A sampling of recording equipment.
The binary recording process begins with analog-to-digital (A/D) conversion. Digital data are generally stored in one of three modes: as magnetic domains on disks or tapes, as pits or holes on the reflective surfaces of optical media or in more esoteric forms involving the interaction of atomic and electromagnetic properties.
TAPES AND DISKS
With magnetic recording, the signal to be captured is present on an electromagnet, the "head," and patterns are imprinted upon a magnetic coating. The process is both familiar and venerable.
The geometry with which data is arrayed on a magnetic tape can be varied. Data streams of zeros and ones can be oriented in a single linear track, in multiple parallel linear tracks, in helical paths or in diagonal stripes across the tape. Helical-scan technology, introduced by Ampex Systems Corp. (Redwood City, CA) in 1961, is found in a variety of recorders. Helical-scan decouples the tape track density from the number of heads in the drive by mounting the read/write (R/W) heads on a drum called a scanner that rotates at high speed. The tape is moved very slowly across the scanner, which is mounted at a slight angle to the tape path. This causes the data to be recorded on diagonal stripes across the tape.
Magnetic recording is divided into two principal classes -- those using rigid disks and those employing flexible tapes. With the rigid disk, data are stored in concentric tracks in magnetic material deposited on one or both surfaces of a thin platter. Reading and writing to the disk is usually accomplished by arrays of movable arms which "fly" at a fraction of a millimeter above the surface.
Although expensive when compared to tapes, the disks have the decided advantage of offering random access; that is, being able to directly access data by moving the reading arm radially to the appropriate track and then waiting for the desired information to spin by. The time to acquire data in this quasi-random manner is about 20 msec in a mainframe-type hard disk such as the IBM 3390.
Flexible tapes, in general, record data by drawing the tape over fixed R/W heads. There may be multiple R/W gaps in a fixed head, but basically all tape systems record and access data in a serial manner. The variety of available flexible tape formats is a testimony to the maturity of this medium. Some of the media encountered in this sampling include:
* Cassettes: Based upon widely used audio cassettes, this 1/8-in.-wide tape is available in 60-MB to 600-MB capacities.
* 4-mm: This tape format, developed for digital audio tape (DAT) cassettes, has storage capacities in the 1.2- to 5- GB range and transfers data at about 180 kB/sec.
* 8-mm: Storage products based upon helical scan 8-mm video camera technology were first introduced by Exabyte (Boulder, CO) in 1987. The initial offering, the EXB-8200, had a capacity of 2.3 GB.
* VHS 1/2-in.: The first product to use the 1/2-in. T-120 cassette was Honeywell's Very Large Data Storage (VLDS) system. This format was used in a helical-scan instrumentation recorder which could store up to 5.2 GB at 2 MB/sec per channel.
* 19-mm: Packaged in three sizes of cartridge shells, the DST line introduced by Ampex Systems Corp. has capacities ranging from 25 GB on a small cartridge to 165 GB on the large.
MIRRORS ON DISKS
Optical media may eventually become the first choice for low-cost, high-density storage. As yet, however, the full potential of the technique has yet to be realized.
Optical disks, in particular the rewritable or magneto-optical disks, have experienced vigorous growth. Starting with the optical video disks in the late 1970s, the audio compact disk and the now familiar "CD player" appeared on the consumer market in the early 1980s. These were followed by CD read-only-memory (the CD-ROM) and the 12-in. write-once optical disk. Writable video disks and rewritable 5.25-in. optical disks appeared in the late 1980s. All of these products share the related technology of measuring reflected light rather than magnetic strength.
Currently two types of optical recording are commercially available -- write-once-read-many (WORM) and rewritable magneto-optic (M-O). WORM is relatively simple. A metallic, mirror-like surface is deposited upon a plastic substrate. To "write" data, lenses focus laser light on this surface and melt a hole or pit into the metal. The tiny holes created during the write operation reflect a different amount of light than areas which are pristine. By measuring this variation in reflected light, the stream of zeros and ones which represent data is created.
Rewritable, or M-O, technology is more complicated and involves a two-step operation. The metal layers in an M-O disk have magnetic properties which disappear when the material is heated to an elevated temperature called the Curie point. In the first step, a laser heats a large portion of one track to the Curie point and effectively erases the track. During the second pass, a magnet in the R/W head is activated and the laser flashes on and off, heating tiny segments in the coating to the Curie point and thus creating tiny magnetic islands.
To read these data, a property called the Kerr effect -- the ability of a magnetic field to rotate light -- is used. A low-power laser hits the surface and if it impinges upon a magnetized island, the light is slightly rotated. These changes in rotation are convened into data.
For data to be processed, tape must be drawn around capstans and over rollers in the reel-type magnetic recorder. Disks must be spun and R/W heads indexed to access data on the platters of a Winchester drive. Moving such physical objects takes time, and time is a commodity in short supply in a high-speed recording system. There are recording techniques in which only electrons or electromagnetic waves must move, and these can move at velocities at or close to the speed of light. Solid-state memories which use the motion of electrons (or their counterpart "holes" -- the absence of electrons) are found in random access memories (RAM) and read-only memories (ROM).
If access time is the paramount criterion, solid-state storage is the choice, with seek-command to seek-complete times of under 150 |micro|seconds~~. Solid-state memories may be divided into two classes -- volatile and nonvolatile. Volatile memories, such as dynamic random access memories, require repeated applications of power to refresh the data stored in memory. Static RAM, while not requiring periodic refreshing, does require a maintenance electrical potential.
THIS MONTH'S SAMPLING
By far, the most mature medium is magnetic storage, and this is reflected in the population of our sampling. In all, the products of 16 manufacturers are listed in the table.
Ampex Data Systems Corp. 401 Broadway Redwood City, CA 94063 415-367-2758 Fax: 415-367-3106
Computing Devices Int. (CDI) 8800 Queen Ave. South Bloomington, MN 55431 612-921-6080 Fax: 612-921-6966
Condor Systems, Inc. 2133 Samaritan Dr. San Jose, CA 95124 408-371-9580 Fax: 408-371-9589
Datatape Inc. 360 Sierra Madre Villa Pasadena, CA 91109 818-796-9381 x2444 Fax: 818-351-0276
Fairchild Defense 20301 Century Blvd. Germantown, MD 20874 301-428-1124 Fax: 301-353-8679
Loral Data Systems PO Box 3041 Sarasota, FL 34230 813-378-6950 Fax: 813-378-6995
Merlin Engineering Works, Inc. 1888 Embarcadero Rd. Palo Alto, CA 94303 415-856-0900 Fax: 415-858-2302
Miltope Corp. 1770 Walt Whitman Rd. Melville, NY 11747 516-420-0200 Fax: 516-756-7601
Mountain Optech, Inc. 4775 Walnut St., Ste. A Boulder, CO 80301 303-444-2851 Fax: 303-444-4431
NAI Technologies, Inc. Military Products Div. 60 Plant Ave. Hauppauge, NY 11788 516-582-6500 Fax: 516-582-8079
Penny & Giles Data Systems Ltd. (P&G) The Mill, Wookey Hole Wells, Somerset BA51BB England 011-44-749-675454 Fax: 011-44-749-676678
Precision Echo, Inc. 3105 Patrick Henry Dr. Santa Clara, CA 95054 408-988-0516 x250 Fax: 408-727-7491
Racal Recorders, Inc. 15375 Barranca Pkwy, Ste. H-101 Irvine, CA 92718 714-727-3444 Fax: 714-727-1774
Raymond Engineering Inc. 217 Smith St. Middletown, CT 06457 203-632-4582 Fax: 203-632-4567
Sony Electronics Inc. 3 Paragon Dr. Montvale, NJ 07645 201-358-4209 Fax: 201-358-4215
TEAC America, Inc. 7733 Telegraph Rd. Monteballo, CA 90640 West: 213-727-7659 Fax: 213-727-7621 East: 508-683-8322 Fax: 508-686-4345
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|Publication:||Journal of Electronic Defense|
|Date:||Mar 1, 1994|
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