Dual-Layer DVD Burning

[filmguy123]

I was wondering, when will there be DVD burners capable of burning DVD-9s I think they are? Whatever the DVDs that are single sided, dual layered, like most movies you buy today. When will there be a burner capable of this? Is this likely to happen anytime soon?
What I am getting at ultimately is, is this a good time to buy a DVD burner, or would it be better to hold off - perhaps for a month, 3 months, 6 months, or even a year?
Lastly, does anyone know when there will be high quality 4x DVD-R media available?
Thanks,

Dan

[Thorn]

The DVD-R and DVD+R technology is different from dual-layer pressed DVDs, so it's not possible for single sided, dual-layer DVDRs. You'll probably see blue laser discs come out before single-side capacity increases again.

4x media will probably be very expensive when it first comes out. Even good 2x media is sortof expensive still. A lot of people opt for super-cheap 1x media when they could burn at 2x for more money, so I doubt the new burners and media will make much of an impact in that segment... so ask how much is it worth to you to burn in 15 minutes instead of 30?

The best time to buy is when you want it and can afford it (maybe look 2 months in either direction and get a good price). If the 4x burner is cheap enough it might drive down the price for 2x burners, though.

[solarfox]

I've posted this explanation once before... maybe I should submit it to the site admins as an article.

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My (somewhat educated) guess is that it will a very long time, if ever, before dual-layer DVD-R will be available... and if they ever do appear, it's a dead certainty that the current generation of DVD-R recorders will not be able to write to them.

I see this question pop up all the time... I think perhaps a lot of people out there don't quite understand what's going on inside CD/DVD burners and players. Let me try to illustrate how this works -- and, by extension, why dual-layer DVD-R is something that, while not necessarily impossible, is much more difficult to make work than you might think.

Note that the following explanation is somewhat oversimplified for purposes of illustration, so as not to get bogged down in unnecessary details. (As the saying goes, an ounce of inaccuracy sometimes saves several tons of tedious explanation.)

Consider the construction of a typical CD-R or DVD-R disc: (warning, bad ASCII art ahead! )

------------------------------ <-top polycarbonate layer 
============================== <-reflective layer 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ <-organic dye layer 
------------------------------ <-bottom polycarbonate layer

The organic dye layer is made up of a substance which is normally transparent (or at least is optically transparent to the particular frequency of laser light which is used to read the media), but which becomes opaque when "burned", i.e. when hit with a burst of laser light at a particular frequency with sufficient power. (Which is why CD-ROM drives cannot be made to write to a CD; their lasers aren't powerful enough to affect the dye layer.) Reading such a disc is a simple matter of bouncing the laser off of the reflective layer behind the organic dye; any "burned" spots in the dye layer will block the laser beam from reflecting back to the sensor and register as a binary "0", while a successful reflection registers as a binary "1".

So, basically, a single-layer DVD is just a refinement of the same CD-ROM technology we've been using for years. Now let's consider how a dual-layer DVD works:

------------------------------ <-top polycarbonate layer 
============================== <-reflective layer 
:::::::::::::::::::::/\::::::: <-data layer 2 
,,,,,,,,,,,,,,,,,,,,/,,\,,,,,, <-semireflective layer 
!!!!/\!!!!!!!!!!!!!/!!!!\!!!!! <-data layer 1 
---/--\-----------/------\---- <-bottom polycarbonate layer 
  /    \         /        \ 
_||_  |_|      _||_      |_| 
LASER SENSOR LASER   SENSOR 
  reading     reading 
  layer 1     layer 2

The way a dual-layer disc is read is by using some very tricky, precise optics to focus the laser to a particular depth, enabling it to either bounce off of the closer, semireflective layer or the deeper, full-reflection layer. This enables the sensor to "see" layer 1 or layer 2, as needed. (Exactly how this is accomplished is well beyond the scope of this simplified explanation - in a nutshell, though, the data layers on a "pressed" disc don't actually block reflection of the beam, the way a burned dye layer does; instead, they change the reflection angle so that the laser either hits or misses the detector.) Notice, though, that in order to "see" layer 2, the laser must pass through layer 1 and the semireflective layer both on its way in and out of the disc.

If you think about it for a moment, I'm sure you can see what this would mean for a dual- layer DVD-R. Theoretically, a dual-layer DVD-R would have to look like this:

------------------------------ <-top polycarbonate layer 
============================== <-reflective layer 
{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{ <-organic dye layer 2 
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, <-semireflective layer 
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}} <-organic layer 1 
------------------------------ <-bottom polycarbonate layer

So: how do you burn the second layer? The obvious answer would be to focus the laser onto the deeper layer, right? Well, maybe - except that how do you keep the laser from burning the dye in the first layer as well, as it passes through it on its way to the second? That beam will have a fair amount of power behind it (more power than would be needed to burn layer 1, in fact, since it has to reach layer 2 with enough energy left to burn the dye and some of the beam's energy will be lost passing through the intervening layers); even if the beam isn't focused on layer 1, it can still "scorch" it enough to corrupt the data.

Also note that since the laser has to go through layer 1 to record layer 2, you'd obviously have to record all of layer 2's data first, or else the burned spots on layer 1 would prevent the writing laser from ever reaching layer 2. And, finally, there's also the question of whether or not the read laser would be able to see layer 2 properly during playback, since you have a layer 1 which consists of spots that physically block the laser beam from reaching the reflector, rather than one which alters the reflection angle -- a small but critical difference between pressed and burned discs!

When you add all of these things up, you can see that a dual-layer DVD-R is a very tough engineering problem, particularly if you want to remain backwards-compatible with existing DVD players. Basically, the only way I can see it being made to work is as follows:

* You need some kind of data-recording material for data layer 1 which, when "burned" by the laser, changes the reflection angle in a manner consistent with a pressed DVD's layers, rather than becoming opaque and blocking the laser entirely.

* The material for data layer 1 will need to be burned at a much higher power, and/or at a significantly different laser wavelength, than layer 2 so that the process of recording to layer 2 doesn't corrupt layer 1.

* Your DVD-R recording software will have to be intelligent enough to figure out what goes on what layer (or give you control over it) -- and smart enough to burn layer 2 before layer 1.

* Your DVD-R(W) drive will need appropriate optics to allow it to focus the recording laser to different depths... (and, as seen above, will probably require two entirely different recording lasers, which further adds to the weight and complexity of the optical block.)

* Your DVD-R(W) drive will also need to know how to record layer 2 in a reverse spiral, since that's the way most dual-layer DVD's are done. (By mastering layer 1 with a center-to-edge spiral, then layer 2 with edge-to-center, you make it easier to switch layers in mid-programme, since the optical block doesn't have to retract all the way back to center before it can begin playback of the second layer.)

While none of these are insurmountable engineering problems, they're not exactly easy to solve, either... and since you would almost certainly need a new DVD-R(W) drive, and new software, to make use of them, it'll probably be a while before any of the major manufacturers even attempt to solve this problem. The "plus-R / dash-R" incompatible- standards brouhaha is already keeping a lot of people from investing in recordable-DVD until the mess shakes out; introducing Yet Another Incompatible Format would only confuse, divide, and probably drive away the market even more.

Of course, I would be happy to be proven wrong... Unfortunately, based on what I know of the process, I don't think dual-layer DVD-R is gonna happen anytime soon.

[vitualis]

Hey, great ASCII art...

Errata: CDs and CD-R/W don't have a top polycarbonate layer...

Regards.

[solarfox]

Uh... since when? I have several examples of CD-R and CD-RW right in front of me at the moment, just to make sure of this, and I'm quite sure that the top surface of the disc is not an atoms-thin piece of metal... It sure looks and feels like the same polycarbonate plastic as the bottom layer to me!

[vitualis]

Take a closer look.

Bottom to top:
- polycarbonate substrate
- organic dye layer (not on a pressed CD)
- reflective layer (on a pressed CD, this is a thin layer of aluminium coating the top surface of the polycarbonate substrate that has been "stamped" with the data.
- lacquer layer
- + other protective coating that is optional

The polycarbonate substrate is BELOW the data layer. The top layer of the CD is highly susceptible to damage from scratching. Get one of those AOL (or similar) discs and run a key across the polycarbonate substrate on the bottom. There will be little damage except for some markings on the polycarbonate.

Do the same to the top surface. Now look at the bottom surface to see the data layer. You will clearly see the distortion on the data layer!

Regards.

[vitualis]

Ah, found it... From the CD-R FAQ:
http://www.cdrfaq.org/faq02.html#S2

Regards.