Dvd - How DVDs and DVD Players Work

How DVDs and DVD Players Work

Nearly every movie produced today is available on DVD, and many older movies are being moved to the DVD format. Often, a movie comes out on DVD before it comes out on video tape, because the manufacturing and distribution costs for DVDs are so much lower!

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, 720 dots of horizontal resolution
• 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.

Storing Data on a DVD
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, totalling about 1.2 millimetres thick. Each layer is created by injection moulding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data.

Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminium 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 printed onto the nonreadable side. Double-sided discs are printed only on the nonreadable area near the hole in the middle completed DVDs.

Diameter=Schicht

Injection moulding = Spritzgießen

Bumps = Unebenheiten

Sputtered = gespritzt

Squeezed = gedrückt

Cured = Vukanisiert,Haltbar gemacht

The DVD Video Format
Even though its storage capacity is huge, the uncompressed video data of a full-length movie would never fit on a DVD. In order to fit a movie on a DVD, you need video compression. A group called the Moving Picture Experts Group (MPEG) establishes the standards for compressing moving pictures.

When movies are put onto DVDs, they are encoded in MPEG-2 format and then stored on the disc. This compression format is a widely accepted international standard. Your DVD player contains an MPEG-2 decoder, which can uncompress this data as quickly as you can watch it.

The MPEG-2 Format and Data Size Reduction
A movie is usually filmed at a rate of 24 frames per second. This means that every second, there are 24 complete images displayed on the movie screen. American and Japanese television use a format called NTSC, which displays a total of 30 frames per second; but it does this in a sequence of 60 fields, each of which contains alternating lines of the picture. Other countries use PAL format, which displays at 50 fields per second, but at a higher resolution. Because of the differences in frame rate and resolution, an MPEG movie needs to be formatted for either the NTSC or the PAL system.

The MPEG encoder that creates the compressed movie file analyzes each frame and decides how to encode it. It also uses information from other frames to reduce the overall size of the file. Each frame can be encoded in one of three ways:

• As an intraframe - An intraframe contains the complete image data for that frame. This method of encoding provides the least compression.
• As a predicted frame - A predicted frame contains just enough information to tell the DVD player how to display the frame based on the most recently displayed intraframe or predicted frame. This means that the frame contains only the data that relates to how the picture has changed from the previous frame.
• As a bidirectional frame - In order to display this type of frame, the player must have the information from the surrounding intraframe or predicted frames. Using data from the closest surrounding frames, it uses interpolation (something like averaging) to calculate the position and color of each pixel.

Depending on the type of scene being converted, the encoder will decide which types of frames to use. If a newscast were being converted, a lot more predicted frames could be used, because most of the scene is unaltered from one frame to the next. On the other hand, if a very fast action scene were being converted, in which things changed very quickly from one frame to the next, more intraframes would have to be encoded. The newscast would compress to a much smaller size than the action sequence.

Fields = Sektoren

Alternating = wechselnd,         newscast = Nachrichten

DVD Audio
DVD audio and DVD video are different formats. DVD audio discs and players are relatively rare right now, but they will become more common, and the difference in sound quality should be noticeable. In order to take advantage of higher-quality DVD audio discs, you will need a DVD player with a 192kHz/24-bit digital-to-analog converter (DAC). Most DVD players have only a 96kHz/24-bit digital-to-analog converter.

DVD audio recordings can provide far better sound quality than CDs.DVD audio discs can hold 74 minutes of music at their highest quality level, 192kHz/24-bit audio. By lowering either the sampling rate or the accuracy, DVDs can be made to hold more music. A DVD audio disc can store up to two hours of 6-channel, better than CD quality, 96kHz/24-bit music. Lower the specifications further and a DVD audio disc can hold almost 7 hours of CD-quality audio.

Reading a DVD
The DVD player has the job of finding and reading the data stored as bumps on the DVD. Considering how small the bumps are, the DVD player has to be an exceptionally precise piece of equipment. The drive consists of three fundamental components:

• A drive motor to spin the disc - The drive motoris precisely controlled to rotate between 200 and 500 rpm, depending on which track is being read.
• A laser and a lens system to focus in on the bumps and read them - The light from this laser has a smaller wavelength (640 nanometers) than the light from the laser in a CD player (780 nanometers), which allows the DVD laser to focus on the smaller DVD pits.
• A tracking mechanism that can move the laser assembly so the laser beam can follow the spiral track - The tracking system has to be able to move the laser at micron resolutions.

The fundamental job of the DVD player is to focus the laser on the track of bumps. The laser can focus either on the semi-transparent 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 'lands,' the flat areas of the disc, and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.

The hardest part of reading a DVD is keeping the laser beam centered on the data track. This centering is the job of the tracking system. As the DVD is 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 an increasing speed. This happens because the linear, or tangential, 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 spindle 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.

DVD Players advantages
DVD players can change the way you watch movies and listen to music at home. They improve the quality of the picture and the sound, and they are more versatile than video cassette recorders. Here are some of the things that you can do with a DVD player:

• Watch high-quality movies with good sound
  You probably know that a DVD player will let you watch DVD movies. These movies have some advantages over video cassette movies. The picture quality is better, and many of them have Dolby Digital or DTS sound, which is much closer to the sound you experience in a movie theatre.
• Skip to your favourite parts of movies
  Many DVD movies have an on-screen index, where the creator of the DVD has labelled 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.
• Play audio CDs
  DVD players are compatible with audio CDs, so if you want to get rid of your CD player to make room for a DVD player, go ahead.
• Watch movies in different picture formats
  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.
• Watch movies with subtitles or in a different language
  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.