[mythtv-users] Resolution
Boyd II, Willy
wboyd at fulbright.com
Wed Dec 3 16:19:20 EST 2003
>-----Original Message-----
>From: James L. Paul [mailto:james at mauibay.net]
>Sent: Wednesday, December 03, 2003 3:08 PM
>To: papenfuss at vt.edu; Discussion about mythtv;
>papenfuss at juneau.me.vt.edu
>Subject: Re: [mythtv-users] Resolution
>
>
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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. :)
Just out of curiousity, do you use a certain brand of cd-r or dvd-r discs
in order to maximize shelf life for those archives? (I'm merely
piggy-backing off your archival
research :-)
- Willy
>
>> -Cory
>>
>> --
>>
>***************************************************************
>**********
>> * The prime directive of Linux:
> *
>> * - learn what you don't know,
> *
>> * - teach what you do.
> *
>> * (Just my 20
>USm$) *
>>
>**********************************************************************
>> ***
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