Seamless Branching

I would like to get some feedback from both sides on seamless branching. Basically, more exact figures for the limitations of each format right before a branch could be taken or not. From what I've read the main track needs to be limited to an average of around 60% (or low 60s) of the peak mux rate for a short period right before the branch point. Hopefully somebody can correct that if it is wrong and provide exact numbers. My understanding is this is so that there will be enough data stored up to continue playing the video while the laser goes to the alternative track, if that track is to be played. Each alternative track would then have 2 branch points. I picture this like a train going down the tracks with a slower speed limit right before a point where the track splits and that speed limit needs to be observed whether taking the alternative track or not. And if the alternative track is taken, then the train needs to come back to the main track later.

An example of where branching can be useful are discs with regular cuts and alternative (like Director's or unrated) cuts where the user can choose which to watch. I believe that "AVP: Alien vs Predator" has been announced to have seamless branching for Blu-ray with 2 cuts. MI:3 for HD DVD has branching, but that shouldn't be confused for seamless branching from what I saw. One spot early had about a 1 second pause going to the alternative track and about a 5 second pause (although some of it might have been in the source) coming back to the main track. Another case where seamless branching can be useful is in changing things like a note or billboard that is shown so that it is in the language of the currently selected language track. With CGI getting better and better, this should be more possible in the future. I recall reading about "True Lies" needing modification to a name of a company on hats and things before it could be released because it was too close to a real company name, so I know they can modify things like that (although that wouldn't have been an example of seamless branching).

Some things we know are that HD DVD has a total mux rate of about 30Mbps for all audio and video (and 36Mbps off the discs) that comes from their choice to spin the discs at 1.0x, while Blu-ray is at 48Mbps total (and 54Mbps off the discs) that comes from their choice to spin the discs at 1.5x, while they have a max video rate of 40Mbps.

I am especially curious how the HD DVD side plans to handle seamless branch cases for average to hard scenes without degradation given their lower limits. Right now we have releases without seamless branching where things other than the main video take up about 11Mbps, leaving about 19Mbps peak for video. I'm pretty sure there are releases where the average (not the peak) for video is higher than 19Mbps. We have also been told that Microsoft has a plan to do dynamic muxing to deal with the 30Mbps ceiling, and that was before seamless branching was even discussed, from what I recall.

If we give them some benefit of the doubt and figure 8Mbps for things other than the main video, that would leave 22Mbps peak for video. Add seamless branching with those 8Mbps still being used and now with the total for everything right before a branch of somewhere around 19Mbps, that would leave 11Mbps for the main video. That would probably work if the video is easy there, but have more difficult video there and that is a whole lot more limiting than the 19Mbps peaks that we are seeing on some titles currently that still take weeks to get encode to the states we get them in. For the above I didn't even really include going to the 20/48 or 24/48 TrueHD that has been discussed, instead of the 16/48 TrueHD currently.

One more thing is that the average right before a branch point shouldn't be confused with averages we see for whole movies. Difficult frames tend to have a correlation to difficult frames around them. Just because a 2 hour movie can average 12Mbps and look good, that doesn't mean every 2 second segment of that movie could look good with 12Mbps.

So, I am curious. What is the plan for seamless branching support given the above issues and how will the more difficult cases be handled? Will the HD DVD team consider spinning the discs at 1.5x when/if they go to 45GB discs so that seamless branch points aren't nearly as limiting as they are with their 1.0x spin rate?

I already know that some won't want us talking about more difficult cases than we've seen or that I might get answers that include BD50s being science fiction, Blu-ray being MPEG-2 while HD DVD is VC-1, or that we shouldn't look at upcoming issues unless current discs compare in a certain way, but those are all to be expected at this point.

--Darin

Random would be really tough. I was talking about cases where the people encoding know where the alternative tracks are. An example might be a scene in one of the Lord of the Ring movies where the extended cut shows some more hits in a battle and if the user selects the extended cut the scene looks to them like it would if the other cut wasn't even on the disc (no pause) and vis versa if they watch the regular cut. I could also see them using something like this for "clean" or "family friendly" versions, although I don't know if we'll see that either. Definitely scenes where there is extra nudity over what was shown in the theaters would be an example of where they could include both versions with seamless branching.

I think they would want to be careful about including two discs of the same movie with different versions, because many people would just sell the version they don't want. Flippers with a version on each side would be one option, but I don't think that is as nice as single sided discs and I think many agree.

BD would be limited also, just as HD DVD would be even if they did up their disc spin rate to 1.5x. But not as limited as with 1.0x spin rate.

--Darin

Are you talking about DVD releases where there are two discs and each disc contains a full version of the movie, or releases where there are extras on a second disc? They are very different things.

And it doesn't matter whether a person will watch the main version or the alternative version if seamless branching is used and the video is degraded at points to handle the branching.

--Darin

I wasn't talking about stores. I was talking about some individuals who will either sell or give away discs they won't use. I don't know about special things like Criterian collections, but studios are concerned about providing people separate discs of a movie where they can pass on the one they won't use. Do you understand why studios prefer combo discs with HD DVD to including an HD DVD and a DVD of the movie separately in the package?

Most consumers won't even know when seamless branching is used. But studios like to put multiple copies of a movie on one disc for multiple reasons. One, so that they don't have to have separate releases for things like a rated cut and an unrated cut. It probably also helps sell it to some consumers as they see that they can choose the version they want to play at any time and aren't locked into one version at purchase. For instance, so they could play the rated version for certain people (like with younger people or their parents ). But they don't have to know whether the versions were included completely separately or whether they were put on with seamless branching. From what I've seen, unrated and Director's cuts have become more common with time.

Why do you think "AVP: Alien vs Predator" will have two versions on one disc?

--Darin

I have heard of interest in it. But even besides that we can look at the fact that "AVP: Alien vs Predator" has been announced to use it on Blu-ray, Paramount used branching on MI:3 (although not seamless from what I could see), and there are DVDs with seamless branching.

Here is a DVD set for "The Lord of the Rings" on Amazon:

http://www.amazon.com/Rings-Trilogy-...?ie=UTF8&s=dvd

that says:

According to the specs, it looks like DTS 6.1 ES was on the original extended discs, but was dropped for the seamless version, which makes some sense given the limitations of branch points there.

I don't see seamless branching as something were there will be less of it with time, but more if anything.


So, do you believe that Amir's comments about studios wanting to use combo discs instead of including an HD DVD and a DVD because people could pass one on were false?

--Darin

Just trying to live in the real world. The studios will do what the studios will do, whether I like it or not. We can argue about what they should do, but that is different than discussing what they will do.

--Darin

Basically, the bitrate limitations before the branch points. Like, is it ~60% of the normal rates as an average for the 2 seconds before the branch? And then whether there are any other relevant limitations related to these branch points.

--Darin

I think I'll be able to get the information. Might not be able to share it, but I think I'll get it at some point.

One thing that will be interesting is if somebody with a way to monitor the bitrate can do it for AVP, when that comes out. I just read that the G2 Toshiba player lists a bitrate monitor as a feature. I hope it is true as it could be very interesting if we can look at instantaneous bitrates.

--Darin