The Technology of DVD’s and DVD players
By: Cedric T.
Science, 2nd Period

2012


Introduction to DVD’s


When you watch a DVD, do you ever wonder what it is made of, or how a DVD player works? Do you ever think about what laser technology is, or even how it does what it was made to do? Well, I’m here to answer those questions for you. DVD stands digital video disc, and is a type of optical disc used to store information and data and display that data onto a screen. DVD’s were invented by the companies Sony, Toshiba, Phillips, and Panasonic in 1995. There are a lot of differences between a DVD and a CD. One of them is that a DVD has much more storage space than a CD. A single-sided DVD disc can hold more than 7 times the information storage area of a CD and a DVD can hold up to 4 ½ hours of data. There is a method of compression that allows a DVD to hold more than 135 minutes of information on only one side of the disc. That compression gets rid of unnecessary or redundant data that has been stored on the disc so there is more storage room for data that needs to be on the disc. Some DVD’s use two sides of the disc so there is more space for movies, recordings, or other types of information.


DVD and DVD players.jpg
This is what happens when you play a DVD disc in your DVD player.


Laser technology is a type of data-reading method that is used to read the information on a disc. It works by the DVD drive emitting low-power lasers to read digitized data that has been encoded onto the disc in the form of tiny valleys. The drive can then read those valleys as signals and numbers, and those signals and numbers can be scanned as data that can be transformed into pictures, words, and music. DVD’s store data in a spiral shape on the disc, which means the data is stored in a circular path around the entire disc. The spiral-shaped data on the disc is easier to read by the lasers than having the data spread out in different places on the disc. If that was the case, the laser would have to move a thousand places at once to read the data that could be anywhere on the disc. There is a method that also uses laser technology that allows information to be re-recorded onto a disc, which is called phase-change recording. It works by the disc being reheated with a laser at a different phase than the initial recording. The laser hits the disc at a different angle, changing the shape of the grooves. The data layer of a DVD does not receive damage because they are read by laser light and never physically touched by mechanics, also because DVD’s are covered by a protective plastic substance. In video disc recording, the image is converted to signals recorded on a disc by a laser, which vaporizes tiny holes or indentations on the disc. When the disc is being played, these holes are being detected by another, not as powerful laser.

DVD Players




A DVD player is a mechanical device that is used to play and read the memory that is stored onto the disc. They have a laser assembly that shines the laser beam onto the surface of the disc to read the pattern of tiny indentations on the disc. The DVD player has the job of finding and reading the data stored as those small bumps. Considering how small the bumps are, the DVD player has to extremely precise on finding and reading every bump on the disc. The drive consists of three fundamental components: the drive motor to spin the disc-the drive motor is precisely controlled to rotate between 200 and 500 rpm, depending on which track is being read, the laser and lens system to focus in on the bumps and read them, and a tracking mechanism that can move the laser assembly so that the laser beam can follow the spiral track-the tracking system has to be able to move the laser at light-speeds. The fundamental job of a DVD player is to read the track of bumps. The laser can either focus on the semitransparent reflective material behind the closest layer, or, in the case of a double-layer disc, through this layer and onto the reflective material behind the inner layer. The laser beam passes through the polycarbonate layer, bounces off the reflective layer behind it, and hits an opto-electronic device, which detects changes in light. The bumps reflect light differently than the flat areas of the disc, and the opto-electronic sensor detects those changes in reflectivity. DVD’s are composed of several layers of plastic about 1.2 mm thick. They can have one or two layers per side and can be single or double-sided. In multilayer DVD’s, the laser needs to be able to concentrate separately on either the first or second layer. To allow this sort this sort of two-layer focusing, the first layer uses a semitransparent coating of gold, while the second layer uses and aluminum mirror, like a CD does. The laser can focus on the gold layer, or it can shine through the gold and focus on the aluminum layer. Each layer of a DVD is made with a piece of injection-molded plastic impressed with millions of tiny bumps. A layer of aluminum or gold is sprayed onto the bumps to create the reflective coating. When all the layers are made, each one is coated with a substance called lacquer, they are squeezed together, and the disc is cured under infrared light. The hardest part of reading a disc is keeping the laser beam centered on the data track. This centering is the job of the tracking system. As the disc is being played, the tracking system has to move the laser continually outward. As the laser moves outward from the center of the disc, the bumps move past the laser at increasing speeds. This happens because the linear speed of the bumps is equal to the radius times the speed at which the disc is revolving. So, as the laser moves outward, the motor must slow the spinning of the DVD so that the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.



Citations
"DVD." Encyclopedia Britannica. Encyclopedia Britannica Online School Edition.Encyclopedia Britannica, Inc., 2012. Web. 25 Sept. 2012.
http://electronics.howstuffworks.com/dvd6.htm
http://www.mediatechnics.com/dvdplayers.htm
McGrath, Kimberley A., ed. “DVD.” The Gale Encyclopedia of Science. Vol. 2. Detroit: Gale Group, 2001. Print.
“Sound and light,” Holt Science & Technology; Orlando, Leila, Dumas; 2007

Glossary
Drive Motor-a device used to spin the disc in a DVD player
DVD-a type of optical disc used to store data
DVD Player-a mechanical device that finds, reads, and plays the data in the form of tiny bumps on a disc
Lacquer-a substance used to coat the layers of a DVD
Laser and Lens System-a device used to focus in on the bumps on a DVD in a DVD player
Laser Technology-a data-reading method used to read, detect, and find the information on a disc
Phase-Change Recording-a method used to re-record information onto a disc
Tracking Mechanism-a device used to move the laser assembly in a DVD player