DVD
A commercially manufactured music or data CD or a movie DVD contains 2K blocks of data encoded as pits in the metal surface of the disk. The CD was invented 20 years ago. Advances in technology permit improvements on that standard. First, the pits on the disk surface that carry the signal can be made smaller, allowing more data to be written in one spiral around the disk surface and allowing the adjacent spiral tracks to be packed tighter. The usable disk surface is extended, and the data is written in a slightly more compact encoding. Combining all these features, the surface of a DVD can hold 4.7 gigabytes of data compared to the 700 megabytes of a CD.
A commercially produced DVD can do even better. In addition to the base data embedded in pits in the metallic base, a second set of pits is added in the semitransparent plastic coating. The DVD sensor can focus either on the pits of the back layer or of the coating layer. This nearly doubles the amount of data on the complete disk and makes it possible to store almost 9 gigabytes of data.
A two hour movie can barely fit on one surface of a disk. Add several sound tracks, including a director's commentary, plus trailers, bloopers, and other extras, and most movies require the capacity of a two layer disk.
Writable disks (CD-R, CD-RW, DVD-R, DVD+R, DVD-RW, or DVD+RW) have a reflective back surface covered by a plastic coating embedded with dye spots. Initially the spots are transparent. A laser operating at high power generates enough heat to change the chemical composition of the dye material and turn it dark. Later, a laser running at much lower power can focus on the surface. Where the dye was not changed, the light still reflects back. Where it was changed, the dark color absorbs the light. This technique only operates on a single layer, so current writable DVD's have only 4.7 gigabytes capacity.
A writable disk is not featureless. It is manufactured with guidance information that enables the drive to position all the 2K blocks and the spiral tracks. The DVD-R(W) and DVD+R(W) standards accomplish this slightly differently. Each has advantages and disadvantages. Fortunately, the difference is more a matter of reprogramming the firmware in the drive rather than changing the hardware. Today most new DVD writers support both families of formatting.
Although some companies produce CD-R disks labeled "music", the CD has the same physical low level structure whether it contains audio or computer data. The difference is in the directory structure written onto the disk by the program that burns the data. Similarly, a DVD video disk is simply a DVD data disk with some directories and files with special names.
The video files on a commercially produced movie DVD are encoded by an encryption system called "CSS". For the movie to play, it must be decrypted by hardware in the DVD player or software on the computer. It is illegal to create software to bypass the CSS protection, but such software is widely available from internet sites outside the US.
A movie DVD compresses video in the MPEG 2 format. Ten years ago this was leading edge technology. To play a movie DVD on a computer required a 333 MHz Pentium II CPU. DVD players were expensive. In those days, DVD players came from Japan or Korea. Today CD and DVD drives and players come from China. Prices have been falling month by month. By Christmas 2003, DVD players were selling for $30, and a DVD writer for a computer costs $100.
All current DVD movies are designed to play on traditional TV sets. They may have a very sharp picture, and some may be designed to stretch the picture horizontally for a "widescreen" display, but the files are still encoded to display a picture with 480 lines of resolution 30 times a second by alternating two sets of 240 lines 60 times a second. New DVD players have a "progressive scan" option to intelligently combine adjacent groups of 240 lines (deinterlace) to provide a 480 line picture on TV sets with enhanced resolution.
High Definition DVD
We are now seeing the launch of two competing systems that substantially expand the amount of data that can be stored on the CD/DVD disk form. HD DVD and Blu-Ray represent slightly different versions of the same basic technology. Exactly what encoding standards and features will be on each format has been the subject of negotiation right up to the wire.
The main difference is the manufacturing technology. The HD DVD is basically the same as a current DVD, only the pits are smaller so they have to be read by a blue laser. Generally speaking, you can take a current DVD production factory and turn it into an HD DVD factory at very low cost.
Blu-Ray, however, requires a new coating on the disks that is much thinner than the plastic coating on a DVD or CD. This is a new material and requires new equipment. To protect the disk surface, this thinner coating must be harder and more resistant to scratches.
There are certainly technical details that people can argue about for hours, but a quick summary is that Blu-Ray is more advanced technology that, today, is much more expensive. HD DVD is less advanced, but it starts at a much lower cost.
Which wins depends on how two predictable technology trends interact. Every previous CD or DVD technology has started out expensive and then become affordable over time. At some point Blu-Ray will become affordable. However, at the same time compression is reducing the amount of space required to hold HD content. So the technical advantages of Blu-Ray over HD DVD may become less important.