Inside DVDs

Just how do DVDs work their multi-gigabyte magic? Ashton Mills peels back the layers.

There are almost as many acronyms for the various standards of DVD as there are companies involved in DVD development. The quagmire of differing formats is partly the result of simple exploration of the technology to its fullest and partly due to members of the DVD Forum separating to create their own standard.

The main formats, and specifically the ones we are interested in, currently include DVD-ROM, DVD-R, DVD-RAM, DVD-RW and DVD+RW. The other formats, designed primarily for the entertainment industry, include DVD-Video (movie DVDs), DVD-Audio, DVD-R Authoring (for the professional market), and DVD-VR. So what's with all the formats?

Keeping in mind that a forum of companies including notables such as Hitachi, Matsushita, Mitsubishi, Philips, Pioneer, Sony, Time Warner and Toshiba designed DVD, it's important to understand these companies all see different potential from a technology such as DVD. Harnessing that potential involves, sometimes, putting a finger in every pie.

While DVDs are in widespread use as a distribution and archival means for movies and software, so far personal recording is only now starting to become a viable solution.

What is a digital versatile disc?
DVDs are, in many ways, more compact 'compact discs'. In the same 120mm diameter form factor, DVDs can store up to 4.7GBs of information compared to the 650MB of CDs. And ultimately, it's all due to the fine print. Information on CDs is stored as a series of reflective bumps (called lands) and non-reflective holes (called pits) running in microscopic grooves in a spiral track around the disc. Information is read by a 750nm wavelength infrared laser, which picks up reflections from the lands and pits and translates this as binary information.

DVDs use the same principle, only the distance between the tracks and the minimal distance between lands (known as pit-length) is much tighter, thereby creating a higher bit-density over the same surface area as CDs. Similarly, the laser required to read DVDs is a narrower 650nm red laser.

Aside from the impressive storage capacity of 4.7GB this achieves the specification that not only supports dual-sided discs but also dual-layer, optionally providing capacities up to 17GB per disc.

Dual-layer discs use a second track layered underneath the first running in the opposite direction and storing slightly less information: 3.8GB compared to 4.7GB. To read the layer the laser simply refocuses deeper into the disc through the semi-transparent upper layer. It's this ability to refocus that partly allows DVD drives to read CDs, which use, of course, different pit and track lengths.

Dual-layer discs are commonly found in DVD-Video form allowing up to four hours of video (and extras!) on the one side. If you have ever watched a movie and seen it pause briefly partway through, this will most likely be the DVD player refocusing the laser onto the second layer. But what about using the capacity of DVDs for personal use?

Pick a format
Firstly don't get excited at the possibility of stuffing 17GB onto a single disc – recordable DVD media is limited to one layer. But you can buy dual-sided media  capable of storing 9.4GB per disc.

A DVD-ROM is composed of two 0.6-micrometre polycarbonate discs sandwiching a reflective (often Aluminium) layer that contains the binary bits. These discs have a range of standards that define their capacities (see What DVD is that?), maxing out at 17GB. These are DVDs as you know them. Writeable and re-writeable discs, due to their manipulative nature, require a somewhat different structure.

DVD-RW is the de-facto re-writeable DVD format endorsed by the DVD Forum, while DVD+RW is a newer format developed by a splinter group consisting of Hewlett-Packard, Philips, Ricoh, Sony and Yamaha. Both formats are compatible with current DVD writers and DVD-ROM drives.

Like DVD-R, an alterable layer is used to record information, but unlike DVD-R this layer is formed from a crystalline compound that changes phase when heated by a laser. Heating an area to melting temperature (between 500-700 degrees Celsius) causes the atoms to move into a liquid state, which, if allowed to cool quickly, is 'frozen-in'. Dialectic layers above and below the recording layer help to dissipate heat. This resultant amorphous state has a different refractive index than the crystalline state, and thus can be distinguished by a reading laser (which uses less power).

As with DVD-R discs DVD-/+RW uses pre-grooved and wobbled tracks and an expected life of up to one hundred years. DVD-RAM is regarded as the most reliable of the re-writeable formats due to the fact it incorporates an error checking mechanism and can randomly access in reads as well as writes, hence its name. Like DVD-/+RW the technology revolves around a phase change layer sandwiched between polycarbonate platters, however the layer in DVD-RAM uses metallic elements that the laser heats at varying intensities to magnetically polarise. This gives DVD-RAM the impressive ability to be rewritten 100,000 times.

As with other DVD writeable formats, a wobbled groove in the polycarbonate substrate provides drive synchronisation, although the DVD-RAM drive is able to change rotation speeds to match the location of data on a disc. DVD-RAM discs often come in cartridges and, understandably, are not compatible with most DVD-ROM and re-writeable drives. DVD-RAM has a life expectancy of 30 years.

DVD-R media costs around $6 a disc, DVD-/+RW approximately $22 a disc, and DVD-RAM almost $60 per disc. DVD recordable drives generally come with the ability to write both DVD-R and DVD-RW and retail for around $600. DVD+RW drives sell for approximately $100 more than this and DVD-RAM roughly $100 more again. Prices can differ greatly by manufacturer and the number of supported formats.

It's on the disc
There's more to DVD burning than the discs and drives. Just as your hard drives are formatted with a filesystem to help the operating system know where and how to locate files, recordable media uses filesystems to do the same. CDs have a variety of filesystems and extensions that include ISO9660 (the standard), RockRidge (Unix), Joilet (Windows), and HFS (Mac). Mostly the extensions are patches to allow CDs to be used in computers with the operating system's native filesystem format.

Recognising the inherent problems that arose out of using mixed filesystems for CDs, the DVD Forum chose to go with MicroUDF, a DVD adaptation of the UDF (Universal Disc Format) filesystem. MicroUDF is cross-platform, interchangeable (readable by DVD hardware such as home theatre gear), supports long file names and extended attributes, and provides maximum possible file sizes and disc usage by minimising multisession overheads.
 
The future holds even greater promise for the humble 120mm diameter disc. Research into the use of shorter wavelength blue lasers promises to create even higher density discs that can store some 20GB per layer, per side. In the near future you may well be able to back up your entire 80GB hard drive to a single, versatile, disc.