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Chapter 21 |
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Applications |
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One of the most important industries built on magnetic technology is magnetic recording. Actually, magnetic recording is so large and diverse a category that we are really talking about several industries and many different technologies. This document will provide only the barest overview of a rich and rapidly expanding area.
First, let's distinguish among some of the basic industries and technologies that fall under the Magnetic Recording umbrella.
The first distinction to make is between analog and digital recording. Analog systems record a continuous signal (such as music or video) by recording a magnetic signal that is proportional to the original. Thus, the magnetic signal is continuous as well. The basic elements of this scheme are:
Data storage systems are always digital. However, they are in some ways less complicated. On a computer, most of the information (except sound files) remains in digital form. Thus, there is no need to convert from analog to digital and back.
The difference between analog and digital can be understood in terms of the difference between telephone (analog) and telegraph (digital). When you speak into your phone, it converts the pitch and volume of your voice into an electrical signal of proportional frequency and amplitude. The other person's phone inverts the process. For telegraph, the operator converts your message into a series of discontinuous sounds, the dits and dahs (dots and dashes) of a morse code message. These are transmitted, and an operator on the other end decodes the message.
Our next important distinction is: random vs. serial access. A cassette tape (or any other tape) is a "serial access" device. That is, only one piece of the data, music, or other recorded information is near the playback mechanism at a time. If the tape is fully rewound, you have to scan through most of it to get to information near the end. On the other hand, a floppy disk is never "rewound" it spins continuously on its axis, and the playback mechanism (read head) can be moved back and forth along a radial line. Therefore, any piece of information can be located quickly. Devices such as the floppy disk, hard disk, CD-ROM, music CD, and vinyl records are said to provide "random access." Other serial devices are VHS tapes, DAT tapes, and open-reel computer tapes (the kind often seen spinning ominously in the background of old science fiction movies).
A third important distinction is the form and content of the magnetic material on the medium. For instance most tapes of any kind have "particulate" media. That is, the magnetic material is in the form of small particles which are stuck to a plastic tape by some kind of binder. The content of the particles has evolved over the years. Early cassette tapes used ferric oxide particles. Since then, chromium dioxide and pure iron particles have been used. Floppy disks are also coated with a particulate medium, and early hard disks were as well. However, the high storage density hard disks that are available today have a continuous coating of magnetic material. In fact, the surface of a hard disk is a complex set of layers each of which has a specific purpose. In a current hard disk, the "recording layer" which is composed of a cobalt alloy such as Co-Ni-Pt is less than 100 nanometers thick.
Underneath the distinctions, at the level where the magnetic signal is recorded and played back, all of these technologies rely on Ampere's law for writing and (with a few recent exceptions) on Faraday's law for reading. The basic device is the "read/write head." In its simplest form, the read/write head consists of a single ferromagnetic yoke with a small gap at one edge and two coils wound around it. When the write coils (right coils) are energized, Ampere's law says a magnetic field is produced. The yoke is magnetized by this field,
producing a very large "fringe" field at the gap. This field magnetizes a small region of the magnetic medium just below. The medium, which is magnetically hard, retains that
magnetization after the head field is removed.
Thus, a bit (or a small segment of an analog signal) is recorded.
When it comes time to read the recorded signal, the process works in reverse. Now, as the tape passes by the magnetic field of the bit, it partially magnetizes the yoke. As the magnetization of the yoke changes, Faraday's law says that a small electrical signal is induced in the read coils. This signal is amplified and decoded (if data) or sent to an output device such as a loudspeaker.
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