Why some dvds duplicate first scene in second pgc?

[Talayero]

Now and then I find some dvd film with this structure:

pgc1 contains the whole film
pgc2 contains the 5 first minutes or so of the film.

These 5 first minutes are exactly the same as the ones that are in pgc1. I wasn't able to find any difference, as for instance the title of the film written in different languages. So I don't understand it. It must have some explanation.

Thanks in advance

[VegasBud]

Talayero,

At this point, I don't have an explanation, having never seen what you're describing. The best I can do is make a suggestion.

If you open the dvd (or its' rip) with PgcEdit, and press ctrl+h, you'll get the "Calls cross references" report. At the bottom of the report is a section that lists the unreferenced pgc's. Using your example, if pgc2 is listed as unreferenced, it does nothing but take up space. If it's not listed there, somewhere in the list above you can find all the commands that branch to pgc2. From there, you should be able to track backwards through the commands sequence to identify what would cause the flow to direct to pgc2.

Do you happen to remember the name of a movie dvd that has this?

[Talayero]

Thanks a lot for answering.

The dvds I remember having this issue are edited in Spain, where I live. The last two I remember are: "Volver" by Almodóvar and "Ask the wind" by Robert Towne. I have this last one on my hard disk and just tried it with Pgcedit, as you said.

It did show the pgc2 as unreferenced. I paste here the line where it's said:
Unreferenced PGCs
VTST 1 , 2 TTN 2 (5:00) Title 2

It' s still a mistery why they do that.

[VegasBud]

Talayero,

I've personally only seen two circumstances (that I can think of) where there is unreferenced material on a dvd.

1.) By far the most common is as part of some form of copy protection.

2.) Authoring error. I once found an unreferenced preview on one of my dvds. The preview was fully functional, and studios love to make you watch previews, so the only thing I could think of was they forgot to link to it during the authoring process.

It sounds like what you're seeing has a pattern to it, which would rule out authoring error, and leave just the copy protection as the most likely reason.

[adam]

If these are dual layered DVDs (DVD 9's) then check the total filesize of the disc. If it is totally full (7.95 gig/8.54 BB) then they probably added this clip to fill up the second layer completely. This is a somewhat common trick that production houses use to minimize the pause during the layer break.

On a dual layered disc, the first layer is read from inside out and the second layer is read from outside in. If the second layer isn't filled then the laser has to move inward some before being able to start reading. If both layers are completely filled then the laser begins reading immediately and there is less of a pause.

[VegasBud]

If it is totally full (7.95 gig/8.54 BB) then they probably added this clip to fill up the second layer completely. This is a somewhat common trick that production houses use to minimize the pause during the layer break.

With dvd-video on dual layer, the size of the first layer must be equal to, or larger than the second layer, and is laid out using Opposite Track Path specifically so the end of layer one is always precisely aligned with the start of layer two. Any size difference between the layers results in the second layer not completely returning to the hub. By doing it that way, the laser just has to refocus to the second layer to continue, regardless of the size difference between the layers.

It's kind of like folding a piece of paper in half. Even if the two ends aren't precisely aligned, the parts on both sides of the crease in the middle are precisely aligned with each other.

There are two problems that become more serious as the disk becomes fuller:

1.) As the disk fills up, the area that can be used for the layer break becomes smaller. limiting where the layer break can be placed. On a completely full disk, there is only one possible place to put the layer break. If that place is in the middle of an action scene, for example, the layer break is quite noticeable.

2.) The area of a dvd that is most prone to errors is the outer edge. Even the worst media will work close to the hub. Only the best media will work at the outer edge. That wouldn't change whether the disk was burned or replicated.

By forcing the weakest link in the playback (the layer break) to overlap with the weakest link in the data integrity (the outer edge), it's a recipe for generating more scrap (coasters).

[adam]

I didn't know that, but after looking up opposite track path it seems not all authoring packages support this feature. So if done in parallel track path mode then filling the second layer would decrease the layer break pause. Maybe the manufacturers of these discs are doing this to get around limitations in their software. I am fairly certain this is why blank dummy tracks are used on occasion. I have come across it far too many times where the size of the dummy track is exactly the necessary size to totally fill up the second layer.

As for the outer edge of a disc being more prone to errors, this is true with recordable media because the disc must be slowed down, and thus data written at a faster speed, as the laser approaches the outer edges. This is simply not the case with pressed media since the whole layer is just stamped. Now PLAYBACK of any kind of disc can be more problematic at the edges of the disc for the same reason as what plagues burners, the data must be read faster. But if you look at the data itself on a pressed DVD you do not find errors more readily on the outside than you do the inside.

[VegasBud]

adam,

The reason PTP isn't used much for dvd-video is it forces the read head to physically track back to the hub after the layer break, to start reading the second layer and continue the playback of the movie. The added delay makes the layer break pause too noticeable to be acceptable. The more full the disk, the further the read head would have to move to get back to the hub, which would make the layer break pause even worse.

As far as the data errors not increasing towards the outer edge on pressed disks...since the glass master is still "burned" with a laser, it would seem that the same problems that our burners have would still exist. Of course, their laser uses a different wavelength, and I suppose the "master burners" could implement more involved (expensive) mechanisms than consumer burners, but I'm not sure the added complexity and cost would be worth it. As you point out, it still has to be played on a consumer unit. Perfection would only increase their cost and the difficulty in producing their product. There is no benefit in going past making it just slightly better than the ability of a good player to read the disk, and relying on ECC to take care of the rest.

Your comment about PTP being used for dvd-video did jog my memory. I've seen it used where the entire movie fit on the first layer, and there was another angle that could be selected at different places. I never completed my investigation, but it appeared the second angle was located on Layer1 "on top of" the first angle on Layer0, so a layer change would let the viewer toggle back and forth between angles quickly. In that case, "padding" on the second layer would be critical to get the angles to line up.

I'm sure there are other reasons why they make commercial disks the way they do...some of the reasons might be perfectly logical indeed, and have nothing to do with mucking up the possibility of making a backup. To be honest, I'm a little leery of slipping into their mindset if I try too hard to figure out why they did something. Dabbling in their madness or their black arts might be a one way ticket. It's safer to let it go as soon as I've found a way to undo what they did, and make my backup all shiny and pretty inside.

[adam]

Ok, you obviously know more than I do about PTP. But as far as mastering DVDs are concerned it does not suffer the same limitations of consumer burners at all. Its million dollar equipment and a multimillion dollar process. Their equipment is calibrated to spin the disc at exactly the correct linear velocity and the movement is controlled to always maintain correct track pitch and linear velocity. The whole process takes place in a light/temp/electrically controlled environment. The data is tested literally the second it is recorded (You can even start playing that data as soon as it is recorded, before the master is finished) and errors are not tolerated. When you are going to stamp 1000 discs min from the master you must get it right or every single one will have those errors, in addition to any errors introduced during stamping.

The data is typically coming in via DLT and it is scanned via a computer for errors before being sent to the laser beam recorder for modulation. It is all an incredibly controlled process. Acceptable error rates exist for every process but disc manufacturing is just a completely different process from conventional disc burning. Exposing the photoresist layer of the master to store the data is just a completely different data storage method than the dye used in recordable discs.

[VegasBud]

adam,

I agree with everything you say about mastering dvds. It's expensive, complicated, and requires skills that only specialists have. The materials used, and the techniques employed make them entirely different from what we use.

The only part I'm not sure we agree on is:

...it does not suffer the same limitations of consumer burners at all.

The only "limitations" I can think of for dvd+/-r are:
Can't encode with CSS
Not 100% compatible with all players

The first "limitation" isn't much of a limitation. Anyone who knows how to use google can defeat css, which makes it "ineffective" (at best).
The second limitation is valid, for now, but as time goes by, and the oldest players cease to function, it will become even less of a problem than it is now.