You hit the button to open the tray on your DVD player, nestle your disc into place, let the tray glide into the box and -- boom -- "Mary Poppins" starts. But what manner of digital magic is employed to make Mary and Bert and the Banks children sing and play on your TV screen? Of course, it's not magic at all, but it is a feat of engineering and technology.
A DVD player is very similar to a CD player, with a laser assembly that shines the laser beam onto the surface of the disc to read the pattern of bumps (see How CDs Work for details). The DVD player decodes the MPEG-2 encoded movie, turning it into a standard composite video signal (see How Television Works for details). The player also decodes the audio stream and sends it to a Dolby decoder, where it is amplified and sent to the speakers.
The DVD player's job is 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. Learn more on the next page.
The DVD drive consists of three fundamental components:
- A 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.
- 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.
Inside the DVD player, there is a good bit of computer technology involved in forming the data into understandable data blocks, and sending them either to the DAC, in the case of audio or video data, or directly to another component in digital format, in the case of digital video or data.
To work properly, the DVD player must 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.
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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.
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An interesting thing to note is that if a DVD has a second layer, the start of that layer's data track can be at the outside of the disc instead of the inside. This allows the player to transition quickly from one layer to the next, without a delay in data output, because it doesn't have to move the laser back to the center of the disc to read the next layer.
DVD Player Features
- DVD movies Just about all players play DVD movies.
- Music CDs Most players also play music CDs.
- Video CDs Some players can handle this format, which is used mostly for music videos and some movies from foreign countries.
- CD-Rs Some players can play content that you create on your own computer.
- Audio DVDs A few players can handle this format for high-quality audio.
- Dolby Digital decoder This feature allows the DVD player to decode the Dolby Digital information from a DVD and convert it to six separate analog channels. This feature is not necessary if you have a Dolby Digital receiver, which has a digital input that carries all of the audio information.
- DTS decoder This feature allows the DVD player to decode the DTS information from a DVD and convert it to six separate analog channels. Again, this feature is not necessary if you have a receiver with a DTS decoder.
- DTS compatible All DVD players are DTS compatible. They pass the digital audio information on to the receiver, which then decodes it.
- Simulated surround If you are going to hook the DVD player up to a TV or a stereo system with only two speakers, a DVD player with simulated surround processing will give you some sense of surround sound without the extra speakers.
- Disc capacity Some DVD players can hold three, five or even several hundred discs. Since most DVD players can also play audio CDs, if you buy a player with a high disc capacity you could store your whole CD collection in the machine.
- 96kHz/24-bit DAC This is the speed and accuracy of the digital-to-analog converter (DAC), which converts the audio information into an analog signal. Most movie soundtracks are encoded in this format, so this is really a required feature, and most DVD players will have at least a 96kHz/24-bit DAC.
- 192kHz/24-bit DAC This is an upcoming format for audio-only DVDs that are recorded at speeds of up to 192kHz and 24-bits. Only the newest DVD audio players have the 192kHz/24-bit DAC required to play these audio discs.
DVD players may come with three types of remotes:
- A dedicated remote, which only runs the DVD player
- A multibrand remote, which can control other components, like VCRs and TVs made by other manufacturers (Usually, they only support the more common brands.)
- A learning remote, which can learn the signals from other remotes and assign them to a button (This feature is useful if you have uncommon brands of components to control.)
Audio and Video Outputs
- Component-video outputs provide the highest quality video signal to your TV. They are quite rare right now; only the newest high-end TVs can support them. But, if you have such a TV, you'll definitely want a DVD player with component video outputs. There are three separate connectors for component video output.
- S-video outputs are more common. S-video provides a very good picture quality, and every DVD player has at least one of these outputs.
- Composite-video outputs are the most common type of output, and they provide adequate picture quality. Usually, they have a yellow plastic insert.
- Coaxial digital output and optical digital outputs provide the highest-quality audio. They send the digital sound information to the receiver for decoding. You can use either one of these if you have a Dolby Digital receiver.
- 5.1 channel is a set of six analog outputs, one for each of the Dolby Digital channels (left front, center front, right front, left rear, right rear and subwoofer). The DVD player decodes the Dolby Digital signal and uses its own DAC to output an analog signal. These are the outputs you'll need to use if you are hooking the DVD player up to a "Dolby Digital ready" receiver. DVD players with 5.1 channel outputs will always have Dolby Digital decoders, and they may or may not have DTS decoders. If you have a "Dolby Digital ready" receiver and you want DTS sound, you will need a DVD player with a built-in DTS decoder.
- Stereo outputs carry only the stereo music signal. You would use these if you were hooking your DVD player up to a TV that has only two speakers.
Connecting the DVD Player
Connecting a DVD player to your stereo receiver (or television, if you don't have a receiver) involves making two basic connections: audio and video.
The first connection to make is for the audio portion of the signal. There will be several options depending on the receiver you have.
- The best choice (if available) is either to use an optical (also called Tos-link) or coaxial (RCA) digital connection. These two choices are equal in quality. In order to use either of these, you will need to have both an output on the DVD player, and an input on the receiver. Only receivers with built-in Dolby Digital decoders will have this type of input.
- If your receiver does not have a built-in Dolby Digital or DTS decoder, but is "Dolby Digital ready," look for the 5.1-channel Dolby or 5.1-channel DTS. This connection involves six cables, corresponding to different speaker channels: left front, center front, right front, left rear, right rear and subwoofer.
- The last option to connect the two components is with analog RCA outputs. This is a two-cable connection, with one cable delivering the left speaker sound, and the other cable delivering the right. This connection will deliver only stereo sound, but it may be your only option if you are hooking up directly to a television, or if you have an old receiver with only two channels.
Now let's take a look at the video connection.
- The best quality choice is to use component connection. This connection consists of three cables: color-labeled red, blue and green. The quality is superb. However, these connections only exist on extremely high-end receivers and television sets.
- The next option is s-video. One cable connects the DVD player to the receiver in this setup.
- The last option, similar to the audio setup, is to use the analog RCA video output, usually color-labeled yellow on both ends. This will deliver the lowest quality, but will suffice for most older, analog televisions.
For more information about DVD players and DVD technology, see the links on the next page.