Double your DVD Storage

Optical storage technology is changing drastically, taking leaps in terms of the amount you can store by a few factors. When people started feeling the crunch of having too many CD-ROMs around, they invented the DVD. But now even DVD capacity falls short of our expectations.

When optical discs were invented, they were single sided, ie, data could be stored only on one side. Then they made them 'double-sided'. However, each side still had only one layer of data. A layer of data is similar to a coat of paint, where more the number of layers, more is the data stored. While a DVD still stores far more data than a CD, it does this using a different technology. And by putting data on both sides, you are in effect sticking two normal DVDs back-to-back, giving it two writable sides. A regular DVD can store 4.7 GB of data and its double-sided variant can store 9.5 GB-exactly double that.

Enter double layers



If instead of sticking two DVDs back-to-back, you stick them with corresponding sides facing the same way, you get a double-layered disc. So, you have two recordable surfaces with the difference that one layer lies below the other one. But how would you write on the second layer? Simple, when a disc spins inside the drive, it generates heat just like any other fast moving object. When you use special organic material on the disc that softens at this temperature, you achieve transparency. This lets the laser beam that writes/reads the data to pass through the top layer ('L0' in the graphic) and does its work on the second layer ('L1' in the graphic). These recordings have different optical properties compared to their unmodified surroundings and read the same as read-only discs, based on the length of the recordings and unmodified spaces between them. A non-writable 'Spacer' layer, bounded on its L1 side by a semi-transparent reflective metal layer, separates L0 and L1.

Dual-layer writers use the same 650 nm laser used by the single-layer drives. Data transfer rates of a DL writer/reader are also fixed at the same 11.0 Mbps and they're able to use the same MPEG 2 video-compression technology. Though they do not yet support the VC 1 or MPEG 4 AVC formats.

Pros and cons



Exciting as it may sound, dual-layer discs are not exactly a brand-new play thing. Dual-layer technology has been around for quite some time, only never available to the common man to do the writing part. Puzzled? All those DVDs that are played at those hi-tech theatres and video parlors are essentially the dual-layer discs. And, these weren't written using the 'pass a laser beam to burn the plastic' method, but had to be 'stamped' on. So, conventional DVD players have also been playing them perfectly all this time.

But you are actually losing out a bit on the storage front by dual-layering-a double-sided DVD holds 9.5 GB against a double-layered equivalent that holds 1 GB less at 8.4 GB. Also, some adjustments have to be made in the way the data is written to the disc to ensure that the players don't detect spurious errors. Since you write on two media layers lying one over the other, you should neither leave any gaps in this data nor should you have two unequally recorded layers. That is, if you put 2 GB on the upper layer, you should also put 2 GB on the second layer. The writer software does this by adding 'dummy data' to fill the discrepancy on the other layer. Yes, when you're storing pre-saved information (with known sizes), it is easy to adjust the storage. But, when you try to save streamed information, such as from a Web cast, the final size is not known. This will create interminable delays in recording times.

The jury is still out



Some years ago when the DVD (single-layer) battles began, there were two opposing camps-the plusses and the dashes. Today, most writer-vendors support both of them. The same battle is on for the dual-layer discs. The plus team seems to be winning today, both with written down standards drafts and products already out in the market, though time will tell who wins.



Similar to single-layer DVDs, which are cross-compatible on different types of players only 88 percent of the time, we can expect the same for dual-layers as well. Also the dual-layer media has to be able to play on the available players. While this has so far been the case with released standards and drives, you have to watch out for the future.

We reviewed the nu and the BenQ DVD dual-layer writers in our last issue (page 146, PCQuest, November 2004). Vendors such as Sony, Philips, Pioneer and Plextor also have the drives available. As for media, Philips, Imation, Verbatim (Mitsubishi Kagaku Media) already have dual-layer DVDs in the market. While we write this article, these dual-layer writers are available only at 2X (twice the normal reading speed) and some of the problems we mentioned are expected to improve when higher versions are released.

Dual-layer DVDs are not the end of the road. There are already higher-capacity technologies such as HD-DVD (High Definition DVD), which again is available in both single and dual-layer formats. HD-DVDs can store upto 30 GB using a 405 nm laser, and can transfer data at the rate 36 Mbps and support MPEG 2 and 4 (with AVC) as well as VC 1 video compression.

Sujay V. Sarma and



Sushil Oswal