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Magnetic Recording/Audiocassette

History Of Magnetic Recording

The first working model of a magnetic recording device was demonstrated in 1898 by a Danish engineer named Valdemar Poulson. He used the mouthpiece from a telephone to convert speech into an electric current. The current went to an electromagnet which recorded the signal on a thin steel wire. The wire moved past the electromagnet very rapidly, a hundred times faster than modern cassette tapes. Recordings could only be very brief and the wires were awkward to handle. A competing technology, the phonograph, had been invented by Thomas A. Edison a few years earlier. It was easier to operate and was already quite popular by 1900, so magnetic recording attracted little interest at the time.

A step forward in magnetic recording was to replace the wire with a thin steel ribbon. A further advance was the development of paper tape with a layer of iron oxide adhering to one side, which was introduced in Germany in 1930. A few years later, plastic tape replaced the paper.

Commercial recording studios and radio stations greatly preferred tapes over phonograph records because they were suitable for editing. For example, if an otherwise excellent musical performance had minor flaws such as a cough or a note out of tune, the tape could be cut and spliced to remove the offending segment. For home use, only phonograph records were available at this time. Records could be mass-produced cheaply by making a master disk and pressing copies from it, while duplicating tapes was a lengthy process.

The invention of the transistor in 1947 revolutionized the communication industry. The subsequent development of microelectronics and a cartridge tape system led to commercialization of audio cassettes for the mass market in the 1960s. Magnetic tapes were designed to have two sound tracks, one to play in the forward direction and the second one when the tape was reversed. Then stereo sound, using two speakers, came into vogue. Four separate sound tracks were needed now, two tracks in each direction.

A further development in ultra-high fidelity music recordings was the introduction of digital audio tape (DAT) in the 1990s. Each second of sound is subdivided into 48,000 time intervals. The sound intensity during each interval is measured and recorded numerically on the tape in a binary, two-valued code. Each magnetic particle on the tape is like a tiny compass needle, pointing either forward or back, so a binary numerical system is appropriate. For playback, the digital information must be decoded before being sent to the speakers.

DATs cannot be played on ordinary tape players. The digital cassettes are smaller in size, although they play for a longer time. They are fragile and must be handled carefully. Digital recordings have the advantage that background noise and distortion are virtually eliminated. The sound quality of DATs is often compared to being present in the concert hall.

Resources

Books

Brophy, Michael. Michael Faraday. Pioneers of Science Series. Danbury, CT: Franklin Watts, Inc., 1991.

Davidson, Homer L. Troubleshooting and Repairing Audio and Video Cassette Players and Recorders. New York: McGraw-Hill, 1992.

Horn, Delton T. Creative Sound Recording on a Budget. New York: McGraw-Hill Professional Book Group, 1987.

Jorgenson, Finn. The Complete Handbook of Magnetic Recording. 4th ed. New York: McGraw-Hill Professional Book Group, 1995.

Periodicals

Shamos, Morris H. "Electromagnetism: Hans Christian Oersted." Great Experiments in Physics Reprint. Mineola, NY: Dover Publications, l998.


Hans G. Graetzer

KEY TERMS

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Capstan

—A rotating metal rod, driven by an electric motor, that pulls the cassette tape along at constant speed.

Digital audio tape (DAT)

—A high fidelity technology developed in the 1990s, where information is stored on magnetic tape in binary code.

Electromagnet

—A coil of wire surrounding an iron core that becomes magnetized when electric current flows through the wire.

Erase head

—An electromagnet operating at an ultrasonic frequency to scramble previously recorded information on a tape.

Iron oxide

—Tiny, needle-shaped particles that can be easily magnetized, coating one side of the plastic cassette tape.

Playback head

—A small coil that senses the varying magnetic field of the moving tape and converts it into an electrical signal that can be amplified.

Recording head

—An electromagnet that aligns the magnetic particles of the cassette tape while it moves by.

Additional topics

Science EncyclopediaScience & Philosophy: Macrofauna to MathematicsMagnetic Recording/Audiocassette - The Discovery Of Electromagnetism, Recording On Tape With An Electromagnet, Operation Of The Playback Head