Okay, I've noticed something recently that I find a bit confusing.

On dual layer pressed DVD-Video disks--should they be region free--programs like CD/DVD Speed show a triangle shaped read speed graph. I assume this is because the read speed builds as the linear speed of the 'head' increases with increasing radius. Then, at the layer switch, the start of the second layer starts on the *outside* and works into the center of the disc.

On a DVD-ROM I have which is DL, I see something different. I see two ramps from low speed up to high, quick jump back to slow speeed, and then back to high speed. This makes me think that the two layers both start at the center of the drive and work out.

I can see why DVD-Video does it this way--you want no extra seek delay on the layer switch as you'll be in the middle of streaming video/audio at the time. But, why is the DVD-ROM different? Is it easier to make this way? Is it more compatable with older drives? Does it decrease seek time or some other parameter which is more important to a data application VS a video application?

Finally, how does this effect dual layer +R and -R media? Which are they? Triangle shaped or 'shark fins'? And, will that make them better for one application over another?

Thanks!

There are two types of DVDs, OTPs and PTPs.

With a PTP DVD-ROM disc, the starting point of the two layers is at the inner part of the disc. The drive reads from the beginning of each layer (inner part) progreesing towards the outer part of the disc.




With an OTP Dual Layer disc, the first layer structure is the same as the first layer on a PTP disc. The drive reads the first layer exactly the same way as PTP discs, and at the same speed. The starting point of the second layer of an OTP disc is located at the outer part of the disc.

(in reply to willmore)

Thanks, Iggy.

So, are DL DVD+R OTP?

Why are there both kinds?

(in reply to Iggy)

The existance for those two? I really don't know.

The DVD+R DL you can buy are only OTP.

(in reply to willmore)

OTP & PTP are governed by the recording software and not the media, As far as I know all the domestic DVD burning software burns at OTP. I have found some dual layer PS2 games that have been pressed as PTP but as yet I know of no software available to burn the media this way.

(in reply to Iggy)

There are no drives that can burn in PTP method. Those are only the pressed. The above OTP and PTP are reading strategies not recording. The recording method is only one and it is like the OTP.

(in reply to Flash)

Okay, now I've gone from ignorant to partially enlightened to confused.

Iggy, you say that the reader determines if a disk is OTP or PTP? Or do you mean something else? Could you clarify?

I would think that the media--even a 'blank' +R DL disc--would be predetermined to be OTP or PTP by the nature of the 'pregrove', right? I'm a little vague on how DVD works at a very low level. I'm fairly knowledgable of the low levels of CD-R media, and I make the assumption that DVD is similar. Speaking of which is there a good tutorial on that?

The DVD-ROM that I have is a Microsoft Developers disc, so it's purely data, maybe that's why they pressed it PTP? Most other 'DVD-ROM' disks that I've seen are really DVD-Video with some computer content, so, for the sake of the video, they're OTP.

I'd still like to know why there are both kinds. If we have to have OTP for Video, why even bother with having a PTP standard--unless there's something useful about it. Like being cheaper to master/press or better seek time in a DVD-ROM drive, etc.

(in reply to Iggy)