magnetic storage devices

The Floppy Disc

2009-12-22T11:17:00.000-08:00

A floppy disk is a data storage medium that is composed of a disk of thin, flexible magnetic storage medium encased in a square or rectangular plastic shell. Invented by IBM, floppy disks in 8-inch, 5¼-inch, and 3½-inch formats enjoyed many years as a popular and ubiquitous form of data storage and exchange, from the mid-1970s to the late 1990s

The Video Tape

2009-12-22T11:01:00.000-08:00

Videotape is a means of recording images and sound on to magnetic tape as opposed to movie film. In most cases, a helical scan video head rotates against the moving tape to record the data in two dimensions, because video signals have a very high bandwidth, and static heads would require extremely high tape speeds. Videotape is used in both video tape recorders or, more commonly, video cassette recorders and video cameras. Tape is a linear method of storing information.

The first consumer videocassette recorders were launched in 1971, but it was not until Sony's Betamax and JVC's VHS were launched that videotape moved into the mass market.

Reel-to-Reel Tape

2009-12-22T10:46:00.000-08:00

Reel-to-reel tape is the form of magnetic tape audio recording in which the recording medium is held on a reel, rather than being securely contained within a cassette. In use, the supply reel or feed reel containing the tape is mounted on a spindle, the end of the tape is manually pulled out of the reel, threaded through mechanical guides and a tape head assembly, and attached by friction to the hub of a second, initially empty takeup reel. The arrangement is similar to that used for motion picture film.

Tape Speeds

2.38 cm/s — used for very long-duration recordings
4.76 cm/s — usually the slowest domestic speed, best for long duration speech recordings
9.52 cm/s — common domestic speed, used on most single-speed domestic machines, reasonable quality for speech and off-air radio recordings
19.05 cm/s — highest domestic speed, also slowest professional; used by most radio stations for "dubs", copies of commercial announcements
38.1 cm/s — professional music recording and radio programming
76.2 cm/s — used where the best possible treble response is demanded

The Audio Cassette

2009-12-22T10:18:00.000-08:00

The Compact Cassette is a magnetic tape sound recorder format. Although originally designed for dictation, improvements in fidelity led the Compact Cassette to supplant reel-to-reel tape recording in most non-professional applications. Its uses ranged from portable audio to home recording to data storage for early microcomputers. Between the early 1970s and late 1990s, the cassette was one of the two most common formats for prerecorded music, first alongside the LP and later the Compact Disc.

Compact Cassettes consist of two miniature spools, between which a magnetically coated plastic tape is passed and wound. These spools and their attendant parts are held inside a protective plastic shell. Two stero pairs of tracks (four total) or two monoaural audio tracks are available on the tape; one stereo pair or one monophonic track is played or recorded when the tape is moving in one direction and the second pair when moving in the other direction. This reversal is achieved either by manually flipping the cassette or by having the machine itself change the direction of tape movement ("auto-reverse")

The cassette was a great step forward in convenience from reel-to-reel audio tape recording, though because of the limitations of the cassette's size and speed, it initially compared poorly in quality. This permitted monaural cassette players to play stereo recordings "summed" as mono tracks and permitted stereo players to play mono recordings through both speakers. The tape is 3.81 mm wide, with each stereo track 0.6 mm wide and an unrecorded guard band between each track. The tape moves at 4.76 cm/s from left to right. For comparison, the typical open reel format in consumer use was ¼ inch wide, each stereo track nominally 1.59 mm wide, and running at either 9.5 or 19 cm/s.

Tape length is usually measured in minutes of total playing time. The most popular varieties are C46 (23 minutes per side), C60 (30 minutes per side), C90, and C120. The C46 and C60 lengths are typically 15–16 µm thick, but C90s are 10–11 µm and C120s are just 9 µm thick, rendering them more susceptible to stretching or breakage. Some vendors are more generous than others, providing 132 meters or 135 meters rather than 129 meters of tape for a C90 cassette. C180 and even C240 tapes were available at one time, but these were extremely thin and fragile and suffered badly from effects such as print-through, which made them unsuitable for general use.

Cassette types

Type I - Ferric Oxide (Fe₂O₃)
Type II - Chromium Oxide (CrO₂)
Type III - Ferrichrome (FeCr)
Type IV - Metall

The Credit Card

2009-12-22T09:54:00.000-08:00

A credit card is a thin plastic card, usually 3-1/8 inches by 2-1/8 inches in size, that contains identification information such as a signature or picture, and authorizes the person named on it to charge purchases or services to his account -- charges for which he will be billed periodically. Today, the information on the card is read by automated teller machines (ATMs), store readers, and bank and Internet computers.

The magstripe from the credit card can be "written" because the tiny bar magnets can be magnetized in either a north or south pole direction. The magstripe on the back of the card is very similar to a piece of cassette tape.

There are three tracks on the magstripe. Each track is about one-tenth of an inch wide. The information on track one is contained in two formats: A, which is reserved for proprietary use of the card issuer, and B, which includes the following:

Start sentinel - one character
Format code="B" - one character (alpha only)
Primary account number - up to 19 characters
Separator - one character
Country code - three characters
Name - two to 26 characters
Separator - one character
Expiration date or separator - four characters or one character
Discretionary data - enough characters to fill out maximum record length (79 characters tota)
End sentinel - one character
Longitudinal redundancy check (LRC) - one character
LRC is a form of computed check character.

The format for track two, developed by the banking industry, is as follows:

Start sentinel - one character
Primary account number - up to 19 characters
Separator - one character
Country code - three characters
Expiration date or separator - four characters or one character
Discretionary data - enough characters to fill out maximum record length
LRC - one character

The track three is not used