In the next few short years, broadcasters all have to make serious choices in digital media technology. Besides the DTV conversion, the migration to video servers is in full swing, with the movement beginning to head toward digital tape archive and near-line storage.
Over the past several months, this column has explored digital tape storage technology from varying perspectives. Advancements being made in digital tape technologies are changing the way the data industry addresses the handling, makeup and structure of advanced digital media asset management.
So far we have reviewed two of the most prominent tape formats, helical scan recording and digital linear tape (DLT). We still need to investigate other structures, including 19mm (DST), and now we'll look at one of the newest kids on the helical scan block - Advanced Intelligent Tape or AIT.
We need to remind ourselves that we're dealing with technologies that weren't here even five short years ago. AIT, for one, was only introduced in mid-1996, and it was just 1994 (when Quantum bought the DLT technology from developer Digital Equipment Corp.) that the usefulness and business potential for DLT truly began in earnest.
And no sooner are we just beginning to hear about AIT and it's off to the extension races with AIT2 - which we'll get back to later in the article.
BACK TO BASICS
To gain a perspective on AIT, we'll briefly review the fundamental basics of the two counterpart tape technologies. DLT was developed to resolve the problems that possessed the earlier forms of QIC (quarter-inch cassette). DLT also provided an alternative to the older 4mm and the newer 8mm tape formats.
Concerns over data capacity, speed and reliability are all factors in the acceptance of these various tape backup technologies. DLT offered speed increases, upward of three to four times that of 4mm or 8mm, but also provided as much as a fourfold increase in storage capacity. Its counterpart, the helical scan concept, provided a higher density - but also multiplied the issues of transport alignment and data integrity as the media aged.
Tape media performance and transport reliability are critical factors when systems must operate at high duty cycles. Anything that simplifies the structure of a tape system in turn reduces the probability of failure.
DLT, which utilizes a linear serpentine recording principle, is promoted as being far less prone to head failure and tape path misalignment. DLT's competitor, the 8mm helical scan recording process, has reported higher rates of failure that are compounded by two factors - the helical scanner, a rotating mechanical component, and the servo-based tape drive motor system. Proponents of DLT say there is seldom a failure of a drive itself.
Recognizing that DLT still had certain shortcomings, Sony Corp., utilizing its expertise in both tape and transport technology, developed AIT - principally for data and networking host backup. Modeled in part after the 8mm helical concept, Sony has managed to increase both the throughput and the capacity of its new tape drive.
The company has also employed a new evaporative metal media that reduces particle shedding and significantly decreases the drive failures so often experienced in 8mm as a result of contaminants clogging the mechanisms themselves.
AIT drives, like those of other tape formats, also can utilize a mild 2:1 compression to increase data storage capacity. Current models store between 25 and 70 GB on a single cartridge, depending upon whether you use the 25 GB (native) or 50 GB (compressed) model. There is also a 35/70 GB model - both can operate with or without compression.
PREDECESSOR SYSTEMS
One of the more known problems with predecessor tape storage systems is the accessing of files and/or data directories. Legacy model tape systems use a beginning of the tape indexing structure that must be written to each time data is appended or eliminated from the tape. This requires that the tape's indexing headers be read every time the tape is loaded and then written to every time it has completed any form of data modification.
Because this header is located at the beginning (sometimes at the end) of most media, the tape must therefore be rewound to the header, advanced for the first reading of the header, then written to and reread for verification. Changing tapes is always compounded by this basic concept in the data tracking mechanism.
Besides taking the additional time for cycling media, any errors in this process could render the entire data tape useless, as this is the principal location for the data about the data storage on the tape.
Sony, possibly from its experiences in some of the company's broadcast tape applications, took to task another method of storing the bits about the bits. Instead of just writing on the header, Sony placed header information at a location that doesn't require the tape be mounted in order to be read.
Sony introduced its Memory-in-Cassette, or MIC, which is a small 16 kb chip that keeps the metadata stored on the physical cassette - and not on the actual media. This reduces file access location time, and searches can be achieved in 27 seconds (for 25/50 GB) and 37 seconds for the larger cassette format. This amounts to nearly a 50 percent reduction in search time vs. DLT.
TRANSFER RATES
Data transfer rates are much improved over previous-generation DLT, although some of the newest DLTs are getting closer. By year-end the newest "SuperDLT" is expected to store up to 200 MB and allow 20 MBps data throughput. Along with AIT, and just around the corner, are some of the others - including Mammoth2 and even the next-generation AIT2.
Fast/Wide SCSI technology allows AIT a 3 GBps (native), 6 MBps (compressed) transfer rate. Rates are dependent upon compression ratio, the size of the cassette and the length of the tape. Burst rates can be as high as 20 MBps (in synchronous mode) between host-to-tape and 5 MBps in asynchronous mode. The higher data capacity is made possible by utilizing an advanced lossless data compression technology (ALDC) previously available only in mainframe systems.
The AIT technology employs a 4,800 RPM helical scan head. Higher-density data is written at an angle relative to the tape's edge. A closed-loop, self-adjusting tape path provides for highly accurate tape tracking. The AIT transport features an auto tracking following (ATF) system whereby the servo adjusts for tape flutter and lets data be written with much closer spacing between tracks.
A redesigned metal-in-gap (MIG) tape head puts head life at an average of 50,000 hours. Combining their tension control mechanism, which maintains tape tension at one-half that of other helical scan tape technologies, and Sony's unique head geometry, head-to-media pressure is reduced even further. This servo system senses and controls tension fluctuations and in turn helps to reduce tape and head wear.
Only advanced metal evaporated (AME) media can be used on these AIT drives. The magnetic recording layer of the AME media contains a pure evaporated cobalt doping that is sealed with a carbon coating and lubricant.
AME contains 80 to 100 percent magnetic recording media that results in a 5 dB increase in shortwave length readback output vs. conventional metal evaporated tape. Average life of the tape allows for 30,000 end-to-end passes.
In the future (due in spring 1999), AIT2 will have 50/100 GB capacities with 6/12 MBps data transfer rates. Now - with a second licensee, Seagate and its Sidewinder products - market acceptance for AIT is improving. Seagate's models include a self-cleaning mechanism and a variable self-cooling fan that automatically dissipates heat. The Sidewinder 70 series has a feature called TapeAlert that constantly monitors both the drive and the media during backup and restore operations.
Currently, most AIT products are used in network-based backup, archive and restore functions. Vendors like StorageTek and other ALSs (automated library systems) are beginning to look seriously at these and other alternative drive systems. Expect to see AIT and other similar next-generation tape technologies in the broadcast server market in the next year or so.
EMERGING TECHNOLOGIES
As new technologies emerge, including DVD-RAM, OAW (optically assisted Winchester), and nearfield recording develops, one continues to ask the question, "How long will tape remain a viable medium, " for either data backup or media-based recording. As long as mechanisms improve, recording media storage densities increase and there are customers looking to OEMs for mass storage products - tape will continue.
In our upcoming columns, we'll continue to explore tape and its relationships to media asset management for video server and broadcast applications - including the latest in the new linear tape open (LTO) specifications developed by Hewlett-Packard, IBM and Seagate.
