered 352x240, although I can see now the 352 is obviously inflated.
> > (Perhaps that explains why all the VCDs I made from my DirecTV looked
> > better than VHS from the same source.) I think it's the way the
> > interlaced frames are helically laid down on the tape, there is
> > significant bleed on VHS, but SVHS used denser tape and probably smaller
> > heads.
>
> 	The 352 is not all that inflated.  At the rated 240 lines, that equates
> to 320 pixels horizonally.
>
> > 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.

I wasn't talking about bleed that way, perhaps bleed is a bad word choice. Of 
course you get properly interlaced video. I meant that the nature of storing 
analog signals on magnetic tape so closely together and the process of 
writing/reading them with a head that isn't fine enough to completely avoid 
the adjacent signal of the interlaced fields on the tape.

The signal on the tape has a strong center and drops off sharply the farther 
you get from the center, and the interlaced field signals are written 
alternately adjacent to each other. It was the signals being so close to each 
other on the tape that I meant by bleed. As each field is laid down on the 
tape, the recording head isn't far enough away from the previous field to 
totally avoid affecting it, and the same thing happens when reading, the read 
head gets the center of the field but isn't totally unaffected by the two 
interlaced fields next to it. This is all part of what gives VHS the quality 
it has. (Or doesn't have, as the case may be.)

Disclaimer: I don't have any references to back this up, I don't remember 
where I learned this, and I may be full of hot air on this one.

As for beelding into each other such that they soften up or being unable to 
distinguish individual fields, this does happen, and it's why VHS has a shelf 
life. I have tapes from 20 years ago that are unplayable now, and others that 
get worse each year when I pop them in to check. (I do this out of curiosity, 
I started archiving my VHS to VCD in 1996, and switched to CVD in 2001, and 
now use DVD. Every year of so I sit down and play tapes I "rescued" to see 
how long they actually lasted. :)

> -Cory
>
>  --
> *************************************************************************
> * The prime directive of Linux:  					*
> * 	- learn what you don't know, 					*
> * 	- teach what you do.						*
> * 						(Just my 20 USm$)	*
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