Digital Storage Dynamos Ease Data Deluge
Laserdisc technology offers greater capacity and enhanced backward compatibility.
The increasingly heavy flow of data within organizational networks is driving the search for better methods to store actively used information and archives. Advances in optical-disc technology are producing greater versatility in multimedia hardware and software. As a result, consumers will soon achieve increased systems interoperability through a more refined focus on equipment compatibility.
The convergence of laser-read disc technologies to create a high-density, recordable and rewritable medium is the product of a merger of industry and government information technology requirements and consumer needs. Employing a combination of real-time video and random data recording capabilities in a single format, the redeveloped digital versatile disc (DVD) will be compatible with the present-generation of commercially available optical-disc players.
Industry viewpoints vary on the direction that DVD technology should take. Some support the production of systems that are backward compatible across the DVD player and recorder spectrum, while others support a greater focus on enhancing computer-based capabilities to interface with technology on the leading edge in multimedia. Regardless of stance, all of the companies involved are attempting to consolidate proven DVD product capabilities within fewer designs to satisfy consumer demand for an all-purpose format.
One of the latest DVD concepts to emerge is DVD plus rewritable (+RW). Proposed by Hewlett Packard, Philips, Sony and Ricoh, +RW is designed for data and video content across personal computer-based business, government and entertainment applications. Dean Sanderson, lead product manager for compact disc (CD) recordables, Hewlett Packard, Loveland, Colorado, remarks, “The key to +RW is in the way the information is laid down. Using a finer data stream than its CD counterparts, the +RW disc is recognizable by any other DVD-formatted [apparatus], allowing the transport of important information from one type of equipment to another.” Employing the same phase-change recording procedure as standard CD-rewritable applications, +RW puts data down in a continuous spiral groove, preventing any disturbances in the laser readout process.
“The boundaries presented by differences in data density and system incompatibility have finally been crossed,” Sanderson suggests. “As the video, audio and information storage products of the commercial and entertainment industries drive consumer demand, there will need to be a product that can bridge the gap between present and past technology. When mature, this could be that product.”
+RW technology is built around the concept of maintaining a constant disc rotation speed and a uniform linear data density. Information is recorded in either a sequential or random location on the disc face, enabling potentially faster access times than is possible with most existing systems. Operating under the same modulation coding as DVD-ROM, +RW incorporates radial tracking as a means to position a recording laser spot precisely on a new disc or to achieve exact synchronization and alignment between old and new blocks of data on a used disc.
In other DVD formats, the insertion of linking layers between old and new data results in a time gap between data blocks. This causes a loss of seamless transition from previously recorded to newly added material. With +RW, the constant angular velocity (CAV) of the disc minimizes the delays caused by the linking layers, Sanderson explains. Normally, maintaining a constant rotation would cause data on the outer radius of the disc to come off the disc faster than data toward the center, but when the whole operation is moved to a computer hard drive, a memory buffer is available to balance out the data flow. The resulting equivalent data densities ensure that the data placed on the disc can be read at a uniform and overall faster rate.
Early DVD prototypes offered consumers limited recording capabilities. DVD-R was developed as an online archiving medium with greater data capacity than a CD-R version. DVD-read-only memory (ROM) is a replicated disc that found a niche in the video industry as a cost-effective medium for mass distribution. Re-recordable versions, such as the proposed DVD rewritable (RW) and DVD-random access memory (RAM), could be options for the business community because of their high-capacity storage potential and quick information accessibility. “There is a lack of compatibility across the DVD spectrum,” Sanderson admits. “The +RW format, when produced, will soon provide users with truly interoperable functionality.”
DVD technology remains a two-sided coin. On the business side is the need for quicker, more reliable network storage capacities. In the entertainment sector, CD-ROM software has provided consumers with visual applications and increased choices for combining home and office equipment. Lou Skriba, owner of Gig Media Consultants, Libertyville, Illinois, a company that identifies and demonstrates new applications of optical-recording media, is involved with phase-change optical recording. “Although not as broadly compatible as the proposed +RW format, the double-sided, 9.4 gigabyte version of DVD-RAM offers consumers increased disc space and twice the reading and recording speed as regular one-sided 4.7 gigabyte formats.”
Panasonic, Hitachi and Toshiba, collaborators in the development of DVD-RAM, have created a technology that offers precise data location by using pre-embossed, formatted information and zoned constant linear velocity (CLV). With zoned CLV, the disc is constantly changing speed to compensate for the increased radius of the outer tracks, but the disc is divided into about 40 concentric rings. Within each ring the speed changes are very gradual. Employing the same phase-change process as the proposed +RW version to write data to a disc, RAM packs data densely to achieve higher random access speed than is possible with other DVD-disc formats. The relatively uniform velocity within an individual zone enables pinpoint accuracy in finding nearby files, Skriba adds.
The recording methods of DVD-RAM and +RW formats are similar. The surface of a blank disc is polycrystalline metal in composition and lacks the organic dye used in DVD-R and ROM formats. During writing, a focused laser beam selectively heats areas of the disc surface to a melting point of between 500 and 700 degrees Celsius. When cooled quickly, the polycrystalline material is transformed from a random liquid state to an amorphous or “frozen in” state. The data etched into the disc during this process can be removed instantly by reheating the recording layer to a temperature between 200 and 500 degrees Celsius. Any etched data in the heated area will be erased when the material reverts to its original crystalline state.
In both the DVD-RAM and +RW approach, recording processes are performed simultaneously once the new data have been programmed into the DVD recorder, Skriba explains. And both DVD-RAM and +RW formats use groove recording techniques. RAM incorporates a very high-density groove pattern to pack data into a small area. The +RW version uses a less compact spiral-groove pattern to complete a write/rewrite cycle. Capable of 4.7 gigabytes of memory per side, DVD-RAM is the only format that will offer a two-sided disc. “For this reason, DVD-RAM has the advantage in capacity over other systems that use one side only,” he continues.
The reflectivity of particular areas of disc surface is important to disc readability. In the crystalline state, the +RW disc strongly reflects laser light back to a data processing center that uses the light signals to locate the spiral groove. When subjected to laser pulses at a higher power level, the recording surface melts to create the amorphous data spots. Amorphous areas have a low level of reflectivity, allowing laser light to read the data that are represented in these portions of the disc. Variations in temperature or disc component materials can affect a proper read. According to Sanderson, DVD-RAM discs have a greater frequency of crystalline link layers between data blocks than the +RW format is proposed to have. The laser beam experiences more light reflection, making RAM discs difficult to read by DVD video players and recorders, he explains.
Traditional DVD phase-change technology institutes a link layer between newly and previously recorded data. This layer represents a suspension in the writing process because of the constant bit rate required for precise data streaming. DVD video players and ROM drives are unable to read re-recorded discs because of these link layers. DVD-RAM formats have large link layers, making RAM incompatible with previously marketed read-only systems. To avoid this problem with the +RW discs, developers have used a process called lossless linking. In writing a data block with precise positioning, they propose the use of a high frequency wobbled waveform to ensure that data are started and stopped at an exact location. “The greater the position accuracy, the less the need for discontinuations in the data stream,” Skriba indicates. “For this reason, a re-recorded +RW disc can be read as if it were a one-time recorded, read-only format.”
Defect management is a shared attribute of DVD-RAM and +RW formats. When a disc has been damaged, all of the data might be lost because older players cannot open the disc. With RAM and +RW formats, selected data that are unharmed can still be read off a damaged disc. If a disc has a manufacturer’s defect, a RAM or +RW drive can rewrite the unaffected data to another disc and, in some cases, can record data over the affected area depending on the extent of the defect.
The DVD-RAM technical standard includes support for the serialization of each disc. This permits unique identity coding for locking and unlocking secure and encrypted data. Each serial number is written to the disc using a high-powered laser in a restricted area of the manufacturing plant. The result is that each disc acts as its own hardware security key to prevent unauthorized use. “The earliest uses of phase-change optical recording by the U.S. Defense Department were for audio-visual and training exercises using high security military applications,” Skriba explains. In future military operations, DVD-RAM, with its 9.4 gigabyte storage capacity and encryption, could be used to avoid signal interception by enabling large-bandwidth communication via highly transportable physical media, he suggests.
According to Skriba, the future of DVD-RAM and +RW technology is in fields demanding high data storage capacity and high data access rates. Government interest in DVD is in the areas of historical and financial record archiving. The potential for large cost savings by switching from current magneto-optic filing to DVD-based document storage is driving federally funded research within the laserdisc industry. Uses for the visual side of DVD will likely be in desktop software development, mechanical engineering and advertising, Skriba adds.
Sanderson predicts that trends in multimedia will continue to show the inevitable merge of television and video with computers. Equipment that today is separate will be incorporated into one system. According to Skriba, consumers and businesses will have to decide which is the better investment: focusing on products that are backward compatible or taking a forward-thinking approach and focusing on products that are independent in the capabilities they offer. DVD manufacturers will continue to develop optical-disc technology, combining the capabilities of videocassette recorders and audio CDs into a single, cross-compatible format. Research and testing on a potential DVD +RW line is ongoing with possible product release set for early second quarter 2001 in Europe.
As DVD continues to bring the worlds of television, video and computing
closer together, the media has branched out into several types to accommodate the growing demand for increased capability and compatibility.
Below are the six DVD media formats and their features: