DVD - So how exactly is it stored?

[BMXTRIX]

Okay, I've looked at DVD Demystified...
http://www.dvddemystified.com/dvdfaq.html

But, the question remains in my head...
For film based DVDs the 24fps film is stored using 2 interlaced frames, not a single 480p frame. But are any frames actually stored at 720x480 or are they stored at 720x240? Since the MPEG encoding method is progessive in nature, and the video output from DVD is interlaced by nature... For a full frame 720x480 interlaced field, half the data being stored would be 'black lines'...

Why?

Why store 240 black lines? Why not mark the field as 'odd' then store it as a 720x240 field? This is not clear to me from the FAQs...

http://www.dvddemystified.com/dvdfaq.html#3.8

It something I will accept, but think is stupid, if they actually store it as 720x480 full frame, progessive, but INTERLACED with half the lines black. But, it just makes far more sense to store it as 720x240 and mark it as an 'odd' field. I do get 3:2 pull down, I do get that the two fields must be deinterlaced (interleaved?) on a decent DVD player (ie: Panasonic S77/S97 or similar/better)... But, if it was so simple, why do so many DVD players struggle so intensively with the basic deinterlacing process for film based content? Odd field, even field, recombine, show using 3:2... done. Should be easy. But, it clearly isn't that straightforward.

I've bent over backward to get this answer 100% straight and it isn't even straight when I read the DVD Demystified FAQ which I agree is the best I've ever read.

 

[mtrycrafts]

Okay, I've looked at DVD Demystified...
http://www.dvddemystified.com/dvdfaq.html

But, the question remains in my head...
For film based DVDs the 24fps film is stored using 2 interlaced frames, not a single 480p frame. But are any frames actually stored at 720x480 or are they stored at 720x240? Since the MPEG encoding method is progessive in nature, and the video output from DVD is interlaced by nature... For a full frame 720x480 interlaced field, half the data being stored would be 'black lines'...

Why?

Why store 240 black lines? Why not mark the field as 'odd' then store it as a 720x240 field? This is not clear to me from the FAQs...

http://www.dvddemystified.com/dvdfaq.html#3.8

It something I will accept, but think is stupid, if they actually store it as 720x480 full frame, progessive, but INTERLACED with half the lines black. But, it just makes far more sense to store it as 720x240 and mark it as an 'odd' field. I do get 3:2 pull down, I do get that the two fields must be deinterlaced (interleaved?) on a decent DVD player (ie: Panasonic S77/S97 or similar/better)... But, if it was so simple, why do so many DVD players struggle so intensively with the basic deinterlacing process for film based content? Odd field, even field, recombine, show using 3:2... done. Should be easy. But, it clearly isn't that straightforward.

I've bent over backward to get this answer 100% straight and it isn't even straight when I read the DVD Demystified FAQ which I agree is the best I've ever read.

OK.

Just as I thought but the link did confirm it:

To make film content work in interlaced form, the video from each film frame is split into two video fields —240 lines in one field, and 240 lines in the other—

The DVD has two field of video for each frame of 24fps, no black lines in one; it would not work otherwise. Then those two fields are processed to work at 60 fields a second, 30 frames, vs 24 of film frames. That is the 3:2 conversion process that you understand.
Not sure where that black field came from.

 

[BMXTRIX]

Well, I believe a lot of it revolves around the concept that DVD is 720x480 - which implies that the MPEG storage method is at 720x480. All the reading I go through actually implies, without specifically stating, that the MPEG encoding is actually at 720x240 with frames marked as odd/even fields.

It seems like it should be easy enough to put into writing, but it never is:

DVD is stored, most often, using MPEG2 compression, at 720x240 lines as a single field. This 720x240 field contains either all the odd lines or all the even lines, and is marked as such. As film is 24 frames per second, it is broken into 48 interlaced fields (odd/even lines) and stored as such. Then 3:2 pulldown is applied for interlaced displays and we have our DVD video from film.

Progressive scan players would put those frames back together before using 3:2 pulldown... which should be a very easy process to perform considering the two fields originated as a progressive source. Then it is a very easy process of putting field A1 & A2 back together to make frame 'A' as a whole.

QUESTION: So why is it so difficult for so many progressive scan DVD players to properly recombine fields A1 and A2 to make frame 'A' from film based content? The Sonys and Samsungs are especially noteworthy of receiving poor marks for this process. Or, is it just poor for DVD content that begins life as 480i video material?

Seems to me that when comparing DVD to HD format discs it is notable that a 1.85:1 film uses about 720x462 pixels of the 720x480 available for content that must be then stretched to fill a 16:9 screen. This is about 332,367 pixels. HD discs are using 1037 lines of the 1080 available - and have about 1,992,648 pixels in use for storage for the same frame! I'm not sure, but I would think that if properly implemented that this jump in quality should be far more noticable than it currently is considering a 6 fold increase in the amount of used pixels for a 1.85 film.

 

[Audioholics]

The part that is missing from these descriptions is the fact that we're dealing with 24fps vs. 30 fps and the interlaced SD format is 60 fields per second... hence my positing that - as far as the information on the disc is concerned you have (the potential of) a 480p source.

The reason so many DVD players can get it right is due to the 2:3 pull down - getting 24fps to fit into a 30fps world...

 

[MDS]

The frames are interlaced and if they go together to form a progressive frame they are supposed to be flagged as such and therein lies one problem. Not all discs are flagged correctly and sometimes the player has to guess based on the 'cadence'. Some players are better flag readers and some are better at guessing the cadence.

Secrets has the best explanation of the whole process I've ever seen.