Video Disk
The long-promised digital video disk (DVD) is here bringing Studio-quality video images and theaterlike sound into the home on a disc that looks and is handled just like the familiar audio on compact disc.
Just as the audio CD eliminated the hisses, pops, and hums that degraded the music on analog records and cassette tapes, the digital videodisc eliminates the dropouts, flickers, and distortions that are common on analog VHS tape recordings. So, for a while, it looked like there would be two competing and incompatible DVD formats-one developed by Sony and Philips, the creators of the compact disc; and one developed by Toshiba, Time Warner, and five other electronics and movie companies Luckily for consumers, the two groups have agreed to harmonize the ii standards, thus avoiding a replay of the VCR format wars that left some eager buyers with obsolete equipment when Betamax lost out to VHS. In December 1995, the two groups finally agreed on all of the technical details for the new format, which the are calling Digital Versatile Disc."
DVD puts VHS to shame. Colors are deeper and brighter; edges, sharper; details, crisper. The DVD picture is also noticeably better than laser disc. And for most material, you can't tell the difference between the DVD picture and the digital broadcast studio tape. The benefits go beyond picture quality. DVD will deliver all the sound effects you've come to expect in a theater. The new discs promise to be easier to handle and store than tape cartridges. And a two-layer recording technology planned for extra-long- play DVDs could put epics like Gone With the Wind on a single disc. Double-sided discs could add even more playing time.
You'll get the chance to make your own comparisons soon. Videodisc players, starting between $500 and $600, and hundreds of movie titles priced to be competitive with VHS movie tapes, are expected to be available fall '96. The new DVD systems use the same technology found in existing audio CD and CD-ROM players. An optical pickup "reads" the reflection of a spot of light shining on a rotating disc. The character of the reflection changes, depending on whether the light falls on a pit formed in a reflective layer or on a flat surface. This on-off reflection is captured by a photo detector, which produces a string of on-off electrical signals that correspond to the zeros and ones of digital code. This digital data is then converted into analog audio or video signals. But DVD goes a step beyond CDs, taking advantage of recent technological advances to squeeze up to 488 minutes of full motion video data onto the same 120- millimeter diameter discs that strained to hold 70 minutes of audio data lust 13 years ago.
The most important hardware development in DVD players is a new generation of lasers. Current CD players use infrared laser light with a wave- length of 780 nanometers, or about one-hundredth the width of a human hair. The lasers in the new videodisc players will have wavelengths of 635 nanometers. The narrower wavelength that are roughly half the size of the pits on current audio CDs. The smaller pits can be placed closer together along the track, and the distance between the tracks can be cut at least in half.
Another factor also affects the number of pits that can be placed on a disc. Discs wobble slightly as they rotate, which means the disc is sometimes slightly tilted in relation to the laser beam. When the laser beam hits the disc's lower surface at even a slight angle, it is refracted, or bent. This is analogous to a spoon that's half in and half out of a glass of water and appears to be bent. The same phenomenon causes a small displacement of the spot where the laser beam hits the disc's reflective layer; the greater the thickness of the disc substrate, the greater the displacement.
To reduce displacement, the Toshiba group cut the substrate thickness in half-from the 1.2 millimeters used on current audio CDs to 0.6 millimeters. This scheme has been adopted by Sony too, and will allow the track pitch to be narrowed to 0.74 microns. The improvement over the 1.6- micron track pitch of conventional audio CDs adds up. The length of a CD's spiral track is about 3.4 miles, and the total data capacity is about 780 megabytes. Tracks on the new DVD discs will be about 7.4 miles long and will hold more than 4.7 gigabytes of data per side. That's enough room to store 133 minutes of full-motion video per side.
The overall thickness of DVD discs will be made the same as current CDs by bonding two discs back to back. One side could be blank. Or manufacturers could produce a double-sided disc holding a total 9.4 gigabytes of data, enough for two feature-length movies, though the disc would have to be flipped over to play the second side.
The tradeoff in choosing a new substrate thickness is in achieving compatibility with current CDs. Since the DVD format puts the reflective layer on a different plane, combination CD-DVD players will need two pickups, or a single pickup with two interchange able lenses, or a special hologram lens that can simultaneously focus light beams at two different distances.
To add even more space for data, the DVD format includes specifications for dual-layer discs, in which a second layer of data-defining pits is imprinted on a semireflective layer that is closer to the surface than the fully reflective layer. (As in today's CD players, the laser strikes the underside of the disc. The pickup will move slightly up or down to focus on one layer or the other, The DVD format allows manufacturers to produce single-sided dual- layer discs holding 8.5 gigabytes of data, and double-sided dual-layer discs holding a whopping 17 gigabytes. The latter option could pack up to four movies on one disc, something movie studios are considering for collectors' editions of a popular actor's or director's works.
In addition to squeezing more pits onto the disc, engineers have succeeded in squeezing more data into fewer pits. One way is through improved error correction methods that eliminate a lot of the redundancy needed previously But major advances have also occurred in data compression. Both DVD formats rely on a digital data compression standard worked out by an international committee known as the N-lotion Picture Experts Group. This standard eliminates redundant data Without compression, there is enough digital data to define the color and brightness of every pixel of the TV screen each time the image is scanned which is 30 times per second. Although e background of a scene may remain unchanged for a minute or more, the same information is repeatedly sent to the screen. With compression, however, pixel information is defined only when it changes from a previous scan.
Previous video compression methods have been based on a constant bit rate that allocates a fixed amount of data to each frame. In effect, this meant there was more data than necessary for some scenes, and less data than would be optimal for scenes with lots of motion and complicated back- grounds. The new compression method uses variable bit rates. When there is lots of movement, there will be lots of digital video data to define it; for quite scenes, the amount of data will be reduced without impairing picture quality.
The effects of variable bit rates were evident at both demonstrations, where small monitors atop the TV sets displayed the number of bits per second being used to define the scenes flashing on the screens bellow. Through most of the scenes from Sleepless in Seattle shown at the Sony demonstration, just 3.2 megabytes per second were enough to fill the screen. But during a Legends of the Fall battle scene full of explosions and quick cuts, the bit-rate meter soared to more than B megabits per second.
High-quality sound is another of the promises of DVD. A Toshiba demonstration clip of the train wreck scene from The Fugitive put me right in the midst of squealing brakes and crunching metal, thanks to a 5.1- channel digital surround sound specification. This specification allows six separate audio tracks-five full- frequency channels for right, center, left, right rear, and left rear; and a subwoofer channel.
In fact, DVDs will have a lot of room for extra data, allowing for multiple language and subtitle tracks. Users will be able to choose whether to listen to the original movie dialogue with or without subtitles, or to a dubbed version. This feature appeals to disc makers, who will be able to put a number of languages on one disc saving manufacturing and handling expenses. But just how to allocate the available space will be left up to the disc manufacturers, and some not-yet-anticipated uses could emerge. Among the possibilities: new ultra high-fidelity audio applications and new types of interactive games.
The increased data capacity of DVD could allow the viewer to select from several camera angles or to choose how the plot develops in the CD-ROM Factor special made- for-DVD video material. Viewers may also be able to choose whether to watch films in their original wide-screen format on a wide-screen TV, or in the letterbox format on a conventional TV, or in a version that has been cropped to fill a conventional TV screen.
First-generation DVD players will have the freeze frame, slow motion, and Fast-forward Features consumers are accustomed to on their VCRs. But the players will also allow random access so viewers can instantly jump to selected scenes. And DVD players will play current audio CDs.
But one Feature will be missing from the earliest DVD models: the ability to record. The developers are readying a standard for recordable DVDs, but the technology will appear in computer components first DVD technology will be at the heart of a new generation of high- density CD-ROM drives, with write once and rewritable capabilities.
Before recordable DVD players appear, an agreement with Hollywood will be required on technology to block copying or to ensure that royalties from blank-disc sales are funneled back to content providers. Just when recordable players for use with a TV will become available is still unclear.
Engineers are also planning for the eventual use of a blue laser in DVD players. The 430-nanometer wave length of a true blue laser would Allow even more pits to be crammed onto discs. With comparable advances in data compression, future generations of DVD could be programmed for HUFV-quality images that require five times as much data as standard TV images. Reliable blue lasers have yet to emerge from the laboratory. But even without high definition, DVE should give videophiles something to be excited about for years.
