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Topic: 480 vs. 486 lines?

Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 05-02-2006 02:37 PM

As I understand it, NTSC video has 486 usable horizontal lines (the remainder of the 525 are used for the VBI).

So, why is it that DVDs can only reproduce 480 lines? Not that it matters much (since the extra lines would be lost due to the overscan in a typical set), but why wasn't the format designed to display the entire usable NTSC picture?

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Mark Ogden
Jedi Master Film Handler

Posts: 943
From: Little Falls, N.J.
Registered: Jun 99

posted 05-02-2006 05:05 PM

Even though a DVD can actually produce about 500tvl (and NTSC, in theory, 486 visible lines), 480 was the standard accepted by the MPEG DVD group as the generally safe display area for consumer sets, given variations in scanning and voltage regulation between manufacturers. It’s basically just what everyone agreed to be MPEG-2 video standards, based on what most sets are capable of displaying. DVD, being an MPEG-2 device, follows suit.

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 05-02-2006 07:13 PM

It's not just DVD, but DV, videogame systems, and other crap like that as well. 486 lines is like high definition compared to 480, and if they gave us all 486 lines, nobody would buy the fancy-schmancy new HDTVs. It all has to do with $$$$$$.

NTSC DVD maxes out at 702x480, I believe. Maybe 720x480. But it's ALWAYS 480. Only CRTs have any kind of image overscan.

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

Posts: 3836
From: Albuquerque, NM
Registered: Feb 2000

posted 05-02-2006 08:43 PM

The vertical blanking interval for NTSC provides for certain other uses of the last few "blank" lines before the top of the picture area. If display devices displayed all 486 lines, these other signals would show up at the top of the image area. That's one of the reasons the 480 number was adopted.

Typical uses of vertical blanking lines 10 - 21 in each field (lines 1 - 9 are reserved for vertical sync and equalization pulses):

12 - 14: SMPTE VITC (Vertical Interval Time Code)
17 - 18: VITS (Vertical Interval Test Signal)
19: VIRS (Vertical Interval Reference Signal)
20: Network source code
21: Closed captioning and V-chip code

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David Stambaugh
Film God

Posts: 4021
From: Eugene, Oregon
Registered: Jan 2002

posted 05-02-2006 08:47 PM

quote: Joe Redifer
Only CRTs have any kind of image overscan

Rear-projection sets always have some overscan, regardless of the display technology. I've read reviews of direct-view flat-panel displays that mentioned minor overscan.

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 05-03-2006 03:06 AM

Paul, where is the 16:9 flag located? People/DVD authors/whoever does not use this flag correctly/at all piss me off!

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Mark Ogden
Jedi Master Film Handler

Posts: 943
From: Little Falls, N.J.
Registered: Jun 99

posted 05-03-2006 05:29 PM

quote: Joe Redifer
NTSC DVD maxes out at 702x480, I believe. Maybe 720x480. But it's ALWAYS 480

. . . but that's only a function of the encoding scheme. The DVD format, of and by itself, is perfectly capable of handling 500 lines. However, in MPEG-2 encoding, only the active lines are compressed and coded, in an effort to save disc space by not bothering with usless information. Hence the 480 spec.

Also, if Paul dosn't chime in: the anamorphic flag begins as a 16 bit metadata signal on the disc, and arrives on lines 20 and 283 of the NTSC vertical blanking signal on the luminance channel only. You'll notice that if you feed component signals from your DVD player to your monitor, and you remove the green "Y" cable, the scan collapses to 4:3.

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
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posted 05-03-2006 06:16 PM

I can get auto-16:9 even on a composite signal. Or maybe it was S-video. I recorded a 16:9 videogame on a videotape once and when I played the tape back it popped into 16:9 on its own.

If I unplug the green "Y" cable, the screen collapses entirely, not just to 4:3. That's because the other two cables only carry color difference info.

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Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 05-03-2006 06:53 PM

On the subject of component video, I should also ask this: why are the inputs and outputs on some decks (Beta SP, usually) labelled as "Y, B-Y, R-Y" and others labelled as "Y, Pb, Pr"? These seem to just be different names for the same thing. Why?

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

Posts: 3836
From: Albuquerque, NM
Registered: Feb 2000

posted 05-04-2006 01:48 AM

Mark answered the DVD anamorphic flag question.

As to component video nomenclature, "Y R-Y B-Y" and "Y Pb Pr" are practically the same thing, analog component video. The only technical difference I can think of is the two color-difference signals in "Y Pb Pr" are scaled to prevent values that are "out of gamut" for the color space, hence they are always "legal" for output to other devices. Many people use these two designations interchangeably.

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Joe Redifer
You need a beating today

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From: Denver, Colorado
Registered: May 99

posted 05-04-2006 02:18 AM

I didn't ask specifically about DVD anamorphic flags. It is interesting that people seem to thing that the only source of 480p and/or widescreen video is DVD. Get out of the house! I can indeed get auto-widescreen on composite (and probaby RF) on properly flagged video, DVD or not (though DVD players tend to be built like ass and some of them only transmit the flag out of the component jacks).

The component jack on the back of my TV reads "Y, Pb, R-y"

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Stephen Furley
Film God

Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002

posted 05-04-2006 02:27 AM

quote: Mark Ogden
Also, if Paul dosn't chime in: the anamorphic flag begins as a 16 bit metadata signal on the disc, and arrives on lines 20 and 283 of the NTSC vertical blanking signal on the luminance channel only.

Why on two lines? You'd never have a situation where the two fields were in different aspect ratios, so I can't see the need to repeat it.

I assume that on PAL different lines must be used; any idea which ones?

quote: Joe Redifer
I can get auto-16:9 even on a composite signal. Or maybe it was S-video. I recorded a 16:9 videogame on a videotape once and when I played the tape back it popped into 16:9 on its own.

But the luminance channel ends up in the composite signal, so I would expect the anamorphic flag to still be there.

quote: Joe Redifer
If I unplug the green "Y" cable, the screen collapses entirely, not just to 4:3. That's because the other two cables only carry color difference info.

Sounds like you're using a three cable system, with composite sync on the luminance signal, and a monitor that blanks the screen if it doesn't see a sync.

Over here, most domestic video equipment doesn't have component connections, though there is more than there used to be. It's usually either composite, or RGB (via SCART).

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

Posts: 3836
From: Albuquerque, NM
Registered: Feb 2000

posted 05-04-2006 02:50 AM

OK OK, I typed "DVD" when I should have typed "MPEG." Sheesh! [Big Grin]

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Joe Redifer
You need a beating today

Posts: 12859
From: Denver, Colorado
Registered: May 99

posted 05-04-2006 04:15 AM

Hot damn! I didn't know my Xbox encoded videogames to MPEG before output. Same with my MiniDV camera. President Bush must be notified about this immediately! [beer]

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Mark Ogden
Jedi Master Film Handler

Posts: 943
From: Little Falls, N.J.
Registered: Jun 99

posted 05-04-2006 10:07 AM

quote: Stephen Furley
Why on two lines? You'd never have a situation where the two fields were in different aspect ratios, so I can't see the need to repeat it.

It has to do with how your TV set/display device knows to display an anamorphic signal. Basically, the way the standard was laid out, the display’s decoder needs to see a two-bit digital “start code” signal (the first part of the“flag”). The flag on line 20 is the first bit, the flag on line 283 is the second bit. Further, it needs to see both bits for four consecutive fields before it switches to wide-screen mode. This delay is why on some poorly authored DVDs, you’ll sometimes see the menus flash in 4:3 before they kick into 16:9 (smart DVD authors leave a few frames of black or fade-up-from-black before the active video to mask this). After the start code, the flag has four other bits that determine whether the video is full frame 16:9, or letterboxed, and where the image is centered.

I’m sorry, I don’t know off the top of my head where all this happens in PAL/SECAM, but I would guess that it is at a roughly equivalent place late in the vertical interval.

quote: Joe Redifer
I didn't ask specifically about DVD anamorphic flags. It is interesting that people seem to thing that the only source of 480p and/or widescreen video is DVD.

I don't "thing" anything of the sort. This thread started with a question about DVD video, and your question included DVD authors. My answer took DVD as an example of the whole system. So video games can be anamorphic too? Huh. Wouldn't know, don't play 'em.

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