Why are magnets born with two poles

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Since the third century BC, Chinese sources have been mentioning the attractive properties of the magnetic stone, which is often called tzhu shih, the "loving stone", as it is said of him: "The magnet attracts iron like a tender mother changes her children This is the reason why it got its name. The first compasses, so-called magnetic chariots or "chariots pointing south" have existed in China since the first century AD at the latest. It was not until the 13th century, twelve hundred years later, compasses appear in the occident.

The magnetic stone * is also known in ancient Greece, but it was difficult to explain its enigmatic power. Plato (428 - 347 BC) considered them divine. So surprisingly greedily the magnetic stone attracted iron that it seemed to be alive. Thales and Anaxagoras even ascribed a soul to him.

Even in later centuries one got no further with the explanation. Alexander Aphrodiseus, a thinker around 200 AD, considered the question of the magnet to be insoluble. Seven centuries later, the resigned answer to the question why the magnetic stone attracts iron is only: "because it has a natural power to do so".

Such puzzles have always fired the imagination, and so the old ship's legend about the Magnetberg, which attracts all iron-clad ships to be smashed, found wide spread in the stories of many cultures. The mysterious magnetic stone remained the miraculous content of legends until the 16th century. The ideas range from the diamond, which weakens the magnetic stone, to the garlic, which, when consumed by seafarers, numbs the compass and thus throws the ship off course.

The magnet can have different effects on people. It was believed that a magnetic stone placed under the pillow of an unfaithful woman had the power to throw her out of bed. Burglars could have used the magical power for their dark purposes. If bits of magnet were scattered on glowing coals and distributed in rooms, smoke would spread that would chase the residents of the house away and give the thief time to take the valuables.

The magnet has also been said to have healing properties. Held in the hand, it cures pains and cramps in the feet; Annealed and powdered, it alleviates burn damage to the skin.

The oldest experimental physical representation of magnetism is considered to be the thirteenth century text by the French scholar Petrus Peregrinus, who was able to establish that the north and south poles of two magnets attract each other while poles of the same name repel each other. If you break a magnet, you get two magnets with two poles each: it is not possible to keep one pole alone. Peregrinus gained even more insights: the researcher had formed a ball from a large magnetic iron stone. An iron needle is attracted to a magnet in such a way that it is always directed towards the poles. He placed them on different parts of the ball and drew lines around the stone along the needle. These intersected at the poles like the meridians of the globe. However, Peregrinus, who tried to simulate the sky with his sphere, not the earth (which was thought to be a flat disk), but could not draw the correct conclusion from this analogy that the earth behaves like a giant magnet. After all, as others did, he did not relate the alignment of the needle to the attraction of magnetic ore pits in the north or to that of the Pole Star, but to a force emanating from the "celestial poles" around which, as was believed at the time, the vault of heaven spun over the still earth.

The systematic, in today's sense scientific research into magnetism began with the Latin work "De Magnete" (About the Magnet) by the English scientist William Gilbert, published in 1600. In the meantime, many basic properties of magnetic materials have been discovered, and the cause and effect of a magnetic field has largely been clarified. But many questions still remain open today.

So the course of the earth's magnetic field and its strength could be measured very precisely, but its origin or its effects on animals and plants have to be explained in more detail.

Particularly puzzling are the "magnetic monopoles", phenomena that, as one learned at school, actually do not exist. Since Petrus Peregrinus one believed to know that the characteristic of all magnets is their bipolarity. It was the English Nobel Prize winner Paul Dirac who in 1931 first toyed with the idea that there could be an elementary particle that corresponds to a monopoly. Experimental physicists are still looking for these exotic species in the hope of finding answers to some as yet unanswered questions in nuclear physics through their evidence.

In the following it shall be outlined how and where the sciences succeeded in unraveling some "secrets" of magnetism, and where further research has to be done.

Changes and polarity reversals - The puzzle of the geomagnetic field

In 1905 Albert Einstein counted geomagnetism among the five most important unanswered questions in physics. Today the question of its origin is still waiting for an all-encompassing explanation.

Naturally, only a few statements about the interior of the globe can be considered certain. The processes in the earth's core can neither be observed, nor can the prevailing pressure and temperature be simulated in the laboratory. In order to be able to make well-founded statements on measurements of a giant seismological sphere the size of the earth's core, one is dependent on measurements that are carried out from the earth's surface. Observations indicate that the Earth's core consists of a core with a diameter of 3485 kilometers, roughly from the planet Mars. In the center of the earth's core is the solid inner core, which has a radius of around 1200 kilometers. The pressure at these depths rises to 1.3 to 3.5 million times the atmospheric pressure on the earth's surface. The temperatures are estimated to be between 4,000 and 5,000 degrees Celsius.

Just as difficult as gaining knowledge about the earth's core is the establishment of a theory about the origin of the magnetic field. The earth's mantle, a layer of matter around three thousand kilometers thick, separates us from the outermost zone of the earth's core. We know and do not know what the field looks like there or how it is created. It is believed that it is ten times stronger near the earth's core than at the earth's surface.

Indirectly, however, one can make statements about the origin of the field by observing today's magnetic field. The measurements provide valuable information about the short-term behavior of the field, for example about its shape and about the so-called secular variations, which are the usual fluctuations in periods of decades and centuries. Historical sources also provide information on this. Scientists have been able to gain knowledge of earlier geomagnetic field strengths with the help of museum exhibits and collectibles. Pottery products, some of which are 6,000 years old, were examined by Chinese researchers who came to the conclusion that around 2,000 years ago the magnetic field reached a maximum that was about 50% higher than the current field.

Detailed maps showing the strength and direction of the geomagnetic field have been produced for navigation purposes since the eighteenth century. But the maps also show how the field has changed over the past three centuries. There is a gradual, steady decrease in field strength, which, if continued, would lead to its complete disappearance in about 1500 years. The maps also show a slow shift of some irregular vortices in the field in a westerly direction by about one degree of longitude every five years. In these eddies, a magnetic needle deviates more or less strongly from the north-south direction.

In order to study the paleomagnetic field (that is the field as it looked millions of years ago) one must study the rocks of the earth's crust. For this purpose, the age of the rock is dated radiometrically and the orientation of its magnetized inclusions is determined, which reflect the direction of the magnetic field at the time of rock formation. This paleomagnetic evidence shows that the earth has had a magnetic field for at least 3.6 million years. During this time, the strength of the field has changed several times. It is also proven that the polarity of the earth's magnetic field has reversed several times.

As early as 1906, the French physicist Bernhard Brunhes, who had discovered such rocks, put forward the theory of the polarity reversal of the magnetic field. His thesis sparked a heated debate for over half a century. It was only when studies of radiometrically dated lava stones were evaluated in the 1960s that geophysics assumed it was certain that the earth's magnetic field could be reversed. Two polarity states can exist. In the one, which is by convention referred to as the "normal" state, the compass needle pointing north points to the magnetic north pole. In the "reverse" or inverted state, it would be directed towards the magnetic south pole.

What is certain is that there have been nine important polarity reversals in the past 3.6 billion years, the most recent of which took place around 730,000 years ago. The earth's magnetic field does not suddenly switch from one polarity state to the other, but gradually disappears until it enters a neutral, magnetic field-free phase, in which it remains for about 1000 years. It can then return to its original polarity state or change to the opposite state. 98 percent of the time the field remains stable.

Analog audio frequency devices

On December 6, 1877, it was possible for the first time to record and reproduce speech using a machine. The American Thomas Alva Edison designed the first model of his phonograph based on his experience with recording and reproducing devices for Morse code, the telegraph code at that time. This speaking machine, as such devices were later called in Germany, used a cylindrical tin foil as an information carrier. The sound waves captured by a funnel move a membrane, on the back of which a steel pin rounded at the dome is attached. This embossing device, moved helically over the cylinder, presses a spiral soundtrack into the tin. It is a mechanical audio frequency recording in the form of depth writing. The same device, guided along the resulting groove, sets the membrane in vibrations, which stimulate the surrounding air to produce sound waves similar to the recording. From the beginning of the century, the Edison phonograph and the industrially manufactured recorded sound rollers began to be sold, while Edison himself had already been selling devices for self-recording and playback.

From 1887, like numerous other inventors, the German Emil Berliner, who had emigrated to the USA, dealt with mechanical sound recordings based on the principle of lateral needle deflection_ As a result of his work, in 1895 he was able to produce the first series-produced records in page-writing and the necessary playback devices, from him a gramophone called, bring to the market. The characteristics of the record, namely easier production compared to the Edison cylinder and playback devices without the need to transport the scanning element through a spindle parallel to the cylinder surface and the longer running time, ultimately led to the victory of the record. At the beginning of the century, the big business with stored music and speech as well as the associated recording and playback devices began.

As early as 1878, the American Oberlin Smith, who had bought a phonograph from his friend Edison, described audio frequency recordings using magnetism. He suggested, among other things. suggest, instead of mechanical deformation, to store the signals under the action of a magnetic field corresponding to the signals by magnetizing a ferromagnetic information carrier. For playback, the permanent magnets created during the recording should deliver the original signals using inductive voltage generation. Ten years later he published the principle of his magnetic sound equipment. The information carrier consisted either of homogeneous steel wire or of a cotton thread into which magnetizable metal dust was spun. Because of other activities, he prematurely ended his experiments.

In 1898, the Dane Valdemar Poulsen, independently of Smith, not only reinvented magnetic signal storage, but he also built the first functional device. At the Paris World's Fair in 1900 he presented his telegraph, a sensation of the event. The information carrier consisted of homogeneous steel wire 1 mm in diameter and 100 m in length, fastened in a spiral shape on a drum, which runs at a speed of approx. 2 m / s. The magnetic head was switchable for the three operating modes “recording”, “playback” and “erasing”.

In 1902 Poulsen received a patent for the direct current bias, which he specified for the first time, which was the state of the art in the field of magnetic pretreatment of the information carrier during the recording process until the HF bias was introduced in 1940.

After the drum apparatus, he constructed the reel apparatus in 1901 with a 3 mm wide and 0.05 mm thick band of homogeneous steel as an information carrier. The maximum recording time at a tape speed of approx. 2 m / s is a maximum of 16 minutes.In contrast to the drum apparatus, in which the wire was magnetized in the longitudinal direction due to the construction of the magnetic head, the magnetization here took place transversely to the running direction.

In the disk apparatus he brought out in 1903, the information carrier consisted of a steel plate 13 cm in diameter and 0.5 mm thick. The magnetic head was guided tangentially over the disk surface by means of a threaded spindle, the disk speed increasing according to the respective head position coming from the outside inwards. With a relative speed between head and disk of approx. 0.5 m / s that is constant in this way, the result is the same wavelengths at a recording frequency, regardless of the recording location.

At the beginning of the century, a syndicate under the management of Mix & Genest AG took over the task of exploiting the property rights that had been granted in the meantime. Initially, dictation machines and systems for recording telephone calls were planned, all with steel wire as an information carrier. Despite intensive efforts, interest in magnetic recorders declined after a brief euphoria, so that production at Mix & Genest as well as at companies that had meanwhile been established in the USA had to be discontinued. The main reasons for this were the unsatisfactory quality of the reproduction due to the lack of suitable amplifiers and operating difficulties, mainly due to the use of steel wire or tape.

The appearance of the AEG Magnetophon K1 magnetic tape recorder at the Berlin radio exhibition in 1935 is retrospectively to be seen as the big bang on the way to today's importance of magnetic recordings. For the first time, it used the 6.5 mm wide magnetic tape, invented by Pfleumer in 1928 and developed for series production by IG Farben, Ludwigshafen plant, as an information carrier. It is characterized above all by inexpensive production, usability of different, each optimal storage materials as well as the possibility of simple cutting and gluing.

For the first time, the tape drive in the drive was provided by three motors: One motor for a constant tape speed of 1 m / s in the area of ​​the magnetic heads and one motor each for winding and for fast rewinding. Ring magnetic heads were also used for the first time as electro-magnetic converters for recording and scanning, invented in 1933 by Eduard Schüller, who later headed magnetic sound development at AEG. A two-stage tube amplifier that could be switched for recording and playback was used to amplify the currents to be recorded or the sampled voltages.

Apart from a few individual cases, with the introduction of magnetic tape in the mid-1930s, first in Germany, and after 1945 also abroad, the use of steel wire and steel tape as storage media, the steel age, came to an end.

In 1940 Hans Joachim v. Braunmühl and Walter Weber, employees of the Reichsrundfunk-Gesellschaft, claim that a high-frequency bias of the tape instead of the above-mentioned direct current bias extends the dynamic range, which was previously a maximum of 38 dB, to 60 dB, while at the same time increasing the upper limit frequency from 5000 Hz to 10,000 Hz. Both gentlemen developed the new process for series production in just a few months.With these features, the HF bias, as the process is called in parlance, remained unsurpassed even in comparison with other audio frequency recording processes until the advent of digital technologies. On June 10, 1941, the HF magnetophone and the voice and music recordings made with it were presented to experts in a demonstration in Berlin's Ufa Palast am Zoo. Due to the hitherto unattained quality of a sound recording process, the preparatory work for the application of this state of the art in radio, sound film and record studios began immediately.

In 1943 Weber created the first magnetic tape apparatus for two-channel stereophony. Of the approx. 300 stereo recordings made at that time, only 2 were in the possession of the SFB until recently, the rest has since been handed over to the SFB by the Russians.

Starting with the Magnetophon K2, which came onto the market in 1936, the tape speed was 76.2 cm for a long time, especially for studio equipment. For the first time in the miniature magnetophone R 26 developed by RRG for reporting purposes, set up for HF pre-magnetization, a belt speed reduced to approx. 18 cm / s was used.

During the war, a large number of magnetic sound recorders for military purposes emerged in both the USA and Germany. While the US devices only used steel wire for storage, the German devices with the Wehrmacht designation Tonschreiber only ran tape as an information carrier. Particularly noteworthy is the tone recorder b, in which four magnetic heads were arranged on the circumference of a head wheel driven by a special motor for time expansion and time lapse during scanning. In this way, while maintaining the relative speed between the tape and the rotating heads, the absolute speed of the tape compared to the recording could be changed without changing the original pitch and in this way the playback time compared to the original could optionally be lengthened or shortened.

It can no longer be determined afterwards why, despite numerous publications in German specialist and daily newspapers, magnetic tape technology was initially limited to Germany. The US Major Jack Mullin, an audio frequency specialist, got to know the HF magnetophone during a tour of Bad Nauheim in 1945. He procured a direct current K4 magnetophone, shipped it to his hometown of San Francisco, added an HF generator to the equipment and demonstrated the German state of the art to experts in May 1946. This was the starting shot for numerous magnetic tape activities, initially in the USA, and from 1950 also in Japan. The German knowledge, procedures and equipment were available to the victorious powers free of charge as "seized enemy property". In numerous "BIOS" and "FIAT" reports from 1945 onwards by the British and Americans as well as in the "reports" of the Russian Technical Office for Cinematography in Potsdam-Babelsberg, the German status, including the magnetic sound technology, was recorded and stored in the archives.

Until 1947, all magnetic tape recorders were intended for commercial purposes in terms of electro-acoustic data, effort and operation. In this year, the US company Brush began selling the first Sound Mirror BK 401 device designed for home use. The developer was Semi Joseph Begun, who was responsible for the magnetic sound area at what was then C. Lorenz AG until 1935.

The steel tape recorder he developed in the early 1930s, intended primarily for broadcasting purposes, was only built in small numbers because of the AEG magnetic tape recorders available from 1936 onwards. The Sound Mirror opened up a market for amateur devices, including the associated magnetic tapes, that has continued to grow today.

Proposals from the year 1941 for the construction of magnetic film drives for recording using HF pre-magnetization come from the company Klangfilm GmbH. At this time both the AEG and the Tobis Studios in Berlin-Johannisthal were engaged in preliminary magnetic film tests. Between Magnetophon GmbH, in which the interests of AEG as a device manufacturer and those of IG Farben as a magnetic tape manufacturer had been combined since 1942, negotiations began in 1943 with Ufa-Film GmbH as a possible buyer of magnetic films. In the agreement signed in 1943, the introductory paragraph I stated: "Both parties will largely work together for the purpose of utilizing the magnetophone method for sound film recordings". The events of the war prevented it from being realized.

Caused by Mullin's magnetophone demonstrations, preliminary tests began in the USA from 1946 with the aim of introducing magnetic films in studios. Due to the excellent results, provisional systems were initially built by installing magnetic heads in optical sound cameras, until in 1948 the US company Hallen Co. brought the first series-produced magnetic film drive onto the market. It was very similar to the magnetophone in terms of film run.

At the suggestion of the former Ufa technician Dr. Martin Ulner developed in 1949 Wilhelm Albrecht in his company MWA GmbH the magnetic sound camera MTK l, which was set up for 17.5 mm magnetic film. The actual film flow in it, the arrangement of the mechanical filters, the quick-start device, to name just a few features, were optimal and forward-looking for the conditions at the time. At the end of March 1950, the English film “The silk loop” was dubbed with this drive in the Ufa Studios in Berlin-Tempelhof.

Magnetic sound was also soon introduced for film screening purposes in cinemas. The immediate reason for this from 1952 onwards were attempts by various US film producers to make the cinema more attractive through wide-screen film screenings. Wide film walls require a multi-channel sound system to assign the picture and the associated sound. According to the state of the art at the time, the usual optical tone method was not suitable for this. As a result of a series of proposals, the four-channel CinemaScope method of 20th Century Fox prevailed from 1953 onwards with four magnetic sound tracks attached to the screening copy. Three of the channels fed the hall loudspeakers set up behind the projection wall, the fourth track the effect loudspeakers in the hall. Due to the greater expense compared to optical sound technology, the magnetic sound could not hold up in the cinema, especially since the Dolby system noise reduction process introduced in 1978 provided good quality with two-channel optical sound reproduction. In addition, digital optical sound methods with excellent sound quality when reproduced via 6 channels have recently emerged.

Another significant step towards the current state of magnetic tape technology concerns the assembly of the tape, which is apparently a minor matter. In the case of magnetic tape drives, which are mostly operated by specialists, the tape is still housed on tape disks using cores. In the Wehrmacht recorders, the tape was on reels. Two reels were used to accommodate the storage wire in the Dailygraph dictation machine, which appeared on the market in 1929, and are housed in a cassette to simplify replacement.

After the war, various cassette designs appeared on the market in Germany and the USA, now for magnetic tape. With the exception of an endless cassette, which is mentioned under 1.1 on p. 11, no product could hold its own. For a large number of people interested in magnetic sound, however, inserting and changing the tape was too difficult - a real obstacle to sales. That changed from 1963. First at the International Motor Show in Frankfurt and then at the Berlin Radio Exhibition, Philips showed the EL 3300 pocket recorder, developed in the Belgian factory in Hasselt, "an interesting tape recorder for the driver", as a press release said.

To accommodate the 3.81 mm wide tape, which ran at a speed of 4.75 cm / s, the now globally introduced and standardized Compact Cassette, abbreviated to CC, was created. This contained not only the tape manufactured by BASF with the necessary winding cores, but also tape guide elements that were previously always device-proof. In the course of time, first at Philips, then also at competitors, to whom Philips allowed the construction of CC devices without a license, cassette recorders suitable for stereo operation with HIFI features were developed. The prerequisite for the achieved playback quality were the tapes, initially developed by BASF and then by numerous other tape manufacturers, with excellent storage and operating properties.

The success of devices and cassettes on the market was initially not based on playback criteria, but on the significantly simplified device operation compared to all previously released devices, especially when inserting and changing the cassettes. An unexpected boom in the magnetic sound business began in all sectors: devices as well as blank and recorded cassettes.

Akio Morita, Chairman of Sony, came up with the idea of ​​developing a portable, lightweight tape recorder for use with headphones that could only be used to play back recorded CC's. The Valkan was born. In the summer of 1979 the first Walkman TPS-L 2 appeared on the market, which was followed by a wide range of other devices, also with other features.

Soon, not only playback but also recording were possible, the built-in receiver allowed radio reception and recording and the like. The business, in which other manufacturers are now also participating, continues to expand. Finally, commercial reporting devices with the features of the Walkman were also created: set up for operation with CC, but small and light.

The N 1000 ZXL cassette recorder from Nakamichi from 1978 contains two striking innovations. The built-in electronics automatically determine the magnetic values ​​of the storage medium before the start of a recording and another automatic system sets the HF bias current.

In 1984 Sony came up with the Beta HIFI Recorder SLHF, which was for the time being the last “first” in the field of analog magnetic sound technology: the carrier-frequency recording of the two audio frequencies of a two-channel stereo signal. It was the first application of helical track recording for audio frequency purposes, which results in a high relative speed between the information carrier and the heads. The helical track method was invented in 1953 by Eduard Schüller, then Telefunken GmbH. It was originally intended for video purposes.


Finally, two magnetic tape recorders are mentioned, which are little known but are of great importance:
Most aircraft disasters can be reconstructed because the flight data recorder records a large number of important operational data such as the aircraft drive, controls, course and, at the same time, the cockpit voice recorder records on four channels the conversations held in the cockpit during the last half hour and the radio traffic become. Both devices are located in the rear of the aircraft and are designed in such a way that even after a crash under the extreme conditions mentioned below, the magnetic tape in the devices can still be scanned: accelerations of up to 1000 g; Fire at 1000 ° C for 30 min; Pressure of 4.5 t; Remain at a depth of 12,000 m for 30 days.

Some details of the Fairchild Cockpit Voice Recorder A 100 from US company Loral Data Systems, which has been in use since 1984: A 6.35 mm wide magnetic tape is used as the information carrier, housed in an endless cassette with a cycle time of 30 minutes for a cycle at a tape speed of 9 , 5 cm / s The DMI Eclipse magnetic tape system from Sonotron GmbH for recording and playing back long-term electrocardiograms differs considerably from devices used in entertainment electronics. The battery-operated Holter cassette recorder, so named after its inventor, is worn on the body during the ECG recording. Some features of the recorder: At a tape speed of 1 mm / s, a C 60 cassette in one direction of travel is sufficient for 24 hours of recording, 2 tracks for ECG recording, 2 time tracks; Frequency response 0.05 Hz to 100 Hz; Wow and flutter 0.010 / o. The Sonotron playback drive type DMI Eclipse is used to read out and evaluate the magnetically stored cardiac current curves. In this the belt runs at a speed of 5. . . 25 cm / s.This results in higher frequencies when scanning compared to recording and the evaluation time is reduced to a minimum of 25 minutes for a 24-hour recording.

1.2 Digital audio frequency devices

Given the advantages of digital processes mentioned under 3.2 p. 42, especially in connection with magnetic storage, it seems strange that it was not until 1977 that a digital system came onto the market with the Sony Processor PCM-1. This first processor works in conjunction with a commercially available video recorder that serves as a magnetic sound drive. The late start of this future-oriented system is due to a long development process and the fact that the implementation of the associated signal amplification, processing and storage requires a considerably higher number of amplifying, switching and storage components compared to analog applications. They also have to perform their functions in extremely short time intervals. In short, digital technology requires special, sophisticated integrated circuits. These have only existed since the 1970s. Laboratory equipment for digital magnetic tape recording was developed by the BBC and NHK from the mid-sixties. The two formats described below are now accepted worldwide. Numerous devices are based on them.

I. DASH (Digital Audio StationaÄ ± y Head)

In 1980 Sony announced a 24-channel digital recorder for studio use based on the DASH format. After the companies Studer and 1989 Tascam also brought DASH recorders onto the market in 1980, DASH, expanded to 48 channels, has become the most widely used digital studio format.

Some features:
Longitudinal track recording, so that mechanical cutting of the tape is possible; fixed, multi-lane heads; Band width 12.65 mm; Belt speed 76.2 cm / s; Sound track width 0.17 mm each; Distance between audio tracks (lawn) 50 µm shortest wavelength = 2 bits = 1.98 µm data rate / channel 1.152 Mbit / s; Recording density 1.52 kbit / mm 2; Tape on reel.

DASH audio frequency data of the Studer D820-48 tape machine:
Total harmonic distortion at 0 dB 0.0060 / o; Frequency response 20 Hz _ _. 20,000 Hz +/- 0.3 dB; Signal to noise ratio 85 dB; Wow and flutter cannot be measured.

2. R-DAT (Rotating Head Digital Audio Tape), for some time now referred to as DAT.

In 1986 Sony presented the prototype of a cassette recorder designed for home and semi-professional use in the USA, based on the DAT format, which was soon followed by devices from other manufacturers.

Some features:

Helical recording; unlike conventional video recorders, the tape wraps around the head drum at just 90 °; Band width 3.81 mm; Belt speed 8.15 mm / s; Relative speed of tape head 3.133 m / s; Track width 13.5 µm no lawn; to avoid crosstalk between the tracks, the azimuth of the two rotating heads deviates by +/- 20 ° from the vertical; shortest wavelength = 2 bits = 0.67 mm; Data rate 7.5 Mbit / s; Recording density 177 kbit / mm 2; Tape in cassette similar to Compact Cassette, but almost hermetically sealed outside the recorder; 2 channels for stereo operation; two 0.5 mm wide longitudinal tracks for special purposes; maximum running time 2 hours or 4 hours with slightly reduced quality.

DAT audio frequency data of the Sony DAT Recorder DTC-55 ES:

Total harmonic distortion at 0 dB 0.0050 / o; Frequency response 2. . _ 20,000 Hz +/- 0.5 dB; Signal to noise ratio 92 dB; Wow and flutter cannot be measured.
The device contains a dubbing lock to prevent multiple copies of compact discs.

1.3 Analog video frequency devices

Intensive work began in the USA in the early 1950s with the aim of using magnetic recording technology for video purposes as well. Initially, an attempt was made to achieve the high limit frequencies required compared to audio frequency applications by increasing the tape transport speed. In 1951, for example, the magnetic tape in a video recorder development started by Bing Crosby Enterprises ran at 18 m / s.In 1953, the tape speed for a test system from Radio Corporation of America was still around 9 m / s for acceptable image quality. A bobbin with a diameter of approx. 42 cm was required to run for four minutes. I.a. Because of this unwieldy amount of tape and other side effects of the high tape speeds, the longitudinal track recording did not lead to a useful result at that time. The process experienced a brief renaissance in 1979 in the form of the LVR recorder from BASF. However, it was too late by then: Devices with both moving information carriers and moving magnetic heads produced the high relative speeds between tape and magnetic head required for high frequencies.

Some typical devices show the further course of development from the mid-1950s to the present day.

In April 1956, the company Ampex, which supplied the first commercial US studio tape recorders from 1948, presented the professional world with the first operational video recorder: Model Mark IV. This laboratory device was the forerunner of the Ampex VR 1000, which was then manufactured in quantities. Inch video tape ran at a speed of 38.1 cm / s.At a small angle perpendicular to the direction of travel of the tape, four magnetic heads attached to a headwheel, optionally used for recording and scanning, wrote transverse tracks on the tape at a speed of 250 rpm. The relative speed between the heads and the tape was thus 3.81 m / s.The system, designed exclusively for studio use, delivered black-and-white images of an acceptable quality. Tape requirement for 1 hz 1440 m. The associated tape with the designation Magnetic Tape # 179 came from the 3M company.

 

In Germany, Südwestfunk was the first broadcaster to acquire a system that was converted by Siemens & Halske for the previous television system.

 

Toshiba took the next step towards the current state of the art with the first laboratory model of a recorder, completed in 1958, based on the helical track recording mentioned under 3.3 p. 48. From 1959 Toshiba supplied the helical track recorder VTR-1, already set up for color operation in NTSC standard. The video tape was 2 "(inches) wide, ran at a speed of 38.1 cm / s and the head wheel rotated at a speed of 60 r / s, resulting in a relative speed of 40 m / s. The shortest wavelength was 6 µm The head recorded a track corresponding to the field of the television frame.

 

All of the video recorders described below use the helical scan method to record the video signals. The tapes are in cartridges. The “shortest wavelength” is to be understood as the recorded wavelength.

 

After a few devices developed earlier for semi-professional purposes, Philips introduced the type N 1500 for the first time in 1970, a color-capable video recorder for amateur purposes.

 

Some features:

 

Band width 12.65 mm; Belt speed 14.29 cm / s; Relative speed head / tape 8.08 m / s; Video track width 130 µm, lawn width 57 µm, shortest wavelength 1.7 µm, 2 longitudinal tracks for sound.

 

Sony, which has also been involved in video recorders since 1962, began supplying the U-Matic series of devices in 1971, intended for semi-professional purposes.

 

Some features:

 

Band width 19.05 mm; Belt speed 9.53 cm / s; Relative speed head / tape 8.54 m / s; Video track width 85 µm; Lawn width 52.3 µm, shortest wavelength 1.581 µm; 2 longitudinal tracks for sound.

 

The following devices represent the current state of the art:

 

1. for studio purposes

 

BTS BVW-75P Betacam SP Recorder for PAL standard, stationary;

 

Some features:

 

Band width 12.65 mm; Belt speed 10.15 cm / s; Relative speed head / tape 5.75 m / s; Luminance track width 86 µm; Chrominance track width 73 µm lawn width 1 µm; shortest wavelength 0.6 µm; Running time 110 min; 2 longitudinal tracks for sound, 1 track each for time code recording and 1 control track; built-in time code generator and extractor; built-in time base corrector for transmission signals in exact synchronism; numerous built-in add-ons for image processing.

 

There are a number of professional camcorders based on the Betacam SP format, including: Sony BVW-300AP.

 

2. for amateur purposes

 

2.1 according to the format Hi8

 

Sony EVS 1000

 

Some features:

 

Band width 8 mm; Belt speed 2 cm / s; Relative speed head / tape 3.1 m / s; shortest wavelength 0.4 µm; Max. Running time 180 min; 2 longitudinal tracks.

 

2.2 according to the S-VHS format

 

Grundig VS 680 VPT

 

Some features:

 

Band width 12.65 mm; Belt speed 2,339 cm / s; Relative speed head / tape 4.87 m / s; Track width 0.5 µm for “long play” 25 µm, shortest wavelength 0.8 µm; Running time 240 min in normal operation; 2 longitudinal tracks.

 

For the amateur formats Video 8, Hi8, VHS and S-VHS there are, in addition to recorders, numerous camcorders with different optical equipment, viewfinders, audio components and video quality parameters.

 

Finally two examples of constructive top performance in the field of consumer devices.

 

1. Camcorder

 

The Mitsubishi HS-C50 camcorder, first shown at Photokina Cologne in 1990, contains all the components of a color video camera, a 470,000 pixel semiconductor image converter, electronic viewfinder, autofocus optics and a complete S-VHS video recorder. Shortest wavelength 0.7 µm the stereo sound belonging to the scene is recorded at carrier frequency using the depth multiplex method. Dimensions: l30 x 145 x 355 mm (WxHxD); Weight including battery for 45 minutes of operation: 2.1 kg.

 

2. Still video camera

 

This ION RC-260 camera, launched by Canon in autumn 1990, does not store the image photochemically, but magnetically on a floppy disk drive built into the device. The video disk with a diameter of 2 inches has a capacity of 50 color images, which can be viewed immediately after recording on the screen of a television set and deleted if necessary. With the help of a special printer, colored paper pictures can be produced. The camera features correspond to those of a conventional high-performance SLR camera. 23,000-pixel solid-state imager; Disc speed 3000 rpm; Track width 60 µm lawn 10 µm shortest wavelength 0.48 µm video output for connection to television sets; Dimensions 111.5 x 42 x 113 mm (WxDxH); Weight including battery for 250 shots: 475 g.

 

With the exception of the cassette system used in the Philips N 1500 recorder, the originally company-specific video formats described ultimately resulted in standards that benefit the amateur video market, which has not yet been assessed in terms of its extent, across companies and countries.

 

1.4 Digital video frequency devices

 

Before a broadcast, the picture and sound information intended for it must be processed in terms of content and technical quality optimization. This usually involves making several generations of tape recordings. Their quality constantly decreases with the number of copies of copies, which considerably limits the range of processing options for video tapes that have been copied using analog methods.

 

As described under 3.4 on p. 49, this handicap of all analog video recordings does not theoretically exist in digital processes. According to the current state of the art, around twenty generations can be achieved without any noticeable loss of quality. For this reason in particular, preparatory work began in the 1980s for two processes that meet the requirements mentioned. Both methods have helical track recordings of the video information in common, the bandwidth of the magnetic tape of 19.01 mm, four digital audio channels and the shape of the cassette housing.

 

First of all, at the suggestion of the responsible EBU and SMPTE committees, after years of in-depth discussions, the Dl component format was created. It is designed for systems in which the components of the television signal - luminance, chrominance, color saturation - are available separately and are not available as composite video signals in PAL-coded form. Using the component method, the individual types of signals can be optimally processed while increasing quality.

 

Some features of the Dl format: belt speed 28.68 cm / s; Relative speed head / tape 35.63 m / s; Video track width 40 µm; shortest wavelength 0.9 µm; Data rate 227 Mbit / s; Recording density 49.38 kbit / mmz. Metal pigment tape is sufficient.

 

The proven advantages of digital technology prompted Ampex and Sony to develop a second digital video system, also with the possibility of flawless copy production up to the twentieth generation. This is able to digitally save FBAS-Pal or FBAS-NTSC signals, which were previously recorded in analog C format systems, for example, on magnetic tape: The PAL D2 composite format or NTSC D2 composite format.

 

Some features of the D2 format: belt speed 13.17 cm / s; Relative speed head / tape 30.4 m / s; Video track width 35.2 µm; shortest wavelength 0.79 µm; Data rate 152 Mbit / s; Recording density 74.76 kbit / mmz. Metal particle tape required.

 

Two examples of digital video recorders currently on offer:

 

1. Sony DVR- / DVPC-1000 recorder for D1 format; stationary;

 

Some features:

 

Running time 94 min; digital inputs and outputs for use in connection with digital component video systems; Additional devices for use in FBAS systems; switchable for 625 lines / 50 Hz - 525 lines / 60 Hz; automatic tracking of the magnetic heads during playback for optimal scanning; numerous built-in additions for image processing.

 

2. BTS DCR 18P recorder for D2 format; stationary;

 

Some features:

 

Running time 208 min; analog inputs and outputs for use in connection with analog PAL systems; Additives for use in digital component systems; Playback of the video and audio signals intended for recording possible before recording; Rewinding speed max. 100 times normal speed; automatic tracking of the magnetic heads during playback for optimal scanning; numerous built-in add-ons for image processing.

 

The relevant companies are also working on the development of suitable digital video recorders for the announced HDTV television processes. For example, Sony demonstrated a system as early as 1989, set up for the Japanese Hi-Vision standard with 1125 lines / 60 Hz, field scanning.

 

Some features of this laboratory device:

 

Band width 12.65 mm; Belt speed 80.5 cm / s; Relative speed head / tape 51.5 m / s; Video track width 27 µm; shortest recorded wavelength 0.69 µm; Data rate 1.188 Gbit / s; Recording density 77.7 kbit / mmz. Metal pigment tape required.

 

European industry is also working on digital video frequency magnetic tape recorders as part of the Eureka 95 project. These will achieve 1250 lines / 50 Hz with full screen display.

 

1.5 Storage and carrier materials for magnetic information carriers

 

From the beginning of magnetic storage technology until the mid-thirties, homogeneous steel in the form of wire, or more rarely tape, was the only storage material used. Because the memory is at the same time a carrier, namely metal, it is not possible to cut in an operational manner in order to process the recorded audio recordings. Wire also has a functional disadvantage compared to the information carriers commonly used today: it twists during the run, so that the defined association between wire and magnetic heads required for optimal magnetization and scanning is impossible. In retrospect, it was one of the striking steps in the development of magnetic storage technology when Fritz Pfleumer not only invented the magnetic tape in its current form at the end of the twenties, but also successfully endeavored to manufacture and market it. In his DRP 500 900 "Lautschriftträger" issued in 1928, it is described that "the. . . Powder with permanent magnetic properties _ _. is applied to the entire surface of the substrate in an even layer thickness and thickness ". According to this recipe, quantities of information carriers that can no longer be recorded have been created. It does not diminish the importance of Pf1eumer's work that in 1936 his patent was null and void because the American O'Neill had received a corresponding property right in the USA shortly before him. O'Neill's idea remained a desk invention. As a storage material, Pfleumer initially used powdered hardened steel, which he fixed in the form of a suspension on paper tape. The research and development work that began in 1932 at IG Farben, Ludwigshafen plant, now BASF, and is still carried out in numerous companies around the world, has resulted in advances in both magnetic and mechanical information carrier properties: improvements in remanence, coercive field strength, particle dimensions, Fixation of the storage materials during production in a preferred direction and, in the case of the carrier materials, improvements in strength, surface properties and flatness as well as reduction in thickness, to name just a few of the essential parameters.

 

Based on steel dust, the following storage materials have been manufactured over the course of time:

 

1934. . 1936 carbonyl iron; it followed in 1936. 1938 Fe3O4; 1938 until today gamma Fe2O3; 1970 to the present day CrO2; 1980 until today metal pigment (pure Fe); Metal vapor deposited from 1980 to the present day (pure Fe); 1972 until today cobalt-doped gamma Fe2 = 3; 1991 until today cobalt-doped CrO2.

 

In addition, there were at times two-layer tapes, for example with a lower layer made of gamma Fe2O3 and an upper layer made of CrO2.

 

Already in the first 50,000 m of type C magnetic tape, acetylcellusose, which was supplied by IG Farben Ludwigshafen in 1934, was used as a carrier for the storage substances, used up to around 196l; it followed in 1943 ._ 1968 polyvinyl chloride = PVC, from 1965 until today, polyethylene terephthalate, abbreviated to PET.

 

1943 ._ 1955 so-called mass tapes were produced, in which the storage material gamma Fe2O3 was mixed with PVC to a homogeneous mass, rolled out and cut into tape form. Because of the customary IG designation "Luvithenn" for PVC, such tapes were designated type L.

 

Perforated carrier material in the form of magnetic film as well as that with an applied light-sensitive emulsion is used where the carrier must either be transported in a form-fitting manner or the perforation is used to determine the expiring carrier length.

 

For example, raw film material with magnetic tracks is used for 16 mm and 8S color reversal film. There are also films that are only tracked after they have been developed, for example magnetic-sound image film copies.

 

1.6 Shapes, formats, tracks and their spacing for magnetic information carriers

 

The first sound carrier ever used in Poulsen's telegraphone was steel wire 1 mm in diameter, which was soon followed by 0.05 mm thick, 3 mm wide steel tape. In 1931, Pfieumer used 16 mm wide magnetic tape with one track in both directions for a demonstration drive. In the magnetic tape era started in 1935 with the AEG Magnetophon Kl, the full width of the tape of 6.5 mm was used as a track. The two-lane tapes, also 6.5 mm wide, were an exception in connection with the push-pull tests carried out by the AEG in 1938 and, from 1943, for the stereophonic system developed by the RRG. When Americans got to know German magnetic tape technology in 1945, they mistakenly assumed that the width of German tapes was 1/4 "(inch) = 6.35 mm. From the beginning, the US tape manufacturers supplied tapes in this width abandoned the 6.5 mm band width and instead set 1/4 ". At the moment there are the following bandwidths are common: l / 8 ”; 1/4 "; 1/2 "; 3/4"; l "; 2". Several tracks, for example 24 on 2 "tape in the case of analog devices and up to 50 tracks on 1" in digital devices, occur frequently.

 

Perforated magnetic tape, referred to as magnetic film, exists in dimensions and perforates corresponding to the 16 mm and 35 mm film formats, in addition as a 17.5 mm wide split film with a row of perforations.

 

In some dictation machines, a perforated, for example 97 mm wide, magnetic sheet is used and is discussed and scanned line by line like a typewriter page.

 

The first magnetic disk had a diameter of 130 mm and was made of homogeneous steel. It was used in Poulsen’s telegraphone recorder, first shown in 1903. In the meantime, there are numerous disc-shaped information carriers with different diameters of, for example, 8 ”, especially for storing data in computers, typewriters and still video cameras; 5.25 ", 3.5" and 2 "with separate carrier and storage materials as with tapes.

 

The storage layers are often applied to both sides of the plate and largely correspond to those of the tapes. The carrier material used for hard disks is aluminum and, more recently, glass, and polyester films for floppy disks. Because of the relative movement of the head position with respect to the disk surface, which is necessary during operation, the drives have special mechanisms for guiding the magnetic heads, preferably radially over the surface of the tape. This is not necessary for magnetic disks with spiral grooves, which are used in some dictation machines. Here the pointed magnetic head slides in the groove, similar to the pickup on a conventional record.

 

Right from the start, efforts were made to keep the memory space required to store an event as small as possible. The specific storage area, characterized by its length in the recording direction and its width across it, depends for the shortest wavelength to be recorded essentially on the coercive field strength of the storage material, the magnetic head data and the scanning voltage required for dynamic reasons.This, in turn, is proportional to the scanned track width, along with other parameters.

 

In order to reduce the area required for magnetic storage of a message, improvements in the properties of the storage material, the magnetic heads and the drive mechanics have been constantly being worked on.

 

The results using the shortest possible wavelength and a dynamic range of 60 dB are impressive, as the following examples show.

 

1941: Shortest wavelength 1 = 100 µm; scanned track width b = 6500 µm; Area F = 650,000 µmz;

 

Apparatus: Magnetophon K4 converted to HF pre-magnetization; Tape: Type C with gamma Fe2O3;

 

In order to generate the upper limit frequency of 10,000 Hz from the stored wavelengths, a belt speed of 100 cm / s is required.

 

1991: Shortest wavelength 1 = 2.35 µm; scanned track width b = 600 µm; Area F = 1410 µmz;

 

Equipment: Studer Cassette Recorder B215; Volume: BASF CrO2 II; In order to generate the upper limit frequency of 20,000 Hz from the stored wavelengths, a belt speed of 4.76 cm / s is sufficient.

 

The improvements mentioned also benefit digital storage.

 

In the case of multi-track recordings, because of the magnetic cross-talk, the tracks must not be as close to one another as desired without special measures: a lawn is required. This can be dispensed with if the head gap angles of adjacent tracks differ from one another.

 

Some examples of track divisions and distances on tapes from the consumer sector:

 

Track positions on a CC tape Fig. 1

 

Track positions on a VHS tape Fig. 2

 

Track positions on a DCC tape Fig. 3

 

Magnetic information carriers are supplied in various configurations:

 

Ribbons and others on cores, on reels, in magazines, also known as single-hole cassettes, and in cassettes; Plates et al. in bags, cassettes and jackets.

 

Fig. 1 Fig. 2 Fig. 3

 

1.7 Trends in competition: procedures and future prospects

 

As a magnetic storage device in the field of entertainment electronics, the compact cassette is of paramount importance. Here are some figures for worldwide sales Year 1989 1990 I.

 

conventional records 340 million 260 million

 

Compact Discs 615 million 780 million

 

Empty cassettes 1,600 million 1,600 million

 

Sprinkled cassettes 1,000 million 970 million

 

As you can see, sales of records and recorded cassettes have declined in favor of CDs. Obviously, among other features, the optimal playback quality of the CD based on digital storage is one reason for this phenomenon Compared to recorded CD's, higher manufacturing costs and thus price are hardly in the trade.

 

The DCC (Digital Compact Cassette) format, developed by Philips and presented in January 1991 at the NAB in Las Vegas and at the IFA in Berlin in August 1991, promises a change in the situation: The associated cassettes can be carried out by fixed magnetic heads, can be copied in the same way as conventional CDs at a much higher speed than in real time. This is not possible with DAT cartridges.

 

Some features of the DCC format:

 

Longitudinal track recording; Band width 3.78 mm; Belt speed 4.76 cm / s; Track width 185 µm lawn width 5 µm; 8 tracks for sound and 1 track for auxiliary code purposes; shortest wavelength = 2 bits = 0.99 µm; Data rate 768 kbit / s; Recording density approx. 11 kbrt / mmz tape in cassette similar to compact cassette, but almost hermetically sealed outside of the recorder, so that the box that was previously used is superfluous for many applications; 2 channels for stereo operation; Running time 2 X 60 min.

 

The lower data rate compared to DAT, a prerequisite for using the longitudinal tracking method at the specified tape transport speed, is based on the PASC (Precision Adaptive Sub-band Coding) method, according to which not all audio frequency components are stored, but only those above the hearing threshold that is dependent on the overall spectrum .

 

Chromium dioxide, which cannot be used with DAT, is sufficient as a storage material. Both CC and DCC can be played in the announced recorders - an attractive way of comparing analog and digital audio frequency technologies.

 

Overall, regardless of the chip competition, a steady increase in magnetic storage is to be expected. This applies to the video recorder market, for example. The need for devices and cassettes is growing by leaps and bounds. Camcorders are increasingly replacing the amateur cine film cameras. This gradually disappears a field of application of photochemistry in favor of magnetic storage. Some figures from the Federal Republic:

 

Year 1984 1990

 

Empty video cassettes in millions of pieces 36 101

 

Video programs in million cassettes 2.1 9.8 *

 

VCR 1.49 3.3

 

Camcorders in millions of units 0.02 0.79

 

* Estimated figures, sales to retailers, shift from rental to purchase cassette, especially in 1990; here without approx. 5 million cassettes in the lowest price segment.

 

At the present time nothing can be said about the future importance of magneto-optical processes and recordable and erasable CDs based on them.

 

Increasingly, data-storing systems and devices are coming onto the market which contain no moving parts and no moving information carriers. The storage is done purely electronically in digital form using highly integrated circuits. Numbers corresponding to the signals are recorded as capacitor charges for the duration of the storage time. In contrast to devices with magnetic information carriers, the memory ICs are generally device-proof. In order to maintain the storage, they need a constant DC voltage, which altogether limits their possible uses.

 

Two examples of electronic data storage in consumer devices.

 

1. CODE-A-PHONE telephone answering machine model 2760

 

(Storage of audio-frequency signals)

 

While a recorder with a micro cassette is used to record the telephone call, the main announcement is electronically stored for up to 28 seconds and the final announcement for up to 4 seconds.

 

2. Fuji Still Video Camera DS-X

 

(Storage of video-frequency signals)

 

Still video cameras from various manufacturers have been around for a number of years, of which a Canon device is mentioned as an example under 1.3 on p. 15. So far, magnetic disks have saved the images in all models. The Fuji-Camera DS-X, which was first shown in the USA in 1990, instead contains an IC solid-state memory with a storage capacity of 18 Mbit, suitable for storing 6 color photographs in "high quality".

 

Electronic storage applications will increase with further reductions in the price of 16 Mbit and later availability of 64 and 256 Mbit ICs. Because of the elimination of the drive mechanism, such devices are characterized by low power and space requirements and the lack of components that are subject to wear and tear. This opens up further areas of application for signal storage.

 

2. Basics and terms

 

2.1 Communications technology

 

It has been a human concern since time immemorial to collect, forward, reproduce and process information. According to Duden, information can mean news, information, instruction, clarification.

 

As described in DIN 40 146 “Terms in communications technology”, message is a colloquial word that has many and varied meanings. The conceptual content of a message can be represented by various, preferably physical, states and events, such as electrical, magnetic, mechanical or optical processes, and also in the form of written, printed or embossed letters, words and sentences as well as in still and moving images .

 

The representation of a message as a whole or a part thereof by physical quantities, e.g. B. by electrical voltages and currents, magnetic field strengths, especially their changes, are referred to as a signal.

 

In many cases, such processes involve vibrations of various types, for example sound. According to its physical nature, this consists of mechanical vibrations of elastic media. The theory of sound, of tones, of sound technology, is called acoustics.

 

Data are standardized messages in a special way.

 

The signals used for message transmission change as a function of time according to a time function. If, as is the case with air pressure fluctuations in the course of a conversation, the time function of the corresponding signal changes continuously in terms of value and time, it is referred to as an analog signal. The principle of a value and time-continuous signal is shown in Fig. 4.

 

Fig. 4

 

Of the other three possible types of time functions, namely continuous-value and discrete-time; Discrete-value and time-continuous as well as ultimately discrete-value and time, the last variety has a special meaning. In the case of these digital signals, the signal parameters containing a message generally consist of samples, as shown in FIG. 5, which are then available as data in series of numbers following one another in time.

 

Fig. 5

 

There are also numerous other forms of discrete-value and time-discrete messages, for example a series of successive phase images, used in cinematography.

 

A location function arises when messages are stored on an information carrier that is moved relative to the recording element. The scanning of the stored information again results in the original time function. The record groove, with its deflections corresponding to the signal that is continuous in value and time, is an example of a position function.

 

A message is transmitted from a source to a sink, often amplified, processed, fed to a memory or information carrier by a recording device, retrieved from this time-independent and often also independent of the recording location by a playback device and occasionally reproduced in a copier.

 

In the course of a message transmission it often happens that, firstly, the physical state which represents a message has to be brought into a different physical state by a converter when the message content is received. Secondly, conversions from time to location functions and vice versa are often required, and thirdly, value and time continuous functions into value and time discrete functions and vice versa, that is, conversions from analog to digital messages / signals and vice versa.

 

For each of these tasks there are categories of transducers. Each converter acts as a sink for the incoming message / incoming signal and as a source for the outgoing message / outgoing signal.

 

The first category combines acoustic, electrical and magnetic in any combination