Technology and Time-to-Market: The Two Go Hand-in-Hand
- The combined strengths of Quantum Corporation and Digital Equipment Corporation's mass storage businesses yield a technology portfolio ideally suited to the present and future demands of the computer
storage market. The technology strengths include advanced ASIC design capabilities, analog and digital signal processing expertise, PRML read channel design experience, and industry-leading magnetoresistive (MR) head design.
- Strong ASIC capabilities lead to the development of lower-cost disk drives with better performance, higher areal densities, and improved reliability. Analog and digital signal processing expertise has already resulted in an "all-digital" embedded servo, advanced read channel designs, and sophisticated Error Correction Code (ECC) schemes.
- The state-of-the-art PRML read channel and MR head technologies will drive the areal density improvements of the future, ensuring that storage densities continue to double every 18 months.
- These four key technologies - ASIC design, analog/digital signal processing, PRML read channels, and MR heads - will help Quantum meet its rigorous time-to-market goals, and give the company an edge in bringing many new user benefits to the hard disk drive marketplace.
This Quantum Technical Information Paper (TIP) examines Quantum's advanced ASIC capabilities, signal processing expertise, PRML read channels, and MR head technology. The paper also discusses Ultra SCSI, an extension to the standard parallel SCSI interface introduced first by Quantum as a result of its extensive in-house ASIC capabilities.
Quantum Corporation entered the computer storage market in 1980, growing to its present position as the worldwide unit volume leader in hard disk drives. Quantum recently acquired Digital Equipment Corporation's hard disk drive, tape drive, solid state disk, and recording heads businesses. Blending the strengths of these two organizations yields a technology portfolio ideally suited to the present and future demands of the computer storage market. Quantum's key technological strengths are:
- Advanced ASIC design capabilities
- Analog and digital signal processing expertise
- Extensive read channel design experience
- Industry-leading MR head design and manufacturing
The technologies found in Quantum's portfolio enable the company to continue offering its hard disk drive customers leading performance, areal density, and cost efficiencies as well as the Quantum hallmark superior quality. Quantum's extensive technology portfolio also gives the company a time-to-market advantage in bringing these benefits to its customers.
A strong in-house ASIC capability is important because advances in ASICs can affect four of the primary goals of disk drive designers: better performance, higher areal densities, improved reliability, and lower-cost designs. For example, custom ASICs enable Quantum to implement unique, proprietary features in hardware rather than firmware, significantly improving a drive's performance. In-house ASIC design also promotes chip integration, reducing the number of chips employed on the printed circuit board (PCB). Fewer parts means increased reliability and decreased cost. Quantum led the low-cost design revolution, when in 1992, it introduced the 3.5-inch ProDrive ELS drives with just 11 ICs on a single-sided PCB, followed one year later by the 2.5-inch Go-Drive GLS drives with just 5 ICs on a single-sided PCB.
A third advantage of in-house ASIC capabilities is that Quantum can add significant value to interface and controller electronics. In 1987, the company pioneered the "intelligent" drive, that is, a drive with an embedded controller. Now, virtually all hard disk drives have embedded controllers, with SCSI and IDE-AT being the most common. Intelligent drives control a number of processes that ensure data integrity and fast data retrieval, including error recovery, defect management, and cache management. Since 1987, Quantum's extensive expertise in interface and controller technology has led to some significant time-to-market wins:
- First to incorporate an embedded SCSI (Small Computer System Interface) controller in a 3.5-inch drive.
- First to offer continuous prefetch read caching and write caching.
- Industry leadership in new SCSI and IDE-AT features or variations: Fast ATA, Fast and Fast Wide SCSI, SCSI-3, Plug-and-Play SCSI, and AT Cable Select.
- First to introduce Ultra SCSI, an extension to the standard parallel SCSI interface. (See sidebar on page 4.)
Quantum's technology portfolio also includes extensive analog and digital signal processing expertise. This has led to all-digital embedded servo designs, advanced read channel designs, and sophisticated Error Correction Code (ECC) schemes. In 1990, Quantum was the first to implement ECC on-the-fly on drives with multiple zone recording schemes. Having designed elaborate ECC schemes for hard disk drives, tape drives, and solid state disks, Digital brings additional ECC experience to Quantum. Its disk drives' Reed Solomon 198-bit ECC scheme is capable of correcting 120 bits within a 512-byte sector or up to 32 bits on-the-fly. ECC, all-digital embedded servos, and advanced read channels can lead to higher levels of electronics integration, improved data integrity and/or increased data bits per inch (BPI) and higher data throughput.
An example of a technology that combines both ASIC design and signal processing is the ASABET chipset that Quantum acquired through its purchase of Digital's drive business. This patented technology optimizes drive reliability and performance by integrating the analog and digital portions from five chips into a two-chip family. The ASABET chipset also automatically executes a number of features in hardware, resulting in very low SCSI command overhead and reduced bus execution times for higher system throughput. The chipset contains on-board power management that turns off selected circuits, such as the read channel, when they are not being used.
Extensive ASIC design and analog/digital signal processing expertise will help Quantum meet its rigorous time-to-market goals. In addition, these capabilities will enable the company to continue developing innovative drive designs that can significantly improve performance or make possible a new capacity point at a lower cost.
Interface wars in the hard disk drive industry come and go. ESDI versus SCSI. IDE-AT versus SCSI. Now a new dimension: serial SCSI vs. serial SCSI. Although serial SCSI interfaces are expected to begin shipping in late 1995, the industry has yet to settle on a single serial SCSI interface standard. The three serial interfaces in contention are:
- Fiber channel. Data transfer rates starting at 100 MB/second, 25m cable lengths, up to 126 devices supported.
- SSA. Data transfer rates starting at 80 MB/second, 20m cable lengths, up to 25 devices supported.
- P1394 (or Firewire). Data transfer rates starting at 12.5 MB/second. Low-cost and ergonomically designed connector.
Since adopting any of these serial interfaces will entail considerable costs and work for systems designers, some will elect to postpone the decision until there is a clear winner. The situation is similar to when designers were making the transition from ST-506 drives and had to choose between ESDI and SCSI implementations. Those that moved to ESDI did so to their lasting regret because SCSI prevailed.
While serial interfaces are costly to implement, the benefits mentioned above are clearly sufficient to appeal to many OEM customers who will eventually integrate them into their future high-end systems. In the meantime, it is possible to extend the existing parallel SCSI interface through a standard called Ultra SCSI. The Ultra SCSI interface, which is expected to begin shipping in early 1995, can deliver considerable performance improvements without locking designers into a potentially obsolete serial interface. Quantum believes that a number of OEMs, systems integrators, and VARs will use the Ultra SCSI interface as a stepping stone to serial SCSI.
Ultra SCSI, which conforms to ANSI's Fast-20 SCSI interface standard, can double the burst transfer rates found in disk drives today without changing the physical connector. For example, the current 50-pin implementation of Fast SCSI can transfer data from the drive's buffer to the host at 10 MB/ second. The 8-bit implementation of Ultra SCSI, using the same 50-pin connector, can transfer data at up to 20 MB/second. Likewise, the 16-bit Ultra SCSI implementation is capable of achieving a transfer rate up to 40 MB/second, compared to 20 MB/second in the current Fast Wide implementation, while using the standard 68-pin or 80-pin connectors. Ultra SCSI accomplishes the higher transfer rates by speeding the clock rate, which of course also entails additional technical work with the I/O drivers - the analog hardware drivers on the driver ASIC - to ensure that the doubled speeds work reliably.
If doubled transfer rates without physical changes are Ultra SCSI's main appeal, cable length limits are its Achilles' heel. While serial SCSI can support longer cable lengths, Ultra SCSI is limited to 1.5m cabling. For box- to-box implementations this may be a drawback, but it is not a problem for most inside-the-box configurations.
Quantum, Digital, and NCR have been leading the Ultra SCSI standard effort over the last year. Because of its in-house ASIC-design expertise, Quantum expects to be the first drive maker to incorporate Ultra SCSI into a drive. The Ultra SCSI interface option will begin shipping in Quantum Grand Prix 4.3 gigabyte (GB) and 2.1 GB drives beginning in the first calendar quarter of 1995.
Quantum's acquisition of Digital Equipment Corporation's head business brings together in one portfolio the two state-of-the-art technologies that will drive the areal density improvements of the future: PRML and magneto-resistive (MR) heads. In 1995, Quantum intends to offer drives that incorporate both, and the impact of merging the two technologies will be felt across the entire computer industry as the race for areal density leadership heats up.
Areal density is the term used for the number of bits of data per square inch of disk space. It's the product of linear, or bit density (BPI) and track density (TPI), on a disk drive. Increasing areal density is important because increasing the data that can be stored on a single disk helps to lower cost per megabyte. In the past, the industry has doubled areal density every 18 months through evolutionary changes to existing technologies. While current technologies can still be pushed to achieve areal density increases, new technologies are required to continue this extraordinary pace of change in the future. MR heads and PRML read channels are those revolutionary technologies the industry will rely on through this decade and beyond to resume doubling capacity per disk every 18 months.
MR head areal densities improve dramatically as the technology matures, while inductive heads become less competitive. MR heads combined with PRML read channels will ensure that areal densities continue to double every 18 months.
PRML, or Partial Response Maximum Likelihood, technology was first used in digital communications and was adopted by IBM for disk drives in 1990. In 1993, Quantum - through its ASIC, digital signal processing, and read channel expertise - became the first independent drive company to ship hard disk drives with a PRML read channel. The company is now shipping its second generation of the technology and plans to introduce a third generation of PRML, a high performance, low power, single-chip read channel, in the first quarter of 1995.
PRML will replace an earlier technology, called a peak detection read channel, which has been in use since disk drives were invented. Peak detection ensures the accurate reading of the magnetic transitions, or flux reversals, that encode the bits on a disk. With peak detection, transitions within a fixed clock "window" are given a 1 (peak) or 0 (no peak) value. Unfortunately, in reading two adjacent transitions - that is, two adjacent 1's - thesignals can interfere with each other, effectively canceling one another out. To avoid this "inter-symbol" interference, data must be encoded, resulting in a coded data string that is about 50 percent longer than an uncoded string. This takes up disk space, hence, limiting bit density.
PRML differs by recognizing multiple transitions and using this pattern of transitions to signal a pattern of 1s and 0s (bits). For example, a 6-bit pattern will be compared with all known, possible patterns to determine its identity. Hence, the technique takes inter-symbol interference into account, rather than trying to eliminate it. The result is a far more efficient coding scheme that permits more bits to be encoded on a track. PRML can increase areal density by 30-40 percent over peak detect read channels.
MR heads employ a magnetically sensitive thin-film resistor element to detect data bits written on the magnetic disk
surface. This type of head can produce a large or easily identified signal even when reading bits just ten-millionths of an inch apart an d one-hundred-millionths of an inch wide. Moreover, the signal or magnetic strength is relatively independent of the disk speed, an important advantage over conventional inductive thin-film read-write heads. With an inductive thin-film read head, the signal strength varies with the speed at which recorded bits move under the head. Thus, when disk sizes decrease, the signal diminishes to a point where it becomes difficult to distinguish the signal from background electrical noise.
An additional advantage of MR head technology is that is employs two closely spaced but separate read and write elements. The writing element is an inductive thin-film head designed specifically for recording data. Typically it is wider than the reading element to minimize crosstalk from adjacent tracks. The dual-element MR head structure avoids many design compromises that conflicting read and write elements impose on currently used purely inductive head designs.
Rocky Mountain Magnetics, now a Quantum subsidiary, is widely recognized as one of the leaders in MR head technology and development. Access to sophisticated MR heads combined with in-house PRML read channel experience give Quantum a time-to-market edge in bringing the combined benefits of these two revolutionary technologies to the marketplace.
Blending the development and manufacturing strengths of Quantum and Digital's hard disk drive, tape drive, and solid state disk businesses results in a technology portfolio that enables Quantum to meet its rigorous time-to-market goals. The combined capabilities also allow the company to continue developing innovative hard drive designs and lead the way to the areal density improvements of the future.