The DVD

Like CD ROM drives, DVD drives are optical storage devices that use lasers to write to and read from their media, and essentially DVDs use the same kind of principles as CDs. When writing to an DVD, a laser creates pits on its surface to represent data. Areas not burned into pits by the laser are called lands. The laser reads back the data on the optical disk by scanning for pits and lands. There are, however a number of differences that make the DVD superior to the CD.

Whereas the CD drive burns the pits onto the CD with a track pitch of 1.6µm, the DVDs track pitch is 0.74µm, which means the density of tracks of the DVD is 4.5 times higher than that of the CD. In fact, the capacity of a single DVD layer is 4.7GB. But that is not where it stops. The DVD achieves even more capacity by having not one single track layer like the CD, but two. Last but not least, the DVD has two sides, very much like a conventional record, increasing the capacity of the DVD to 17GB (it's not exactly 4 x 4.7GB = 18.8GB because the inner layer needs some extra lead-out space.

In order to read the two layers correctly, the outer layer of the DVD is 18 to 30 percent transparent, i.e. semi-transparent, whereas the inner layer is 50 to 80 percent transparent. This allows the laser to re-focus and read the desired layer.

There are two ways to read the layers - PTP (Parallel Track Path), and OTP (Opposite Track Path). PTP reads both layers from the inside of the disk outwards, whereas OTP reads the outer layer from the inside out and then back for the inner layer (i.e. outside in). This allows the DVD drive to read both layers almost continuously, with only a short break for refocusing. This is especially useful for movies where a long refocusing time can interrupt the movie.

DVD technology has been around for years but has not caught on because of some format differences. There are three types of format, each not compatible with the other.

DVD-ROM

DVD-ROM only reads factory manufactured DVDs and cannot record new data. It is the only format currently available for an affordable price. Microsoft Encarta and the Encyclopedia Britannica, as well as a huge variety of movies, have already appeard on the market.

DVD-RAM

DVD-RAM uses s phase changing recording method to give re-writable disks a capacity of 2.6GB each side. This works by using a recording medium that can exist in a crystalline as well as an amorphous state. The recording layer (usually crystalline) is changed to the amorphous state to represent the change of data. The laser does not burn pits into the DVD but simply changes the state of the layer.

DVD-R

DVD-R is a recordable format that is compatible with DVD-ROM and disks can be interchanged between devices of both standards. DVD-R are still very expensive though (several thousand pounds), so DVD-RAM will take the market before DVD-R prices can fall.

DVD+RW

DVD+RW has yet not even been accepted as an official standard by the DVD Forum, therefore it is difficult to say when and for how much we will get to see it.

Read Compatibility of DVD drives
	CD-ROM	CD-R		CD-RW	DVD-ROM	DVD-RAM		DVD-R	DVD+RW
CD-ROM
CD-R
CD-RW
DVD-ROM
DVD-RAM
DVD-R
DVD+RW
- compatible			- not compatible				- drive dependent

Integrating the DVD drive into your system is basically done just like the CD ROM drive, with one addition. Because of the enhanced digital sound capabilities of the DVD, there is an additional sound cable leading from the DVD drive into the sound card. It's a simple two way cable called SPDIF and if you ever want to enjoy mpeg movie sound quality, then you must not forget it.