What's a DVD and how does it work?

Benjamin Britton, an associate professor and electronic artist at the University of Cincinnati, provides the following explanation:

The tools of technology can seem like a confusing alphabet-soup. Terms such as ZIP, CD or DVD are commonly used to describe the means available for storing and sharing information, ranging from text to audio to full-length films. Of them all, DVD, which stands for digital versatile disc, is poised to become the most popular and reliable means for storing data, especially high-quality digital video. DVDs are now rapidly making their way into American homes as the state-of-the-art entertainment option for watching Hollywood releases.

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Before going into the mechanics of how a DVD works, it might be best to explain the why. In other words, why is the industry replacing previous storage-unit standards, including the CD (short for compact disc)? Quite simply, the DVD's storage capacity is much greater. Thus, the DVD makes sense for video and other industries in a way that the CD never could. A high-quality digital video could simply never fit onto a CD. It can't even fit comfortably on a computer hard drive.

Indeed, a high-quality digital video requires up to 100 megabytes (MB) of data space each minute, depending on the amount of compression used. MPEG2, a format commonly used for playing video via computer, compresses one minute of visual data into about 30 MB. So a two-minute video requires 60 MB, and a two-hour movie requires 3,600 MB. Compare that to the maximum storage space of a CD which is about 640 MB.

The encoding of data onto a DVD may seem like a black art, but it's really not that much different from encoding data onto a videotape. A videotape stores and plays back information based on the physical placement and arrangement of iron oxide particles on the material of the tape. A DVD similarly stores and plays information based on a particular "dot pattern" on its surface. A very precise laser burns these dots--actually, incredibly small pits--on to the master DVD. It is the smallness of the dots that allows for the roomy storage capacity of the DVD.

Practically speaking, let's say I had a virtual reality project of 75 MB on a disc. If I want to release my project to the public or exhibit it in museums across the country, I'm going to need a reliable means for viewing. Thus, I need to place it on DVD, and not only on one DVD, but on thousands for distribution.

So I make a single DVD at home and then take my disc to a replication house, such as Panasonic, CinRam, Nimbus or Diner. Most of the optical industry service providers are, not surprisingly, located on the West Coast. But there are regional houses as well. They will feed the information from my disc via software algorithms to what is called a burner, which guides the laser that brands a glass-topped DVD with the data pattern of dots that vary in terms of spacing and in terms of brightness and darkness. The spacing and variation of brightness and darkness of the dots are what makes the data readable to a computer or to a television set DVD player in the same way that the variety of shapes and spacing of these letters and words enables you to read this explanation.

Then, a photograph is taken of this master DVD and an etching is made from that photograph. In turn, the etching is used to create a metal stamper. Let's say I want to make 1,000 DVD versions of my virtual reality artwork or 2 million DVD copies of my latest Hollywood flick. The replication house simply uses the metal stamper to imprint the pattern into the plastic coating of all those DVDs.

What most people find at the local movie rental or video store is called a DVD-5. It's so named because it stores nearly five gigabytes (GB) of information. There are also DVD-9s and DVD-18s. A DVD-9 has double the capacity of a DVD-5 because it's basically two discs, placed one right on top of the other. The laser that "reads" the information first interprets the data on the top disc and then changes angle to translate the data on the second one. The DVD-18 works in the same way, except that both the discs are double-sided. Instead of just reading one side for information, the laser reads both sides, something like our old vinyl records.

Two final notes about why Hollywood is turning to DVDs as the medium of choice to release its products: First DVDs are more durable than the standard VHS tape. If you're in the movie rental business that means more long-term profits as you won't have to pay for replacement versions after multiple plays. (It has the added advantage that the local franchise won't have to worry about whether you rewind it or not.) In addition, because of special features within the television set DVD player units, it will be impossible for these sets to "read" and play pirated copies of movies on DVD, a problem that has plagued Hollywood's video market abroad.

More information on DVDs comes from Sylvia Moats, Xiao Tang and Victor McCrary from the National Institute of Standards and Technology (NIST):

DVD (digital video disc) is a technology based on optical data storage similar to compact disc (CD). Optical data storage is a method of storing digital information (1's and 0's) by using light to read the information. Analog information is converted into digital information, which is then encoded onto the disc from the inside edge out. Digital data are encoded by means of pits on the recording layer of the disc. The encoding is done using a technique referred to as EFM, eight-to-fourteen modulation, in CDs and EFMPlus, eight-to-sixteen modulation, in DVDs.

The pits and the separations between pits, called lands, vary in length to represent the digital information stored in the disc. The pits are arranged in a track that forms a spiral pattern on the recording layer of the disc. The disc revolves in a circular motion inside the player, while an optical head laser slowly moves outward and remains focused on the pits. The laser beam is reflected back to a detector when it hits the lands, and it is scattered away from the detector by the pits. The transition between a pit and a land corresponds to a "1" in the digital bit stream.

A DVD can store over two hours of video on one layer of the disc. In comparison to the 74 minutes of data that a CD can hold, this capacity marks a vast improvement. (Seventy-four minutes is just enough to hold Beethoven's Ninth Symphony on an entire CD.) The smallest pits on a DVD are about 0.4 microns (0.0001575 inches) wide, whereas CD pits have a minimum width of 0.83 microns. The pits are arranged in tracks (called track pitch) that are spaced much closer on a DVD (0.74 microns) than on a CD (1.6 microns). With this construction, a single layer, single-sided DVD can hold 4.7 GB versus the 650 MB that can be held on a single CD.

The smaller pit size on a DVD means the laser beam used in a DVD player must have a shorter wavelength than that in a CD player. But this requirement allows for dual compatibility: DVD players are designed to play DVD discs as well as CDs. Unfortunately, this compatibility is backwards and so CD players will not play DVDs.

With such small pit areas, the number of possible errors in reading the disc is limitless. As a result, error correction storage has seen rapid development. In CD technology, there is error correction coding (ECC) on the disc as a means of correcting problems when they occur. The ECC algorithms are configured to detect and perfectly correct errors in the data. This same method is used for error correction in DVD technology.

The greater amount of information stored on a DVD video is chiefly due to the smaller pit size and tighter track spacing, but another factor comes into play--namely, compression. The Moving Picture Experts Group has come out with an advanced video compression algorithm referred to as MPEG2. This development is based on temporal compression that compares each frame of video to the following frame and only stores the changes in the scene. It stores a single frame in a delay so as to continually put out the background of a scene and have more room to store the changes in the scene. MPEG2 also uses the discrete cosine transform, which takes images and finds their frequency and then compresses any repetitive frequencies.

Video compression allows more room to store additional viewing options, a big part of what makes DVD such an incredible advancement in technology. Allowing for the full theater experience, DVD-video comes in Dolby Digital (AC-3) Surround Sound [Dolby Digital (AC-3) is a trademark of Dolby Laboratories Licensing Corporation]. Other options include the choice of wide (16:9) or full-screen (4:3) viewing, sound tracks in different languages and closed-captioned viewing. DVD video can also let the viewer decide the rating, R or PG, thanks to preprogrammed scene cuts on the disc itself. Viewers can also opt to watch an uncut version of the film or see out takes from the original. All of these selections can be chosen at the start of the film. The DVD then makes seamless skips from one scene to the next in the arrangement the viewer has chosen. The director and the producer decide upon the amount and type of options available, so each individual DVD may have some, all or none of these options.

Another big improvement of DVD video storage is its 525-line resolution, which is far greater than the resolution offered by laser disc media and almost twice the resolution of standard VHS videotape. This resolution is dependent on the capabilities of the television monitor used, but the monitor need not have high resolution for the viewer to enjoy the benefits of DVD. One more advantage is that a DVD disc is not physically touched while it spins in the player, so there is no wear and tear or loss of fidelity over time. In contrast, videotapes do touch a playback mechanism and eventually break down, degrading the quality of picture.

That said, DVDs and CDs are not impervious. Because data is stored in a spiral fashion, users should avoid wiping the disc in a circular motion; when cleaning a disc, wipe from the inside radially out to the edge. Compared to video tapes, the longevity and easier storage of DVDs makes them a much more reasonable and user-friendly media.