05.25.08
How DVD’s Work
DVDs are the more technologically advanced relatives of CDs, designed to store large amounts of both video and audio information. Because a Digital Video Disc can hold up to eight hours worth of movies or seven times the amount of music that a compact disc can store, it has been rapidly replacing VHS cassettes and CDs since its debut in 1997. Although it works similarly to its audio forerunner, the DVD has several notable differences. The technology used to create and play DVDs is far more sophisticated. Beyond just holding a movie, DVDs often have menus, language/subtitle options, commentary, and massive amounts of bonus features. They allow us to flip to any part of the film we wish with the touch of a button, getting rid of the hassle of fast-forwarding and rewinding. Although the newest format for movie viewing is being embraced, few people have any idea about how they work. The following will explain how a DVD is made, how it is played, and what it can do.
How DVDs are made
Digital video discs are comprised of up to four layers, which hold expansive storage room. A disc is made up of incredibly thin (only 0.6 mm. thick) plastic layers, each containing a tightly spaced spiraled track. The tracks have bumps that hold data in the form of MPEG-2 format compressed bits. Once the bumps are laid into a layer of track, it is joined to the next layer with plastic resin, after which the disc is pressed together and cured under infrared light. Each piece has an internal coat of reflective aluminum and an exterior coat of slightly reflective gold, which a laser can look through to see inner tracks. Finally, a single sided disc receives a silkscreen label on the unreadable side. On a double-sided DVD, the label is placed only on the interior plastic ring of the disc.
How DVDs are played
Although the closely lain tracks and potential multiple playable layers on a DVD make it capable of storing far more information than a CD, it cannot hold much video information that remains in its current form. This is why MPEG-2 technology is used to condense each frame of video and encode it onto the disc in the form of bumps. Depending on the type of film, different methods of encoding are used. For instance, a movie scene in which a casual conversation is taking place requires little change from frame to frame. In this case, the disc would begin by encoding the scene in intra-frame, a method that contains every detail of the clip. If the scene went on with very little change, the next frame would use predicted encoding. Rather than storing every detail, this method only includes the information that has changed in the scene. Thus, time and effort are minimized during the decompression process. Another type of encoding frame called bidirectional takes details from surrounding frames to produce the current picture. The encoding software chooses which method is best, depending on how much action is occurring in the video. Thus, when the DVD player decompresses the frames, it can do so in as little time as possible.
As was mentioned earlier, DVDs can store music as well and are reputed to provide exceptional sound quality. However, many factors are involved in achieving the best audio in your home. The DVD track contains a compressed version of an exact piece of music, but it may not always be translated that way. DVD music is designed to sound best in Dolby Digital surround sound, the equivalent of movie theater soundtracks. If the DVD player does not contain Dolby Digital technology, it will not be capable of decompressing and presenting the music properly. Most DVD players on the market also have a DAC (digital-to-analog converter), the device responsible for translating sound, that is too imprecise. For the best music quality, look for a player with the highest available DAC grade. In addition, the basic two-speaker television cannot properly simulate DVD audio. For this purpose, it is advisable to hook up a set of five Dolby Digital speakers as well as compatible cables. If you only use your DVD player for music occasionally, you will still find that it sounds better than it would on a CD player. However, the music DVDs that have only recently begun to be available will perform optimally if you consider these suggestions.
Once a DVD is placed in the player several devices work together to extract the information from the disc. A small drive motor rotates the disc at a steady rate of two hundred to five hundred rotations per minute, depending on what information the disc contains. At this point, a laser beam — infinitely shorter and thinner than that of a CD player — begins reading the track. Starting at the outside of the DVD and moving inward, the laser reads the bumps and reflects onto a hologram lens. Here, the beam is split so that it can be read. Information is sent to the player’s MPEG-2 decoder, where it is decompressed and played on the television screen as quickly as viewers can watch it. Once the laser reaches the inside of the disc, it will start reading the second layer of track if present, which begins in the center of the disc. This saves time, as the tracking device does not have to move the beam out again to continue playing. Only when the second layer is played will the disc need to be flipped over if it is two sided.
Because of their compact size, storage capacity, and exceptional audio and video capabilities, DVDs make the bulky inconvenient VHS movie seem antiquated. DVD players and discs are becoming increasingly affordable, a change which has most homes replacing their VCRs for their high-tech successors. Not only do they provide superior picture and sound quality, but they are also more resistant to wear than taped movies. Kept in their cases, they can play virtually forever without decreased performance. Best of all, they are becoming available in recordable form, which means that you can copy your favorite TV show or use a DVD recordable camcorder. Continued advances on the part of manufacturers promise that the abilities of DVDs will only grow with time.
