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Ever since the introduction of the CD-DA format, audio purists have insisted that it was not good enough. They could hear a definite digital sound (whatever that might be) that interfered with their enjoyment of music. They were quick to point out that tradeoffs made in the design of the CD-DA format slighted sound quality. The relatively low sampling rate required to pack enough information on a disc required high-order low-pass filtering to keep digital artifacts out of the audio, and even then the upper cutoff frequency must be too low to accommodate everything that old analog tape recorders could capture.
The huge capacity of the DVD system eliminates the need for the tradeoffs of the CD-DA system and unleashes the potential for superb audio quality—good enough to satisfy listeners with 24-karat ears. The audio-only implementation of DVD—termed DVD-Audio—isn't just for purists, however. The system can use the multigigabyte storage of the medium for super-quality audio, additional channels, or both.
Whereas CDs are built around a 44.1KHz sampling rate, DVD supports both this rate (and the whole Red Book standard) as well as 48KHz, the same as professional audio systems, and a super-high quality 96KHz sampling rate. In addition to today's 16-bit digital audio, DVD will also support 24-bit audio as well as several compressed multichannel formats to accompany video. The multichannel audio standards vary with the video standard used, with Dolby AC-3 (eight-channel audio) for NTSC video.
The options allowed the producer for linear PCM are wide. The systems support up to six channels with bit-depths of 16, 20, and 24. Recording may use either of two bit-rate families—the 44.1KHz of Red Book audio, supplemented by 88.2 and 176.4KHz multiples, or a 48KHz base rate supplemented by 96 and 192KHz multiples. The highest sampling rate and bit-depth (192KHz at 24 bits) allows response to nearly 96KHz, with a dynamic range approaching 144 dB. The maximum data rate of the system, 9.6Mbps, constrains the system to two channels of these highest-quality signals, and the storage capacity of the DVD system limits playing time (at this quality level) to about 67 minutes.
To extend playing time, the DVD-Audio standard allows for data compression of various sorts, including MPEG. Most intriguing is MLP, which stands for Meridian Lossless Packing. As the name implies, this system works like file compression and reduces data redundancies in the digital audio but allows the original signal to be perfectly reconstructed during decoding. Compression extends playing time enough to allow up to about 135 minutes of six-channel 24-bit audio sampled at the 96KHz rate.
To make six-channel audio compatible with two-channel stereo sound systems, DVD-Audio incorporates a special System-Managed Audio Resource Technique, or SMART (entirely different from the S.M.A.R.T. hard disk failure-prediction technology). This system allows the producer of an audio DVD to determine the optimum mix-down that combines the channels together for each selection on each disc. The mixdown information is stored as a table of coefficients—the level assigned to each channel in the mix—and each selection on the disc can have a particular table assigned to it. Each disc can accommodate up to 16 tables.
DVD-Audio discs also accommodate other kinds of data in addition to digital audio. The standard allows for including up to 16 still images in each track and for synchronizing a display of lyrics with the music. In addition, the DVD-Audio system provides for computer-style navigation through a screen-oriented menu system (or a simplified control system for audio players without computer displays).
Write-Once DVD Formats
The A in the name stands for authoring. DVD-R(A) was the first rewritable DVD format, marketed by Pioneer in 1997. The initial drive stored only 3.95GB per disc and was priced at $17,000—hardly a mass-market item. The format was revised in 1999 to accommodate the full 4.7GB of other DVD systems. Both the 3.95GB and 4.7GB formats remain in use.
The DVD-R system is aimed at professional applications. It has been widely deployed to store images of documents used in commerce and for mastering DVDs, where it replaces the Digital Linear Tape systems once widely used for the purpose. Using the new Cutting Master Format on 4.7GB discs allows the use of part of the lead-in track to hold the Disc Description Protocol header information that was used on DLT tapes.
The DVD-R(A) system is centered around a 635-nanometer laser that's used for writing discs. The dye media are tuned to this wavelength, so only discs designed for DVD-R(A) can be recorded in DVD-R(A) writers. The resulting discs can, however, be read by most DVD players, including both DVD-R(A) and DVD-R(G) drives.
The DVD-R(A) standard is sanctioned by the DVD Forum and is an official standard of ECMA International (which changed its name from the European Computer Manufacturers Association in 1994) titled ECMA-279.
The G in the name stands for general purposes. DVD-R(G) is both an offshoot and refinement of the older DVD-R(A) technology. The chief change is the substitution of a less-expensive 650-nanometer laser for the shorter wavelength used in the DVD-R(A) system. This laser allows for less expensive drives and media but inhibits compatibility. DVD-R(G) discs can be written only in DVD-R(G) drives, although they can be read in almost any DVD player, including those designed for DVD-R(A) discs. The change in laser does not affect capacity. DVD-R(G) discs hold up to 4.7GB.
Because DVD-R(G) is a consumer-level medium, it incorporates content protection to the extent that a DVD-R(G) drive cannot make a copy of an ordinary DVD—for example, a motion picture—that uses CSS encryption. Moreover, the DVD-R(G) system does not support the CRF system used in mastering DVDs. The DVD-R(G) standard is sanctioned by the DVD Forum and is part of the DVD-Multi specification.
DVD+R is a relatively recent addition to the write-once repertory. An offshoot of DVD+RW technology (hence the use of the plus sign in the name), DVD+R discs can be created in any DVD+RW drive manufactured since the introduction of the format in April 2002.
As with CDs, the write-once DVD+R media are less expensive than the rewritable media, so DVD+R is a more affordable way of making permanent records. In addition, the dye-based medium is more reflective than the rewritable medium, making it more easily read in commercial DVD players. Whereas DVD+RW playback may be marginal in some early DVD players, DVD+R discs will likely work better.
The foremost consideration in developing the DVD+R format was compatibility with both DAT drives and video drives. As a consequence, the media hold the same 4.7GB as commercial DVDs, and there are no plans for alternate formats. The DVD+R specifications do allow for two-sided media, but the technology precludes the use of multilayer discs.
Rewritable DVD Formats
The same phase-change technology that makes CD-RW possible has been applied to DVD to make a number of rewritable formats. The first of these came from the DVD Forum (and all were delivered months later than promised).
The first rewritable format, DVD-RAM, was aimed particularly at data applications. It uses a combination of phase-change and magneto-optical technologies to produce a long-life medium with the ruggedness further enhanced by putting each disc in its own cartridge/carrier, much like a floppy disk shell. As a result, the DVD-RAM system produces discs projected not only to be reliable data archives for more than 30 years, but also the medium itself should endure more than 100,000 write/rewrite cycles.
When DVD-RAM drives first appeared in June, 1998 each disc packed only 2.6GB per side. Two-sided discs doubled that capacity but added the need to flip the cartridge over to access the second side. By October 1999, the specification had been extended to a capacity of 4.7GB per disc side with the first drives arriving in June 2000. The revised specification also included cartridges using discs 80 millimeters in diameter, aimed particularly at high-end digital video camcorders.
The DVD-RAM system uses a wobbled groove to generate the clock used for synchronizing data writing with the spin of the disc. The wobble, a periodic side-to-side twist of the spiral track, is pressed into the blank disc along with fixed sector headers during manufacture. Initially all DVD-RAM discs were encased in cartridges, but the latest design makes the disc removable from the cartridge so you can use it in other formats of DVD drive. Drives can write only on discs encased in their cartridges. Most sources list the DVD-RAM system as incompatible with most other drive formats. DVD-RAM drives write only DVD-RAM discs, and reading the discs in other kinds of drives is chancy. DVD-RAM drives, on the other hand, readily read all other DVD formats.
DVD-RAM drives have built-in defect management, making them the best choice for computer data. In professional camcorders, DVD-RAM has proved impressive, and the defect-management system built in to the drives makes them a good choice for random-access use, their original design intent. The chief handicap of the DVD-RAM system is its inability to make discs that are playable on other kinds of DVD drives.
Developed originally as a companion to and offshoot of the DVD-R(A) format, DVD-RW is the DVD Forum's choice for a sequential read/write DVD medium. It uses about the same track pitch, data length, and speed control (constant linear velocity) as both DVD-R formats and differs chiefly in medium. DVD-RW uses a phase-change medium, which has somewhat less reflectivity than do the write-once dyes, 18 to 30 percent for DVD-RW, compared to 45 to 80 percent for single-layer DVD-ROM. As a consequence, DVD-RW's initial slow start was further impeded by incompatibilities. Ordinary DVD drives had difficulty reading DVD-RW discs, often mistaking the lower reflectivity as two-layer media in that the disc's 18-to-30-percent reflectivity is the same as that of two-layer DVD-ROMs.
Blank DVD-RW discs actually have two kinds of data prewritten to them. To synchronize the spinning disc with data being written, the disc has address information written on the land area between the grooves of the spiral. In addition, each disc has a prewritten lead-in track that's used by copy-protection systems to prevent the pirating of movies. This lead-in data takes three forms:
The first DVD-RW systems were designed specifically for recording video. They operated as sequential media and supported only disc-at-once writing. Although editing was possible, it was cumbersome because the entire disc required rewriting for a single simple change. Initial drives operated only in real time—that is, 1x speed—although faster drives are now available. Discs must be formatted before use, although preformatted discs and a quick-format process help minimize the pain.
From the start, DVD+RW was designed to be a random-access data medium. At the same time, the system was designed to be capable of producing discs compatible with most DVD players. The DVD+RW format is compatible with standard DVDs, and the same 650-nanometer laser reads and writes the medium. The only significant difference is the medium itself, a phase-change compound with somewhat lower reflectivity than standard DVD discs, comparable to that of DVD-RW. As a result, some older DVD players may mistake DVD+RW media for two-layer discs.
Although DVD+RW discs are compatible during reading, the system incorporates a number of enhancements to make DVD+RW useful in data storage. The standard allows drives to use constant linear velocity recording (as with DVD-RW) as well as constant angular velocity recording (constant spin rate) for faster random access. Discs made with either spin-control method are playable on standard DVD players (which use CLV).
For synchronizing disc spin and writing speed, DVD+RW uses wobble tracks—a periodic radial shift to the spiral groove. The wobble produces a frequency of 817KHz during 1x recording, correspondingly higher at faster write speeds. The wobble clock provides four addresses for each 32KB block of data on the disc, allowing the drive to accurately locate any 2KB data cluster. This addressability allows for lossless linking, a term used by the DVD+RW promoters to describe the ability of a DVD+RW drive to resume writing within one micron of the place it left off whenever it is interrupted for whatever reason.
This ability to locate accurately any 2KB block of data makes the DVD+RW system truly random access. It can change any data cluster independently of the others without the need to rewrite the entire disc. This ability fits with the Mount Rainier drag-and-drop writing process, so making Mount Rainier DVD+RW drives requires little more than new firmware and certification.
Initially DVD+RW allowed for only one kind of disc, a 12-centimeter disc holding 4.7GB. The same disc can be used both in computer drives for storing data and in dedicated DVD recorders for motion pictures. In August, 2002, the DVD+RW alliance approved 8-centimeter discs holding 1.46GB for applications such as digital camcorders.
DVD+RW discs require formatting before use, but the DVD+RW system supports automatic background formatting. As soon as you insert a new disc into a drive, it starts formatting the lead-in. This way, the drive is immediately ready to use the disc. If you interrupt the formatting process (for example, by removing a partially formatted disc), the drive will resume formatting the disc the next time you insert it. DVD+RW drives support automatic defect management, hiding bad storage areas from your computer and operating system.
The DVD+RW system allows for standalone DVD video recorders. In such applications, it supports four writing modes or quality levels, differing chiefly by bit-rate and playing time. Table 18.6 summarizes these speeds.
Initial DVD+RW drives for computers operated at up to 2.4x normal DVD speed and allowed writing at their highest speeds. In August 2002, the DVD+RW Alliance announced the approval of 4x drives.
In February 2002, a consortium of nine electronics firms (none American—the list includes Hitachi, LG Electronics, Matsushita, Philips Electronics, Pioneer, Samsung, Sharp, Sony, and Thomson Multimedia) announced it had agreed on the specifications for the successor to the DVD. Called the Blu-ray Disc, the innovation resembles nothing more than a DVD—it's the same size and color and uses the same basic technology. The disc spins in a drive and a laser reads tiny digital spots to play back movies or music. The difference is capacity. The initial Blu-ray discs will be able to store up to 27GB of data, about 5.75 times more than today's DVDs. Moreover, plans include more than doubling that capacity with two-layer discs (a technology already used by DVDs). The specification also allows for discs holding 25 or 23.3GB with relaxed pit-length requirements.
Although initial products will likely only play commercially recorded material, Blu-ray technology allows for erasing and rewriting discs, much as computers already do with CDs. Existing DVD drives will not be able to play Blu-ray media, although Blu-ray drives should be backward compatible with current formats.
The Blu-ray name explains the secret to the new technology. The laser that reads the disc uses shorter wavelengths (at 405 nanometers, it's closer to violet than blue) that can pick out digital dots that are closer together than can the yellowish laser used by DVDs or the red laser used by CDs. The tracking pitch—the distance between adjacent grooves in the single-track spiral—is only 0.32 micrometers, and the shortest data pits are only 0.138 micrometers long.
All Blu-ray discs use a phase-change medium coated with a 0.1 millimeter optical transmittance protection layer. To further "ruggedize" the discs, they are housed in a tough plastic cartridge that measures 5.1 by 5.2 inches and about one-quarter-inch thick. (Exact dimensions: 129 by 131 by 7 millimeters.)
Blu-ray is specifically aimed at putting high-definition television onto DVD-style discs. The system encodes video into MPEG-2 data streams and writes data at 36Mbps. A single disc can hold over two hours of HDTV or 13 hours of VHS-quality video at a 3.8Mbps rate.
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