From DDS-1 to DAT 72: a brief history of DDS media.
The emergence of DAT 72 is good news for a technology segment that is often overlooked with much of the industry attention focused on the high-end battle between SuperDLT and LTO. However, DDS technology is the most popular tape drive technology ever--with an installed base of nearly 7 million drives--and still dominates the low end of the tape drive market with a 58% market share in 2002, according to Freeman Reports.
At the heart of DDS technology is the compact 4mm data cartridge, such as that supplied by Maxell Corporation, who for the last 30 years has been dedicated to being first to market with next generation product. The media improvements over five product generations form the basis of the continued capacity improvements for DDS. The evolution of DDS media is closely linked to the continuous improvements in metal particle coating technology. In fact, metal particle media dominates digital data storage media today thanks to the inroads made with the first generations of 4mm DDS and 8mm helical scan data recorders when most computer tape products were based on standard ferric oxide media.
DDS media introduced metal particle as a viable technology for high-density digital data storage and, today, MP media dominates the digital data storage market. DLT, Super DLT, LTO, IBM Magstar and other popular media formats, in addition to 4mm DDS, rely on MP formulations for reliable, high-density data recording.
The move to MP media from the gamma ferric oxide and chrome tape formulations that were widely used at the time was necessitated by the higher recording densities demanded by the 4mm format to compensate for the small form factor of the cartridge. The compact 4mm DDS cassette was a true breakthrough that allowed multi-gigabyte capacities in a 3.5-inch form factor. The downside of this small size was the limited amount of tape inside the cartridge. The DDS-1 format utilized 90-meter length, meaning that there were less than 50 square feet of available tape on which to record data. The quarter-inch cartridge tapes at the time offered only one-fourth of the capacity as a DDS-1 tape, but packed more than five times the physical recording area inside the cartridge.
The physical properties of MP media were quite attractive to meet the demands of DDS helical scan recording. Metal particle media offered twice the coercivity and twice the remnant flux density of alternative media pigments, producing the potential for significantly higher recording densities compared to quarter-inch or half-inch cartridge formats.
Although MP media provided much higher magnetic output, there was also a major issue with technology. MP formulations use a pure metal particle, making the media particles more environmentally sensitive and susceptible to oxidation. When MP media oxidizes, it destroys not just the recording layer of the tape, but any data that was stored on it as well.
To prevent oxidation problems and still take advantage of the high recording density potential of MP media, metal particles used in DDS media are typically coated with a passivation layer material. Maxell's Ceramic Armor coating is an example of a passivation layer that protects the metal particles against oxidation and corrosion while maintaining high signal output. This coating technology provides a thin Ceramic Armor protective layer around a pure iron core that effectively localizes the particles' energy and maintains their stability over the long term. In addition to oxidation prevention, coating metal particles with a protective layer offers two other major advantages: excellent heat resistance and increased durability. Extreme heat conditions can build up inside a DDS drive, leading to deterioration of the magnetic properties of metal particles. The thin Ceramic Armor securely protects metal particles from the effects of extreme heat, preserving the particles' magnetic properties and signal output. Similarly, the protective coating layer preserves metal particles from damage that high-speed head scanning can cause and makes a significant contribution toward improving durability.
In addition to conquering oxidation problems, another issue faced by the DDS industry was how to prevent lower-grade audio DAT media from being used inadvertently to store critical data files. Audio-grade DAT media cassettes are physically compatible with DDS-qualified media, yet cost much less. To preclude customers from being tempted to save money by using inferior grade media for data grade applications, the Media Recognition System was developed. MRS-enabled media embeds a section of stripes on the tape leader that the drive recognizes as media certified for digital data storage. DDS-MRS drives that detect MRS media will enable data-write operations. DDS tapes without MRS stripes become read-only.
Consistent Generational Improvements
Each successive generation of DDS media technology has been accompanied by improvements in the MP formulation to support increased recording densities. As DDS media has evolved from the initial MP formulation to the MPI++++ use in DAT 72 media, the magnetic particles have become smaller with higher coercivity, making a major contribution to the 18x capacity increase over the last decade.
Evolving DDS media from first-generation 2GB products to the 36GB capacity of the new DAT 72 format is the result of consistent improvements in metal particle formulations, thinner media and improved base films. In fact, the DDS-1 media was the first example of using longer-length tapes to extend capacity. The first implementation of the DDS-1 format used 60-meter tapes for a 1.3GB capacity. Capacity was upgraded to 2GB by extending the tape length to 90 meters, with no other changes in the tape formulation or the recording format.
The first major upgrade to DDS technology doubled the capacity from 2GB to 4GB. Media length was increased to 120 meters and a new higher-coercivity MP+ formulation was introduced. To reach the 120-meter target, a thinner tape was required. DDS-2 media was reduced to 6.9 um thickness, compared to 90 um for DDS-1.
The transition to DDS-3 Brought the largest capacity jump in the history of DDS technology, a 3x improvement over the previous generation. The 12GB native capacity of the DDS-3 format was derived from two key media improvements: a higher-coercivity (1700 oersted vs. 1530 oe) version of the MP+ formulation used in DDS-2 media, and a nominal increase in tape length to 125 meters. The enhanced magnetic properties of the higher-coercivity MP++ media supported a doubling of the linear bit density to 122 kbpi. DDS-3 also benefited from a significant non-media technology enhancement: the DDS debut of highly efficient Partial Response Maximum Likelihood (PRML) read channel technology, contributing further to increased capacity.
DDS technology reached the 20GB per cartridge threshold with DDS-4. Following a familiar model, DDS-4 media included an improved MP media formulation and longer length tape. A higher-output MP formulation, designated MP+++, was developed to handle the higher track densities of the DDS-4 format. The reduction in track pitch required a media formulation capable of producing greater output signal.
DDS-4 media was upgraded in several areas other than the MP formulation. The media thickness was reduced to 5.9 um, enabling an increase in the tape length from 125m to 150m. However, while making the DDS-4 tape thinner, it was also necessary to maintain the same strength and rigidity as previous DDS media, and increase its durability to withstand the higher rotational drum speeds required for increased data transfer rates. These seemingly conflicting requirements were addressed by improvements in the base film and backcoating layers of the media.
In order to overcome problems of modulation noise and signal dropout at the very high head-to-tape speeds of helical scan recording, it is essential that the tape's magnetic base film be exceptionally smooth. At the same time, the base film must be strong enough to withstand high-speed searching and editing. The base film used in DDS-4 media satisfies both conditions, reducing modulation noise and improving the signal-to-noise ratio. The base film is also extremely tough, limiting the tape's expansion and contraction even at very high and very low temperatures.
Although a smooth base film is necessary to achieve low noise performance, the smoother the base film, the higher the coefficient of friction. To maximize the performance of the base film, it is necessary to use a backcoating in the tape that realizes a coefficient of friction most suitable for the required format. DDS-4 media backcoating, designed so that the tape achieves an ideal amount of friction with the DDS drive, satisfies this condition perfectly. The DDS-4 backcoating is also very conductive, making it free from dust-attracting electric charges and the dropouts and errors that dust can cause.
DAT 72 (DDS-5)
DAT 72 builds on the technology foundation of previous DDS media format to achieve native capacity of 36GB using a new MP++++ formulation. DAT 72's 80% capacity boost over DDS-4 is the result of technology enhancements in four key areas: increasing tape length to 170 meters, transitioning to a higher output MP formulation, a substantial boost in recording density from 122 to 162 kbpi, and increased track density.
Dan Murphy is a freelance writer specializing in storage issues.
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|Title Annotation:||Tape/Disk/Optical Storage|
|Publication:||Computer Technology Review|
|Date:||Jul 1, 2003|
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