Comments

JaysonHolovacs wrote on 7/29/2004, 2:17 PM
Commercial DVDs are just sneaky about switching tracks. It's an artistic solution to a technical problem. You need a pause in the music or dialog or whatever and you just have to slip the switch in there. You don't know exactly how long it will be but on most modern DVD players it's not bad. To put it in perspective, think of the time it takes when you hit a button on a CD player to jump to the next track. There is a noticeable delay there, too.

This is especially evident on layer transitions on commercial DVDs. I guess(not sure on dual layer spec) that the same title cannot span layers. So, somewhere in the middle of the movie you will see a layer transition. Usually, it's during a black fade where there's no music so that you can't notice the video or audio jump, but if watch, you can usually spot when it happens. You can just tell by the longer than normal pause between scenes, or you might hear your DVD player jumping around the disk.

Like others have said, I don't think there's much you can do about it. A normal track is designed to be read sequentially, and DVD players are good at reading a constant track. But a new title can be in a new file, and it is not necessarily located in the same area of the disc. This means the laser needs to move from the end of the track it's on to another spot on the disk where the new media is stored. This causes a delay.

This was especially prevalent in the early days of CD-ROM drives on PCs. CDs, like DVDs, are good at reading long files of data in a straight linear fashion. They are not as good at jumping around to grab smaller items from different places. Early CD-ROMs were heavily affected, performance wise, by their average "seek time", or the time it took for the laser to jump from its current file to some other file at some other random location on the disk.

One reason that hard disks are better at random access and CD/DVD is better at streaming continuous data is the way the data is organized. A hard disk's tracks ares composed of concentric circles split up into sections. Jumping to a particular section is fast because the exact distance the head must move is known by the particular circle it is located on, because a particular cylinder(circle) is the same distance from the center no matter how far you spin the disk. But, if you try to put a long file, it might not fit on one cylinder, so at some point you have to jump to a different cylinder.

CD/DVDs are different. They are organized as spirals. So, you can start from the beginning and read without stopping all the way to the end of the media. You will slowly spiral inward(or outward, can't remember whether they read inside to out or vice versa) as you read. But, if you now need to jump to another piece of media, you need to find it. And since the distance from the center to the location you want can vary depending on which position the disc is in, it is more difficult to find a particular location, hence longer seek times.

Of course, there are other factors, and my memory may be less than perfect, but I think this is pretty accurate.

-Jayson
JSWTS wrote on 7/29/2004, 6:21 PM
Other apps do it by delaying the appearance of the subpicture, which allows for seamless playback. You can't do this in DVD-A, so you are left with 'faking' it with the work arounds mentioned above. With a delayed subpicture appearance, you can have your video and graphics ramp up, buttons or text appear and once the final text is in position, have the subpicture/highlight appear to allow the viewer to make a choice. You can have a continuous audio stream all throughout this, and would not have a pause occur when the highlight appears. Since the track is continuous, the laser doesn't have to 'jump around'.

As far a dual layer switches are concerned, the pause that one sees from that is what it takes for the laser to switch from one layer and refocus and start on the the next. It can and generally is within a single track, and it typically placed at a spot in the video where it has the least likely chance of being noted (like a fade to black and audio cut). Most Hollywood dvd's use OTP (opposite track path) to minimize the time it takes for the laser to refocus.

Jim
JaysonHolovacs wrote on 7/29/2004, 7:48 PM
Jim,
OTP? Opposite track path? So does that mean if the DVD reads from the inside to the outside on one layer, then the layer switch occurs at the outside and the path continues back inward on the new layer? Neat little trick. Is this the way the new DL writeable media works, too?

-Jayson
bStro wrote on 7/29/2004, 8:58 PM
So does that mean if the DVD reads from the inside to the outside on one layer, then the layer switch occurs at the outside and the path continues back inward on the new layer?

(Boy, that part of Jim's note just flew right past me.)

Is that what you mean, Jim? That's....just weird. Useful, obviously, but weird.

Rob
JSWTS wrote on 7/30/2004, 4:14 PM
There are two ways to author a dual layer break--OTP and PTP. With PTP (parallel track path), the first layer is written from hub to edge, the second layer is then written in the same direction. Since the laser has to refocus and go all the way back to the beginning, it will take some time. Because of that, you typically would choose OTP if your layer break comes within a movie so the viewer doesn't see the pause. With OTP the second layer is written from edge to hub, so the refocus time is much less. There are several rulles that one must follow when creating layer breaks, and fortunately most of them are made by the authoring app to make the break 'legal'. The first layer must be larger in size than the second with OTP, otherwise the laser would have to refocus and move out closer to the edge to start the second layer.

The layer break can be set specifically in only a couple of apps (like Scenarist and DVD Studio Pro on the Mac), but there are apps that support DL, like Encore from Adobe. However the app itself does all the calculating and doesn't give much in terms of user control. The current DL burners are not directly supported from within the authoring apps to make these sorts of discs, but I'm sure over time they eventually will. Nero supports DL discs, but it remains to be seen just how well it will work with taking a finished project and creating the layer break. It certainly doesn't offer any direct control from the user. Data dual layer discs will be relatively easy to make, because you aren't dealing with the visual issues of pauses creatiing during layer switiches, and for that reason, PTP type burns are most likely (and are the easiest to create).

Jim