A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD does. This huge capacity means that a DVD has enough room to store a full-length, MPEG-2-encoded movie, as well as a lot of other information.
Here are the typical contents of a DVD movie:
- Up to 133 minutes of high-resolution video, in letterbox or pan-and-scan format, with 720 dots of horizontal resolution (The video compression ratio is typically 40:1 using MPEG-2 compression.)
- Soundtrack presented in up to eight languages using 5.1 channel Dolby digital surround sound
- Subtitles in up to 32 languages
DVD can also be used to store almost eight hours of CD-quality music per side.
The format offers many advantages over VHS tapes:
- DVD picture quality is better, and many DVDs have Dolby Digital or DTS sound, which is much closer to the sound you experience in a movie theater.
- Many DVD movies have an on-screen index, where the creator of the DVD has labeled many of the significant parts of the movie, sometimes with a picture. With your remote, if you select the part of the movie you want to view, the DVD player will take you right to that part, with no need to rewind or fast-forward.
- DVD players are compatible with audio CDs.
- Some DVD movies have both the letterbox format, which fits wide-screen TVs, and the standard TV size format, so you can choose which way you want to watch the movie.
- DVD movies may have several soundtracks on them, and they may provide subtitles in different languages. Foreign movies may give you the choice between the version dubbed into your language, or the original soundtrack with subtitles in your language.
DVDs are of the same diameter and thickness as CDs, and they are made using some of the same materials and manufacturing methods. Like a CD, the data on a DVD is encoded in the form of small pits and bumps in the track of the disc.
A DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. More on the bumps later.
Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers. After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, the label is silk-screened onto the nonreadable side. Double-sided discs are printed only on the nonreadable area near the hole in the middle. Cross sections of the various types of completed DVDs (not to scale) look like this:
Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside. That the spiral track starts at the center means that a single-layer DVD can be smaller than 12 centimeters if desired.
What the image to the left cannot impress upon you is how incredibly tiny the data track is — just 740 nanometers separate one track from the next (a nanometer is a billionth of a meter). And the elongated bumps that make up the track are each 320 nanometers wide, a minimum of 400 nanometers long and 120 nanometers high. The following figure illustrates looking through the polycarbonate layer at the bumps.
You will often read about “pits” on a DVD instead of bumps. They appear as pits on the aluminum side, but on the side that the laser reads from, they are bumps.
The microscopic dimensions of the bumps make the spiral track on a DVD extremely long. If you could lift the data track off a single layer of a DVD, and stretch it out into a straight line, it would be almost 7.5 miles long! That means that a double-sided, double-layer DVD would have 30 miles (48 km) of data!
To read bumps this small you need an incredibly precise disc-reading mechanism.
For more Details: How DVDs Work