Basics

• A DVD player operates according to the same basic principles as a CD player, a laser disc player and similar types of electronics. The only difference is the amount of information encoded and the overall efficiency of its encoding. Sandwiched between the plastic of the disc is a thin piece of aluminum, upon which is encoded a series of divots. Those divots create a binary code, which stores all the information of the movie's sound and image.
  
  

Binary Code

• When the DVD is placed in the player, it spins around very quickly. A laser light then scans the disc and reads the divots, which are interpreted as a series of 1s and 0s (similar to information stored on a computer). An MPEG-2 decoder device translates to code into sound and images, which then get sent to your screen. The laser starts out on the inside of the disc and then gradually moves to the outside, following the divots the way an old LP player followed the grooves in the record (though LP record needles went in the reverse direction as the DVD laser, starting on the outside of the record and moving in). Most DVDs are dual-layered, which means they possess an additional series of divots: when the laser reaches the end of the disc, it moves backward to read this additional layer. That's why some DVDs include a brief pause in the middle of the film; the laser is switching to the second layer.
  
  

Tracking and Measuring

• Other devices in the DVD player control the tracking, the position of the laser on the disc and the rate at which it moves up and down the length of the DVD. Most DVDs take advantage of that with the chapter listings. They essentially tell the laser which point on the disc to begin scanning, allowing you to move backwards and forwards to different points on the film.
  
  

Advantages

• The central advantage to this kind of technology is that the disc itself contains no moving parts and the surface never actually comes into contact with anything (unlike VHS tapes and similar technology). That makes DVDs extremely durable. While they may become scratched or damaged, they won't "wear out" from excessive playing, and the image will be as sharp and clear the millionth time you play it as it was the first.
  
  

Scratches

• With most DVDs, the aluminum layer which contains all the information is closer to the top of the disc than the bottom. That's why you can sometimes buff out minor scratches in the bottom of the disc and still have a playable movie.