Magnetic Recording/Audiocassette
Magnetic Recording/Audiocassette
The discovery of electromagnetism
Recording on tape with an electromagnet
Operation of the playback head
Motor drive for constant tape speed
Audiocassette tape recorders have been widely used to record and play back music or speech. They are sometimes also called compact cassette recorders, cassette tape recorders or, simply, tape recorders. Information is stored on a narrow ribbon of plastic tape that has one side coated with a magnetic material, such as iron oxide. An electromagnet aligns individual magnetic particles in a pattern that corresponds to the loudness and frequency of incoming sounds. In order to play back the recorded information, the magnetic tape moves past a pickup coil that generates an electrical output signal. After being amplified, this signal causes a speaker to vibrate, which produces sound waves for the listener. A tape recording can be erased by using a rapidly changing magnetic field that scrambles previously recorded patterns of particle alignment. From the 1970s to the early 1990s, audio-cassette tapes were one of the most popular formats for prerecorded music. The LP (long-playing record) and the CD (compact disc) were the other two popular formats. In the 2000s, the compact disc is the most popular format for prerecorded music. As of November 2006, blank cassette tapes are still being manufactured and sold, however their availability continues to decline as other recording technologies emerge. Some musicians continue to prefer using magnetic tapes for their master recordings.
The discovery of electromagnetism
Before 1820, magnetism and electricity were two completely separate fields of science. Magnetism was associated with the attraction of magnets for iron objects and the use of a compass needle to locate north and south. Electricity was of practical interest in connection with the hazards of lightning. Some scientists experimented with static electricity in the laboratory by rubbing a wool cloth against glass, but no useful applications came about from that experimentation.
In 1821, Danish scientist and physics teacher Hans Christian Oersted (1777–1851) made a remarkable discovery while doing a demonstration for his class. He had made a crude chemical battery by placing strips of copper and zinc into an acid solution. By connecting the two metal terminals with a wire, he provided a path for electric current to flow. A magnetic compass was lying on the table nearby. To his great surprise, Oersted noticed that the compass needle would deflect whenever current flowed through the wire. Apparently, the electric current created a magnetic field around the wire. His discovery was the beginning of electromagnetism, a joining of these two sciences.
Other scientists followed up on Oersted’s discovery. For example, it was found that a much stronger magnetic field could be produced by winding the electric wire into a coil. In addition, an iron core at the center of the coil intensified its magnetic field even more. Joseph Henry (1797–1878), a Scottish-American inventor and scientist who later became head of the Smithsonian Institution in Washington, DC, made an electromagnet that was powerful enough to support a load weighing 2,000 lb (908 kg).
Recording on tape with an electromagnet
Information becomes stored on magnetic tape as it passes by the so-called recording head, which is a small electromagnet. There must be a narrow gap in this electromagnet so that its magnetic field will extend over the nearby section of tape.
The signal coming from the audio input is an alternating, back-and-forth current. An audio sound with a frequency of 1,000 cycles per second, for example, reverses its electric current direction every one-thousandth of a second. When the current is reversed, the north and south poles of the recording head electromagnet are interchanged. Consequently, the nearby magnetic particles embedded in the tape will become reoriented in the opposite direction.
When a loud sound is being recorded, the current to the electromagnet is large and its magnetic field will be relatively strong. Therefore, a large number of magnetic particles in the tape will become aligned. A soft sound produces a weak field, so only a small fraction of magnetic particles will be affected.
For audiocassette players, the tape is designed to move at a standard speed of 1-7/8 in (about 4.8 cm) per second. During one cycle of a 1,000-cycle note, the tape moves only about l/500th of an inch (about 0.005 cm), which is a distance smaller than the diameter of the period at the end of this sentence. Several magnetic particles in a row must fit into such a short distance on the tape.
The human ear normally can hear sounds up to about l5,000 cycles per second. To record such a high frequency without distortion requires extremely tiny magnetic particles. The magnetic material must be easy to align and should retain its pattern of orientation indefinitely. Better quality audio tapes use very fine grains of chromium dioxide instead of iron oxide. Inexpensive tapes are adequate for recording the spoken voice because its frequency range is much less than for music.
An audiocassette has a built-in erase head to remove previously recorded information. The tape has to be blank before it can be used again to make a new recording. The erase head normally is an electromagnet that operates at an ultrasonic frequency, much higher than the human ear can hear. It effectively randomizes the alignment of magnetic particles. Audiocassettes are designed so that the tape passes by an erase head just before the recording head. Sometimes musicians at a recording studio want to record a second sound track over the first one. In that case the erase head has to be deactivated, so the original sound is not lost.
Operation of the playback head
How can the information, which was stored in a pattern of magnetically aligned particles on tape, be converted back into sound waves? The magnetic pattern
must be transformed into an electric current, which then can be amplified and cause a speaker to vibrate.
The operation of the playback is based on a discovery made in the 1830s by English physicist and chemist Michael Faraday (1791–1867). He knew about Oersted’s earlier observation that magnetism is created by an electric current. Faraday wondered if the opposite process might occur, where an electric current could be created from magnetism. By experimenting with magnets and coils of wire, he was able to show that a moving magnet did create a small current in a coil. His discovery was called electromagnetic induction because current was induced in the coil by the moving magnet. The induction principle, combining magnetism and motion, is the basis for the operation of the generators that produce electricity at power plants.
The tape of an audiocassette has a weak magnetic field around it that varies from point to point depending on the orientation of its magnetic particles. The playback head contains a coil of wire. When the magnetized tape moves past the coil, Faraday’s condition for inducing a current in the coil is fulfilled. The induced current will alternate in direction depending on the orientation of the magnetic particles as they pass by the playback head. The magnetic pattern originally recorded on the tape is transformed into a precisely corresponding electrical signal.
The electric current from the playback head is amplified and sent to an audio speaker, which vibrates in synchronism with the varying current. The back-and-forth motion of the speaker creates pressure waves in the air. This causes the listener’s ear drums to vibrate, producing the sensation of sound.
When someone wants to listen to a previously recorded tape, only the playback head is activated. However, to record new information on a tape requires two operations: the erase head must be activated, followed by the recording head. It is possible to activate all three heads, so that the first one erases, the second one records and the third one plays back what has just been recorded.
Motor drive for constant tape speed
In an audiocassette player, the tape must move from the supply reel to the take-up reel at constant speed. Otherwise, the sound becomes distorted. It would not work to pull the tape along simply by rotating the take-up reel, because each successive revolution would pull a longer section of tape past the heads, causing the tape speed to increase.
To obtain a constant tape speed, a motor is used to turn a small metal cylinder, called a capstan, at constant speed. When the tape player is switched on, a roller presses the tape against the rotating capstan. The tape is pinched between the roller and capstan, forcing it to move toward the take-up reel at constant speed, as desired.
The main problem with this tape drive mechanism is that it may generate a background hum in the output sound. One can listen for hum by playing a tape that is blank and turning the volume control up to maximum. This provides a helpful comparison test when trying out several models in an audio store for possible purchase.
History of magnetic recording
The first working model of a magnetic recording device was demonstrated in 1898 by Danish engineer Valdemar Poulson. He used the mouthpiece from a telephone to convert speech into an electric current. The current went to an electromagnet that 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 American inventor Thomas Alva Edison (1847–1931) 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.
KEY TERMS
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.
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
Baneriee, Sumanta. Audio Cassettes: The User Medium. Paris, France: Unesco, 1977.
Brophy, Michael. Michael Faraday. Pioneers of Science Series. Danbury, CT: Franklin Watts, Inc., 1991.
Horn, Delton T. Creative Sound Recording on a Budget. New York: McGraw-Hill Professional Book Group, 1987.
Huber, David Miles. Modern Recording Techniques. Boston, MA: Focal Press, 2005.
Jorgenson, Finn. The Complete Handbook of Magnetic Recording. 4th ed. New York: McGraw-Hill Professional Book Group, 1995.
Vasic, Bane, and Erozan Kurtas, eds. Coding and Signal Processing for Magnetic Recording Systems. Boca Raton, FL: CRC Press, 2005.
PERIODICALS
Shamos, Morris H. “Electromagnetism: Hans Christian Oersted.” Great Experiments in Physics. Reprint. Mineola, NY: Dover Publications, l998.
Hans G. Graetzer
Magnetic Recording/Audiocassette
Magnetic recording/audiocassette
Audiocassette tape recorders are widely used to record and play back music or speech . Information is stored on a narrow ribbon of plastic tape that has one side coated with a magnetic material, such as iron oxide. An electromagnet aligns individual magnetic particles in a pattern that corresponds to the loudness and frequency of incoming sounds. In order to play back the recorded information, the magnetic tape moves past a pickup coil that generates an electrical output signal. After being amplified, this signal causes a speaker to vibrate which produces sound waves for the listener. A tape recording can be erased by using a rapidly changing magnetic field that scrambles previously recorded patterns of particle alignment.
The discovery of electromagnetism
Before 1820, magnetism and electricity were two completely separate fields of science. Magnetism was associated with the attraction of magnets for iron objects and the use of a compass needle to locate north and south. Electricity was of practical interest in connection with the hazards of lightning . Some scientists experimented with static electricity in the laboratory by rubbing a wool cloth against glass , but no useful applications came about.
In 1821, a Danish physics teacher named Hans Christian Oersted made a remarkable discovery while doing a demonstration for his class. He had made a crude chemical battery by placing strips of copper and zinc into an acid solution . By connecting the two metal terminals with a wire, he provided a path for electric current to flow. A magnetic compass was lying on the table nearby. To his great surprise, Oersted noticed that the compass needle would deflect whenever current flowed through the wire. Apparently, the electric current created a magnetic field around the wire. His discovery was the beginning of electromagnetism , a joining of these two sciences.
Other scientists followed up on Oersted's discovery. For example, it was found that a much stronger magnetic field could be produced by winding the electric wire into a coil. Also, an iron core at the center of the coil intensified its magnetic field even more. Joseph Henry, an American inventor who later became head of the Smithsonian Institution in Washington, D. C., made an electromagnet that was powerful enough to support a load weighing 2,000 lb (908 kg).
Recording on tape with an electromagnet
Information becomes stored on magnetic tape as it passes by the so-called recording head, which is a small electromagnet. There must be a narrow gap in this electromagnet so that its magnetic field will extend over the nearby section of tape.
The signal coming from the audio input is an alternating, back-and-forth current. An audio sound with a frequency of 1,000 cycles per second, for example, reverses its electric current direction every one-thousandth of a second. When the current is reversed, the North and South poles of the recording head electromagnet are interchanged. Consequently, the nearby magnetic particles embedded in the tape will become reoriented in the opposite direction.
When a loud sound is being recorded, the current to the electromagnet is large and its magnetic field will be relatively strong. Therefore a large number of magnetic particles in the tape will become aligned. A soft sound produces a weak field, so only a small fraction of magnetic particles will be affected.
For audiocassette players, the tape is designed to move at a standard speed of 1 7/8 in (4.8 cm) per second. During one cycle of a 1,000 cycle note, the tape moves
only about l/500th of an inch (0.005 cm), which is a distance smaller than the diameter of the period at the end of this sentence. Several magnetic particles in a row must fit into such a short distance on the tape.
The human ear normally can hear sounds up to about l5,000 cycles per second. To record such a high frequency without distortion requires extremely tiny magnetic particles. The magnetic material must be easy to align and should retain its pattern of orientation indefinitely. Better quality audio tapes use very fine grains of chromium dioxide instead of iron oxide. Inexpensive tapes are adequate for recording the spoken voice because its frequency range is much less than for music.
An audiocassette has a built-in erase head to remove previously recorded information. The tape has to be blank before it can be used again to make a new recording. The erase head normally is an electromagnet that operates at an ultrasonic frequency, much higher than the human ear can hear. It effectively randomizes the alignment of magnetic particles. Audiocassettes are designed so that the tape passes by an erase head just before the recording head. Sometimes musicians at a recording studio want to record a second sound track over the first one. In that case the erase head has to be deactivated, so the original sound is not lost.
Operation of the playback head
How can the information, which was stored in a pattern of magnetically aligned particles on tape, be converted
back into sound waves? The magnetic pattern must be transformed into an electric current, which then can be amplified and cause a speaker to vibrate.
The operation of the playback is based on a discovery made in the 1830s by an English physicist, Michael Faraday. He knew about Oersted's earlier observation that magnetism is created by an electric current. Faraday wondered if the opposite process might occur, where an electric current could be created from magnetism. By experimenting with magnets and coils of wire, he was able to show that a moving magnet did create a small current in a coil. His discovery was called electromagnetic induction because current was "induced" in the coil by the moving magnet. The induction principle, combining magnetism and motion , is the basis for the operation of the generators that produce electricity at power plants.
The tape of an audiocassette has a weak magnetic field around it that varies from point to point depending on the orientation of its magnetic particles. The playback head contains a coil of wire. When the magnetized tape moves past the coil, Faraday's condition for inducing a current in the coil is fulfilled. The induced current will alternate in direction depending on the orientation of the magnetic particles as they pass by the playback head. The magnetic pattern originally recorded on the tape is transformed into a precisely corresponding electrical signal.
The electric current from the playback head is amplified and sent to an audio speaker, which vibrates in synchronism with the varying current. The back-and-forth motion of the speaker creates pressure waves in the air. This causes the listener's ear drums to vibrate, producing the sensation of sound.
When someone wants to listen to a previously recorded tape, only the playback head is activated. However, to record new information on a tape requires two operations: the erase head must be activated, followed by the recording head. It is possible to activate all three heads, so that the first one erases, the second one records and the third one plays back what has just been recorded.
Motor drive for constant tape speed
In an audiocassette player, the tape must move from the supply reel to the take-up reel at constant speed. Otherwise the sound becomes distorted. It would not work to pull the tape along simply by rotating the take-up reel, because each successive revolution would pull a longer section of tape past the heads, causing the tape speed to increase.
To obtain a constant tape speed, a motor is used to turn a small metal cylinder, called a capstan, at constant speed. When the tape player is switched on, a roller presses the tape against the rotating capstan. The tape is pinched between the roller and capstan, forcing it to move toward the take-up reel at constant speed, as desired.
The main problem with this tape drive mechanism is that it may generate a background hum in the output sound. One can listen for hum by playing a tape that is blank and turning the volume control up to maximum. This provides a helpful comparison test when trying out several models in an audio store for possible purchase.
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
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- 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.
Magnetic Recording/Audiocassette
Magnetic recording/audiocassette
Audiocassette tape recorders are widely used to record and play back music or speech. Information is stored on a narrow ribbon of plastic tape that has one side coated with a magnetic material consisting of magnetically active particles, most commonly iron oxide and chromium dioxide. As the tape passes around the five magnetic heads of a tape recorder, sound is recorded, replayed, or erased according to the heads that are activated.
A recording head (a small electromagnet) magnetizes the passing tape in such a way that the magnetic particles on it are realigned in a pattern that corresponds to the loudness and frequency (rate of vibration) of
incoming sounds. The resulting pattern remains on the tape until erased or changed.
The tape of an audiocassette has a weak magnetic field (an area where a magnetic force is present) around it that varies from point to point depending on the pattern of its magnetic particles. The playback head contains a coil of wire. When the magnetized tape moves past the coil, an electrical current is created. The current will alternate in direction depending on the alignment of the magnetic particles as they pass by the playback head. The magnetic pattern originally recorded on the tape is transformed into a precisely corresponding electrical signal.
The electric current from the playback head is then amplified and sent to an audio speaker, which vibrates simultaneously with the varying current. The back-and-forth motion of the speaker creates pressure waves in the air. This causes the listener's eardrums to vibrate, producing the sensation of sound.
Words to Know
Digital audio tape (DAT): A technology developed in the 1990s, by which 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 (rate of vibration) to scramble previously recorded information on a 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.
An audiocassette has a built-in erase head to remove previously recorded information. The tape has to be blank before it can be used again to make a new recording. The erase head normally is an electromagnet that operates at an ultrasonic frequency, much higher than the human ear can hear. It effectively scrambles the previously recorded magnetic particle patterns. Audiocassettes are designed so that the tape from the supply reel passes by the erase head just before the recording head.
History of magnetic recording
The invention of magnetic recording tape is attributed to both American inventor J. A. O'Neill and German engineer Fritz Pfleumer (1881–1945). Pfleumer filed the first audiotape patent in 1929. In 1935, the German electronics firm AEG produced a prototype (first version) of a record/playback machine, called a magnetophon. It was based on Pfleumer's idea, but used a plastic tape. Another firm, BASF, went on to refine the tape AEG used, presenting the first usable magnetic tape in 1935.
Types of magnetic recording machines
At one time, audiotape was used in a reel-to-reel format. This was a complicated and awkward procedure. Eight-track cartridges were another innovation in magnetic recording. These used an endless-loop format so the tape could be played continuously without being flipped over by the listener.
The audiocassette was introduced in 1963 by the Philips Company of the Netherlands. The audiocassette made inserting, advancing, and rewinding a tape fast and easy. The tape could be stopped and ejected at any point. Because of this ease and economy, magnetic tape recordings could compete with long-playing records (LPs). The invention of the microchip allowed audiotape players to be made smaller and more portable. With the introduction of products such as Sony's compact Walkman™, cassettes became universally popular.
Although the audiocassette is economical and still widely used, digital technologies are revolutionizing the industry. Digital audio tape
(DAT) recorders became widely available in the United States in 1990. A digital system enables a home recorder to make a tape copy that is an exact replica (not just an approximation) of the original sounds on a cassette that is half the size of a typical audiocassette. Digital technology records sound in a code of binary numbers (a series of zeroes and ones), so each subsequent recording is an exact copy of the code. These kinds of recordings do not suffer from the sound distortion that was the problem with the recordings they replaced.
[See also Electromagnetic field ]