Is it technically possible to burn double layer DVD-/+R

[patten]

Can the generation do this?
I have a DRU500 and i was wondering about it.

[Vejita-sama]

No, DVD-9 discs are actually two single layer discs pressed together. They are not burnt. I'm sure that 'someday someone' will find a way around this (not counting DVD-10) but that day isn't going to come any time soon.

[vandakeg]

There already is a way around it... blue laser. It's only a matter of time until it's commercially available.

[patten]

I know that the "Bleu Ray" is coming. This can burn discs upto 17GB (i'm not 100% sure, it may be more).
But two emty layers (unburned) can also be put together, the dvd recorder is software controlled. When it can read such a disc, way not burn such a disc?
Laser intensity can be adjusted by software, and many other things.

[HyperYagami]

that's gotta be some technical limitations. otherwise why wouldn't they do it with their expensive equipments during manufacturing to start with, instead of pressing 2 single layer disc together?

Of course, technically it is possible. I think there's already dual-layer Blu-Ray DVD-R prototypes (from Panasonic, I think). But if you do it on the old form 4.7GB DVD-R the existing equipments are not gonna be able to read it so the point is moot.

[solarfox]

that's gotta be some technical limitations. otherwise why wouldn't they do it with their expensive equipments during manufacturing to start with, instead of pressing 2 single layer disc together?

For one thing, because one stamper can turn out a hell of a lot more discs in an hour than a DVD burner could; for another, recordable discs don't play in all DVD players; for a third, it would increase production costs since you still require a stamping machine to turn out the blank recordable discs before you can burn them.

As to why dual-layer DVD+/-R(W) is not currently possible... I can see it's time to repost this explanation once again.

============================================

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! )

------------------------------ <-protective 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 (and the fact that some set-top players still won't play the recordable discs at all) 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.

============================================

[HyperYagami]

thx solarfox for the repost! I never knew such a simple explanation exists!

now someone please make it as sticky...

[Zmeister]

Great writeup solarfox! One thing you could add is an explanation of how RW discs can be re-written to.

[Thorn]

Thanks for the repost solarfox... we need to have this added to the FAQs, since it's asked all the time and people always argue over it...

Just a quick note on how RW discs work: I read (I'm sure someone has a more accurate explanation somewhere) about CDRW that to burn a bit as on, the laser heats up that section to around 500-700 degrees Celcius, so when it cools quickly the atoms aren't ordered and are therefore opaque. The erase laser heats it up to only about 200C and that allows the atoms to reorder and become transparent again in the crystalline state (so light reflects off of the reflective back coating). These two states are the equivalent of the pit/land of pressed discs and of burnt/unburnt spots on a R disc. I assume that DVDRW technology works in a similar fashion to this.

And to summarize why we won't see dual layer DVD+-R discs: They would be too expensive to develop and incompatible with current burners (maybe even players). If a company is going to invest a lot in an upgrade, it will be for results on the order of 4x or 5x (like blueray) rather than 2x the current capacity.

[crusian]

so how do minidiscs work, they came out before cdrw right? And why the hell didnt they ever put them in computers? 700 megs on something that small and protected? Sounds great!!

[boss]

minidiscs for computers? from what i remember, sony tried it, but pc minidiscs never took off. i think i read somewhere that it was because they were unrealible for storing computer data.
A shame, they would have been a great replacement for floppy disks!

[solarfox]

Minidiscs are a magneto-optical technology. The disc itself is similar in construction to a CD or a CD-R, except that instead of a reflective layer of etched aluminum or an organic dye, MD's have a layer of an exotic material called Yttrium Terbium Oxide. This material exhibits a property called the Kerr Effect -- when struck by laser light, the beam is reflected in one of two directions depending on the material's magnetic polarity. It also has a fairly high Curie Point -- the thermal temperature at which its Coercivity (resistance to having its magnetic polarity switched by an external magnetic field) drops radically, allowing it to be repolarized. Together, these properties form the basis of the MiniDisc's Magneto-Optical Recording system -- while recording, a laser heats a microscopic spot of YTbO to the Curie point from below, a magnetic recording head positioned above the disc flips the state of that spot, and the new state is "frozen" into that spot when the laser stops heating the spot. (All of this takes place in a few brief microseconds!) Later, the recorded data is read with a lower-power laser beam, by detecting the small deflections in the returning beam caused by the Kerr Effect.

700 megs on something that small and protected? Sounds great!!

Actually, the storage capacity of an 80-minute MD is more like 150Mb, not 700. Minidisc uses a psychoacoustic compression scheme (called ATRAC) to compress the audio data into a smaller space, similar to MP3.

Basically, the reason MD didn't take off as a computer-data storage format is that Sony botched it. First, they decided to split the product line into Data-MD and "normal" MD, and made the two physically incompatible so you couldn't use one kind in the other's drive. Then, they did hardly any promotion of either format here in the U.S., and priced it way too high for the market.

There's no reason why it couldn't be a data-storage format now -- their newest feature, NetMD, provides you with a way to connect your NetMD-equipped MD player to your PC via a USB connection, and some Sony Vaio laptops (mostly Japanese models) come with a NetMD drive already built in...

...OK, there's one reason. Sony refuses to do it. The current generation of NetMD only lets you send data to the device, to make audio discs; it won't let you get daya back out again. You can also only send down ATRAC-encoded audio data; there's no provision for generic data storage. If you think this makes no sense, you're right -- and the consumer-electronics side of Sony doesn't like it either. Unfortunately, the media-and-entertainment side of Sony has the CE side by the short hairs, since M&E puts far more money into Sony's coffers than CE does, and M&E doesn't want you, the consumer, to have those capabilities since -- oh, horrors! -- you might actually want to use them to copy things with.

Gee, where have we seen this behavior before?

[patten]

Solarfox, thanx for the very nice posts. This makes things very clear.

[crusian]

Whoaz, complicated... I just figured that you could record audio on them over and over again, that there had to be a way for a computer to use them... and MD were out long before cd burners and all the memory sticks and stuff, so they would have been way far and above all the other options at the time. Agreed now they wouldnt command much market, but they sure could have.

[kitty]

Yes Sony is like a bad marriage. You have a hardware side that often rocks and is masculine and a software (movie) side that is soft, feminine, and sometimes sickly. But the soft feminine side says "don't make hardware that will lessen our movie value" (read: if you mess up you get no sex). The hardware side rolls over & plays dead (complies).

So sad.

[Gil T Pleasure]

Good analogy, kitty.

[LanEvo7]

One reason Sony doesn't want you to be able to send data back in to the computer is because of that crappy copyright software it has. I actually owned a lot of sony's products. Hardware is great and stylish. No other company compares to it. Their software sector sucks though. They don't make mp3 players, they make atract 3 players. Granted both of the formats sound quality is about the same, they way you have to record atract 3 to the player is very annoying. First you have to use their crappy software to convert your mp3/wav/etc to atract 3 and then the software puts a code onto it so you can't copy it onto a different computer. Sony thinks this is helping against piracy. Yeh, ok. When all the other companies are making 4gb mp3 players...

[sammie]

I always figured that Sony could take a huge chunk of the market share against miniDV (or vice versa) if they released a ATAPI or firewire or USB2.0 (or better yet all the above) "drive" for Digital 8. Especially if they could make it for around $200 - 300. Nearer $300 you'd want it to be firewire and proably to work as a standalone digital vcr. Have it work as a DV reader/recorder and a tape backup and it sounds like a killer piece of hardware that the essentially should already have all the parts except for the case in house already.

(oops, sorry, just realized how off topic this is. Just got to reading about a computer minidisk drive.....)

[SLK001]

Back to the original subject: Good write-up, solarfox - once again!

[fravel]

Check this out!

http://www.supermediastore.com/dvd-r-primedisc.html

They say it's a recordable double sided dvd9. I'm puzzled. Is this what you are looking for?

[MOVIEGEEK]

patten was looking for dual-layer DVD blanks(DVD-9)not dual-sided(DVD-10).

[HyperYagami]

They say it's a recordable double sided dvd9.

Double sided, 4.7GB each side. Also a "9.4GB DVD" does NOT make you a DVD9, and nowhere on that page claims that's DVD9.[/code]

[Pinchas]

Do they even have double sided DVD+R's? If so, where could I find some, I've been lookin everywhere and can't find any...

[fravel]

Sorry, my error. My intention was to help. But, don't you think you can use them for burning a double sided -9.4 dvd project? And if you can't, what are they good for?

[snorkel64]

dual layer dvd recordable that retains compatibility to the point it could still be called DVD is impossible plain and simple

as outlined in post above plain laws of physics dictate that you cant burn at two different temparatures without affecting the other layer in a dye based system

but even more simply and final - to trick the systyem into some compatibility with the recordable media the decision was taken to combine aspects of the two layers of a factory pressed dual layer dvd in the make up of recordable discs - ie the focal length is close to that of one layer of a factory dual layer while the refractive indice is similar to the the *other* layer of the pressed disc By borrowing aspects from both they made recordable single layer work but in process shut the door on dual layer DVD-R forever

alternatives are there for dual layer system by avoiding need to anodise a dye (eg by photoflouresence where materials react to different light wavelengths etc in order to record/playback at multiple levels or magento optical) but.. end result is a 9.4 gig hybrid (the media for which would bear no relation to DVD other than physical size and capacity) If you were going to make something *that* incompatible with existing DVd you might as well go for a higher capacity system (ie DVDBlue)

snorkel64

[HyperYagami]

Sorry, my error. My intention was to help. But, don't you think you can use them for burning a double sided -9.4 dvd project? And if you can't, what are they good for?

practically pointless. you get two 4.3GB DVDs for the physical space of one, that's the only benefit. you'll need to flip the disk to get to the 2nd side, i.e., it's not like you get a combined 9.7GB.

[solarfox]

Actually, the dual-sided DVD-R discs are great for putting together multiple related programmes -- such as, for instance, making your own "Season x" DVD sets for TV shows you may have recorded. Having to flip the disc over isn't such an issue when you're dealing with TV show episodes, since odds are you don't plan to watch the entire season's worth of shows in one sitting anyway...

They're also useful for situations where you want to crank up the bit rate to the absolute highest quality you can get. Having to flip the disc over halfway through is no worse than what we had to do with the big 12" laserdiscs, after all! (And personally, I never minded it all that much; the break between sides was a perfect opportunity to get up from the couch to replenish the snacks, or whatever...)

[Gil T Pleasure]

I guess the hardest part is labeling the dual sided disc. And another thing, it's hard to know which side you're burning. Also, what if you coaster one side?

[solarfox]

I guess the hardest part is labeling the dual sided disc.

Either Neato or Avery (I forget which at the moment; I haven't had my morning coffee yet ) makes center-ring-only labels which work well for this kind of thing... They're about 1.5 inches in diameter, with a small hole in the center just larger than the spindle hole of the CD, so all they cover up is that clear non-data section in the center.

And another thing, it's hard to know which side you're burning.

Never been a problem for me... then again, I typically stack up the converted MPEGs in an appropriate subdirectory on the hard drive (D:\TVSHOWS\GregTheBunny, or whatever) until I have enough to fill out both sides of the DVD-R; then go into DVD Workshop, create both projects, then do both sides of the disc right there, one after the other.

Also, I rarely manage to burn a disc right out to the very edge; usually there's a little bit of unrecorded space at the outer rim, so it's not too hard to visually ID which side is already recorded.

Also, what if you coaster one side?

Then you need to go back to practicing with DVD-RW's until you get your encoding/authoring methods sorted out.

[Gil T Pleasure]

I heard that!! Or slap on a regular label on the coaster side and pretend it's a single sided disc. No one will know the difference.