DVD TECHNOLOGY

DVDs and CD-ROM's serve the same purpose, but in different ways. DVD's and CD's have the same physical characteristics, but are internally different. They are also both read the same way, using pits and lands to reflect a laser.

A DVD is exactly the same size (physically) as a CD, but can store seven times the data that a CD-ROM can. The disk itself is 120mm in diameter and 1.2mm thick. It can store 4.7 gigabytes while CD-ROM can only hold up to 800 megabytes. DVDs also come in four versions:

• DVD 5 is a single sided, single-layered disk with a capacity of 4.7GB
• DVD-9 is a single sided, double-layered disk with a capacity of 8.5GB
• DVD-10 is a double sided, single-layered disk with a capacity of 9.4 GB
• DVD-18 is a double sided, double-layered disk with a capacity of 17GB.

 

There are also five physical formats to DVDs:

• DVD-ROM is a high capacity data storage medium
• DVD-Video is a digital storage medium for a feature-length motion picture
• DVD-Audio is an audio-only storage format, similar to CD-Audio
• DVD-R is similar to a CD/R which offers a write-once, read many storage format
• DVD-RAM was the first re-writable (erasable) DVD.

The tracks in a DVD are placed much closer together than a CD with the pits and lands also being much smaller, thus allowing for more data storage. A DVD also allows information to be scanned from more than one layer of a DVD by changing the focus of the read laser. Instead of using an opaque reflective layer is uses a translucent layer with an opaque reflective layer behind carrying more data. However, this does not double the capacity of the disk because the second layer cannot be as dense as the single layer.

An interesting feature of DVD is that the disks' second layer can be read from the inside of the disk out, as well as from the outside in. In an ordinary CD, the data begins reading from the center (the hub) of the disk and goes out.

A DVD also allows for double-sided capacity. This means that data can be stored on both sides of the disk. To facilitate the focusing of the laser on the smaller pits, manufactures used a substrate thinner than that used in CDs, thus reducing the depth of the layer of plastic the laser has to travel through to reach the pits. Because of this, the resulting disk was only .6mm thick, which was too thin to remain flat and withstand handling. The solution was to bond two disks back-to-back, effectively doubling the potential storage space.