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History of Magnetic Recording
History of Magnetic Recording
History of Magnetic Recording
Myth and folklore abound with tales of how the human voice has been captured or preserved, but the only known early attempts to realize this idea were the efforts to construct mechanical devices which would "talk." As early as the third century B.C., Heron of Alexandria is said to have devised automata which would talk or emit the cries of animals. The earliest such device of which we have any actual knowledge is the one constructed by De Kempelein of Vienna, who, in 1791, published a description of it under the title "The Mechanism of Speech, Followed by a Description of a Talking Machine." De Kempelein's device could be made to speak short sentences.
The construction of a machine that would record the human voice, however, had to wait on the progress of the science of sound. Acoustics has always been more or less the stepchild of the physical sciences. Man's earliest attempts to understand the physical world were almost necessarily confined to those aspects of it which he could see or measure directly, and the basic phenomena of acoustics are all too minute and too rapid to be dealt with other than indirectly. Although relationships between the frequencies of vibrating strings had been known since the time of Pythagoras (550 B.C.), the absolute frequency of a musical note was first measured by Mersenne, who published his results in the "Harmonie Universelle" in 1636.
It was an amateur who invented the device that ushered in the modern era of acoustics. On June 26, 1857, a French typographer, Leon Scott, deposited with the Acad-mie des Sciences a paper describing an invention which he named the "Phonautograph." This ingenious device consisted of a horn with a thin membrane stretched over the end, to which was attached a stiff bristle. Rotating beneath the bristle and advanced by means of a lead screw was a smoked drum. Sounds directed into the horn caused the bristle to move back and forth across the smoked paper and trace out a waveform. For the first time in history it was possible to see sound, and the waveform traced by the machine was the first side-to-side, or "lateral," recording.
The dazzling possibility that seems totally to have escaped both Scott and the great acoustic experimentalist Koenig, who actually constructed the first
phonautograph, was the simple idea of reversing the process and getting back the original sound which had been recorded. Within a few years of Scott's announcement, however, a number of people suggested the principle of the phonograph, although there is no evidence that any of them actually constructed a machine. One of them, I'Abb- Lenoir, even used the name "phonograph" in a paper he wrote on the subject in 1877, the year Edison built his machine.
Edison's phonograph operated by indenting, or embossing, a tinfoil-covered cylinder to a varying depth corresponding to the sound pressure. The sound track was, therefore, a spiral of varying depth around the cylinder. This is "hill-and-dale," or "vertical," recording as opposed to the "side-to-side," or "lateral," recording that Scott's phonautograph produced.
In 1888 Oberlin Smith wrote an article in the magazine Electrical World in which he suggested, probably for the first time, the use of permanent magnetic impressions for sound recording. Smith visualized a cotton or silk thread in which steel dust or short clippings of fine wire were suspended, these particles to be magnetized in accordance with the undulatory current delivered from a microphone. Smith already discussed the possibility of employing a hard steel wire, but thought it scarcely possible "that it would divide itself up properly into a number of short magnets" to establish a magnetic pattern that is a replica of the microphone current. Smith never built an instrument.
Around the turn of the century, Emil Berliner made his great contributions for the further commercial development of the phonograph by working out the methods of mass-producing disc 'records.
When Berliner was producing his first commercial shellac records, a young Danish engineer, Valdemar Poulsen, was working on magnetically recording sound on steel wire. Poulsen has been called the Danish Edison, and his early history bears out the title. He came from a good family-his father was a judge in the highest court in Denmark-and there could hardly be any question as to his intelligence, but he was not a good scholar. The only subjects in which he was really interested were physics and drawing, which he pursued night and day. Since, however, they were not offered in the curriculum of his school, this absorption in science hardly advanced his scholastic record. He had no interest in mathematics, a trait he shares with many of the great experimentalists.
His father, being a professional man himself, wanted Valdemar to be a doctor, and Valdemar accordingly enrolled in medical school; but the only aspect of his studies that really captured his imagination was the lectures on chemistry. Finally, in 1893, at the age of twenty-four, he gave up the struggle and obtained an appointment with the technical section of the Copenhagen Telephone Company. In those days any boy who was interested in electricity thought of telephone work as the highest aim in life, just as, in a generation before, telegraphy had been the great career for young scientists. Poulsen's
work at the telephone company consisted mainly of trouble shooting, and there was plenty of time and apparatus for carrying out his own experiments.
How he ever got the idea of recording sound by varying the magnetization of a steel wire will probably never be known. It is believed that Poulsen was not familiar with Smith's article. What makes the idea so intriguing is that it was so counter to all accepted theory and to normal intuition. If a bar magnet is broken into little magnets, they will all be of equal strength. If they are stuck back together again, just one magnet results. But Poulsen thought that perhaps it was not necessary to magnetize a whole bar at a time. Maybe, if the steel were hard enough, it would be possible to magnetize just one spot on a bar and leave the rest unaffected. And maybe it would be possible to magnetize a wire to different degrees so close together that sound could be recorded on it by running the current from a microphone through an electromagnet and by either drawing the wire rapidly past the electromagnet or drawing the electromagnet rapidly past the wire. Since for most practical purposes magnetization is a reversible process, the wire could be used over and over again by the simple process of demagnetizing it whenever desired. Also, since no demagnetizing force need be present during playback, the records would be much more permanent than mechanical recordings. Poulsen himself probably had no idea how permanent the records would actually be. The early literature abounds with references to an expected life of hundreds of playings, but states that it could be expected that the quality would drop off thereafter. As a matter of fact, measurments have subsequently shown that hundreds of thousands of playings can be made without substantial loss of quality.
It was an amazing idea, and it was Poulsen's alone. Time and time again, as the files of the Patent Office amply attest, an invention has been made independently and almost simultaneously by two men. In view of the litigation that plagued Bell for years, hundreds of people in the United States and Europe were prepared to rove in court that they had invented the telephone. But nobody else, so far as is known, ever claimed to have invented the Telegraphone, as Poulsen named his machine.
The Telegraphone was exhibited at the Paris Exposition of 1900, where it won the Grand Prix, and it was as much a sensation as Bell's telephone had been at the Philadelphia Centennial twenty-four years earlier. People flocked to see it, and one can hardly pick up a scientific journal of the time without finding a reference to it. This response is hardly to be wondered at; now, nearly fifty years later, the idea of magnetic recording is still one of man's more intriguing conceptions.
A sketch of Poulsen's device, as described in his first United States patent, is shown in Fig. 1-I. The large cylinder d was made of brass, with a spiral groove running the length of the cylinder. Lying in the groove was a steel
wire g, against which rested the two poles of an electromagnet p, which carried the voice currents generated by the microphone. The cylinder is stationary while the electromagnet-or "recording head"-is rotated by the structure e following the wire and magnetizing it by amounts corresponding to the strength of the voice currents. When the recording was complete, the microphone was switched out of the circuit and a telephone receiver connected in its place. Then, by placing the recording head, which now served as a reproducing head, at the beginning of the wire spiral and playing the record through again, the original message was heard, as the varying magnetization of the wire generated current in the windings of the electromagnet.
By the standards then prevailing, it worked extremely well. If contemporary reports can be believed, everyone who heard it was enchanted by the naturalness of the reproduction and the freedom from noise. It might not be quite so impressive today, from the vantage point of nearly half a century more of acoustical -sophistication, but those auditors were comparing it to the phonographs of their time. The only regrettable feature of the original Telegraphone was the necessity of using earphones, but everyone assumed that this difficulty would be overcome shortly.
Actually, it took about twenty-five years, because it had to wait for the development of electronic amplifiers. The low playback level was the besetting weakness of early magnetic recording. When it became obvious with the passage of a number of years that this obstacle was basic, magnetic recording dropped out of sight, and interest was not revived until amplifiers became available.
Poulsen realized that he had made a great invention and acted accordingly. His original patent application, filed in Denmark in 1898, was followed in the next two years by applications in the United States, England, Germany, Austria, Hungary, France, Belgium, Italy, Spain, Switzerland, Russia, Norway, and Sweden. Together with his associate, Peder Oluf Pedersen, who was also an excellent engineer and in whose name many of the early patents were filed, he looked about for capital to develop and exploit the Telegraphone, and inevitably their eyes turned to America. And there, in November, 1903, was incorporated, with $5,000,000 capital stock, par value $10 a share, the American Telegraphone Company. It had a most involved history.
Serious charges were leveled against Charles D. Rood, for many years president of the company, by certain stockholders. It is not the intention here to enter into a discussion of their merits and demerits, but they read like a cloakand-dagger dime novel. A record of the testimony presented at a Senate hearing on the subject is cited in the bibliography at the end of this chapter.
When the company was organized, the intention had been to manufacture Telegraphones for use as dictating machines and telephone recorders, an ap-
plication where their ease of operation and the "erase" feature made them very attractive and where their low playback level placed them at no great disadvantage. The instrument had been redesigned with reels to hold the wire instead of the brass drum of Poulsen's first model. Despite the large diameter, by present standards, of the wire (0.01 inch) and the high velocity of 7 ft. per sec., at which it was reeled, it still had a recording time of half an hour. The low distortion (compared with early mechanical dictating machines), the longer playing time, and the fact that the wire could be used over and over again made strong selling points.
Some of the early machines are still in existence, and perform remarkably well. Looking at it fifty years later, the company might have been successful if everyone connected with it had done his part. The machines had some disadvantages. The speed at which the wire traveled in recording was just about as fast as was practical at that time. Consequently, it was impossible to rewind at any significantly greater speed, and rewinding before playback required an annoyingly long waiting time. Furthermore, the playback level was quite low, and threading a Telegraphone was a somewhat more involved operation than placing a cylinder on a mechanical dictating machine.
Eventually, the company went into receivership, and the stock, which had been mostly sold in blocks of one to ten shares to small investors all over the country, became worthless. A Danish company had been formed in 1909; but it never manufactured any machines, and it was dissolved in 1916.
Poulsen had left the telephone company in 1900 to pursue investigations in wireless telephony. However, he did not lose interest in magnetic recording, as proved by the numerous patents he filed during later years. One of them, U.S. patent No. 873,083, issued to Poulsen and Pedersen on Dec. 10, 1907, describes the extremely important discovery of d-c biasing. The details of this process are dealt with in another chapter; suffice it to say here that its introduction effected a great improvement in the quality of magnetic recordings, and it is still used in some applications.
Many honors came to Poulsen in his lifetime, among them an honorary degree of doctor of philosophy from the University of Leipzig in 1909 and the Medal of Merit of the Danish Government, which he shared at that time with a most distinguished company: Nansen, George Brandes, Sven Hedin, and Amundsen. At his death he was a fellow of the Danish Academy of Sciences, the Danish Academy of Technical Sciences, and the Swedish Instifor Engineering Research. Another honor, of more than passing interest, that fell to him was his election in 1936 to the vice-presidency of the Jnstitute of Radio Engineers. He died in Denmark in 1942.
Following the debacle of the American Telegraphone Company, magnetic
recording dropped into almost complete oblivion. The great problem of amplification had to be solved before it could hope to compete successfully with mechanical recording, which was technically far advanced before magnetic recording was even born. The United States Navy, however, carried on investigations of magnetic recording at the Naval Research Laboratory because of its applications to the high-speed transmission of telegraph signals. W. L. Carlson and G. W. Carpenter, working at the Laboratory, made the next great discovery in magnetic recording, the so-called "a-c biasing" method, described in U.S. patent No. 1,640,881, issued in August, 1927.
The mechanism of a-c biasing effected a basic improvement in the quality of magnetic recordings and is today in nearly universal use. With some of the newer recording mediums-powdered-iron tapes, for example-it is usually desirable because the use of the older d-c biasing method frequently results in so much more background noise that recordings are rendered almost worthless.
Magnetic recording was ripe for "rediscovery." Electronic amplifiers, the key to the great problem of sufficient output level, were coming into general use, and even without the use of a-c biasing, it was possible to make quite satisfactory magnetic recordings and play them back at any desired volume.
What magnetic recording needed at this time-the middle twentieswas a press agent, and he appeared in Germany, in the person of Kurt Stille. His persuasive powers were remarkable, and he succeeded in getting backing from a group of very substantial German financiers who organized a company known as the Telegraphie-Patent-Syndikat. This company existed for the purpose of selling licenses to manufacture magnetic-recording equipment.
Armed with a wire recorder, which was probably a slightly modified American Telegraphone, Stille would expand on its wonderful possibilities and then, as a climax to his sales talk, would draw out of his pocket a beautifully polished steel tube about 8 inches long.
"Do you see this?" he would say. "On this little tube I can record a whole symphony"
Despite such sensationalism, the effect in the long run was a good one, because the persons to whom he succeeded in selling the right to manufacture magnetic-recording equipment actually went ahead and did so.
About 1930 Telegraphie-Parent-Syndikat sold the right to manufacture magnetic-recording equipment for entertainment purposes to Blattner, who was a promoter in the motion-picture industry and whose chief interest in magnetic recording was its use as a sound-recording medium for talking pictures. Blattner went to England, Where he actually made several movies with
a sound track recorded on synchronized steel tape. Thereafter, he sold out his interest to the Marconi Company. The Blattnerphone, as his machine was called, has been used quite extensively by the British Broadcasting Company.
Karl Bauer, to whom Stile had sold a license to produce dictating and telephone-recording equipment, organized the Echophone Company for the production of the Dailygraph, which was a combination dictating machine and telephone recorder. The Dailygraph was the first magazine machine, which is to say that both the supply and take-up reels constituted a single unit detachable from the machine. This scheme conferred a distinct advantage, since the reels could be removed from the machine at any time without the necessity of first rewinding the entire spool. The Dailygraph also contained an automatic-volume-control device, which was necessitated by the unpredictable level of long-distance telephone conversations of that day. This was a long time before the introduction of stabilized feedback repeater amplifiers, and the signal level was likely to vary as much as ten to one from one conversation to another.
The use of these machines for telephone recording had some success, and permission was obtained from a number of the European telephone companies-many of which were government-owned-to connect these machines to their lines. In this connection, it might be pointed out that magnetic-recording machines were the first instruments which had ever made it possible to record a telephone conversation directly, because at the time of their invention, they were the only machines that could record sound by means of an electrical current. The Telegraphone had antedated by considerable time the development of the present-day electrical record cutter.
Like the old Telegraphone, the Dailygraph employed a wire speed of 7 ft. per sec., but its magazine held sufficient wire to record for a full hour. This characteristic was in no small measure due to a reduction of the wire diameter from 0.01 to 0.008 inch, for a given volume of wire the total recording time is approximately inversely proportional to the square of the diameter of the wire.
A number of different models were manufactured, and the sales literature of the company listed the price of the least expensive unit as 2,250 reichsmarks. The exchange value of this sum at the time equaled nearly $600.
Despite the fact that the Dailygraph worked very well and a large numher of machines were sold, it had several weaknesses, and Bauer, being somelung of a salesman himself, sold the Echophone Company to the Internarional Telephone and Telegraph Company in 1932. They in turn passed it on to the C. Lorenz Company, an associated company and one of the biggest German communication firms. Lorenz completely redesigned the Dailygraph
and marketed it under the name of Textophone. Both the names Textophone and Dailygraph, it might be added, had been chosen to help the sale of the machines on the international market. The Textophone represented a re-al improvement over the Dailygraph both from the standpoint of reliability of operation and the ease with which it could be mass-produced. The vacuumtube amplifier that it contained was also considerably better than that of its predecessor.
The Textophone was placed on the market in 1933, about the time Hitler came to power. The Nazis needed all the recording equipment they could get, and the Gestapo bought huge numbers of Textophones for the German government. The market was not entirely a domestic one, however, for they were sold all over Europe, several hundred installations having been made in Switzerland alone.
The Lorenz Company also developed a steel-tape recorder, which it marketed under the name of Stahltonmaschine. It likewise had considerable success and in 1935 was adopted by the German Broadcasting Company, which used it extensively in mobile pickups.
One other German development in magnetic recording, which is of considerable interest, was the Magnetophone. Pileumer, in 1927, had conducted numerous experiments with magnetic-recording mediums consisting of paper or plastic tapes coated with powdered magnetic materials. The grain size of the materials was rather large, and the resulting tapes somewhat resembled sandpaper. However, in 1931 he managed to get the Ailgemeine Electrizit-ts Gesellschaft (A.E.G.) to take over this work. Thereafter, A.E.G., together with I. G. Farben, undertook the development of his idea; I. G. Farben continued the development of the tapes, and A.E.G. developed the associated electronic and mechanical apparatus.
The Magnetophone had been designed as a dictating machine, but it was considerably inferior to the Textophone and Stahltonmaschine. It was first demonstrated in 1935 at the German Annual Radio Exposition in Berlin, and despite its mediocre performance, it was something of a hit, and for a very simple reason. Steel tape such as was used in the Stahltonmaschine in 1935 was so expensive as to make 1 minute's recording cost about $1, while the coated tape employed by the Magnetophone cost only 15 cents per mm. Not only did this represent a saving in the cost of the material, che nature of the plastic tape made it possible to wind it on a single reel, instead of on the bulky magazines that the Textophone required, and thus effected another substontial saving.
During the war the Germans continued to work on magnetic recording, despite serious shortages of manpower and material. Equipment seized after the Allies entered Germany shows a high degree of development, for both
broadcast and military applications. Magnetic powder-coated tapes were greatly improved and the Magnetophone redesigned to take full advantage of their capabilities.
In the meantime, research in the United States had by no means been at a standstill. In the early thirties the Bell Telephone Laboratories started an extensive research program on magnetic recording.
At the meeting of the American Acoustical Society at Washington in 1937, C. N. Hickman of the Bell Telephone Laboratories demonstrated the results of his work, a magnetic-tape recorder of excellent quality and with a tape speed of only 16 in. per sec., or about one fifth the speed of Poulsen's wire recorder. The introduction of the new material, Vicalloy, which had been developed as a magnetic-recording medium, marks the true beginning of what might be called the modern period in magnetic recording and has led to the development of any number of new recording materials and vastly improved performance.
Apart from the use of some steel-tape recording equipment in Europe, almost the only other application which had been made of it was for telephone recording and dictation. It had never competed with mechanical recording in the entertainment field, although it had two features that made it potentially very atractive for this service: continuous records up to 1 hour or more in length could be made; and the number of times a single recording could be reproduced was almost without limit. A recording made on steel tape, for example, has been played more than 100,000 times with no measurable deterioration after a slight initial falling off in output level.
These advantages, in the late thirties, were more than offset by other factors. Magnetic records could not be changed with anything like the facility of phonograph records, and the ecording medium itself was a great deal more expensive. Mechanical difficulties connected with the high speed at which the wire had to be spooled were still a problem. Even so, the obvious advantages and the fact that almost no technical skill was required to make a recording led a number of radio-receiver manufacturers to consider seriously at this time the inclusion of magnetic-recording equipment in their higher-priced sets, and plans had gone forward to do so when the war intervened.
The outbreak of the war greatly stimulated the further development of magnetic recording. Its ability to operate under extremes of heat, cold, and vibration made its use mandatory under battle conditions and in ships, planes, tanks, and other mobile equipment where disc recording could have been employed only with great difficulty. The fact that magnetic-wire recording equip inent can be made small enough to be carried in the pocket and still contain enough wire for a considerable recording time made it indispensable for scouting and reconnaissance use.
The stringent requirements of military equipment present tremendous
problems to the engineers, but the very wastefulness of war makes many things possible that would have to wait for years for their development in peacetime. To a nation fighting for its existence, monetary cost can never be a serious consideration. The widespread applicability of magnetic recording to wartime use made its rapid development an urgent necessity, and it can be fairly said that more significant advances were made during the war years than during any period of equal length in its history.
Spectacular gains were made in the development of new recording mediums. The stimulus to this development was threefold. First was the eternal problem of compressing a recording into smaller and smaller space, a consideration of the utmost importance in many wartime applications. Improved recording mediums, as Hickman had shown, held promise of making possible significant reductions in the velocity of the recording medium. Also, although heat and cold had little, if any, effect on a steel wire or tape, humidity was extremely damaging. Tungsten- or carbon-steel wire rusted so quickly in a humid environment that it shortly became useless. Finally, the fact that previous to the war almost all the steel wire and tape used for recording purposes had been imported from Sweden made the development of domestic sources urgently necessary because the submarine blockade rapidly dried up this source. Vicalloy, the alloy that had been developed at the Bell Laboratories, afforded considerable improvement over carbon or tungsten steels, both in its magnetic properties and its resistance to high humidity. However, it was found difficult to produce in the quantities required by the services.
The stainless steels were the subject of intense investigation as to their suitability for magnetic-recording mediums, and the alloy known as "420" stainless was found to answer the requirements quite well after proper heattreatment. Thousands of miles of 420 stainless wire were produced during the war in diameters of 0.004 and 0.006 inch.
Even after the use of this material in magnetic-recording equipment had become quite general, the search for still better mediums continued, but along somewhat different lines. The first of these consisted of an investigation of the possibility of plating magnetic materials on a ductile base wire, such as brass. The second line of approach was a return to Pfleumer's idea of binding particles of magnetic material to a paper or plastic base. Both these methods of attack resulted in the development of magnetic-recording mediums that have markedly superior characteristics and are, moreover, a great deal less expensive to produce. Parallel with these developments, much time was devoted to the rruhlem of finding a stainless steel with properties more suitable for magnetic II(&)rding than the 420 type and which would at the same time be easier to produce. Several companies active in the production of thin wires focused their attention on 18-8 type stainless steel, which was known to obtain superior
magnetic properties by suitable cold-working. Wire of this material is now made for use in many magnetic recorders.
Magnetic-recording equipment itself had not been manufactured in the United States from the time of the American Telegraphone Company until 1937, when the Soundmirror* was placed on the market. First produced by Acoustic consultants and later by The Brush Development Company, it was an endless-tape machine with a recording time of about one minute and had considerable application as a voice-training device. Shortly thereafter, the Western Electric Company marketed a similar device under the name of Mirrophone, one version of which was extensively used for telephonic weather announcing.
Prominent in the rebirth of interest in magnetic recording was The Brush Development Company. In 1939 this organization inaugurated a research program that has resulted in many contributions to the art. During World War II this company built large quantities of magnetic-recording equipment for use by the armed services and, in cooperation with the Office of Scientific Research and Development, perfected the magnetically coated paper-tape and plated-wire recording mediums which hold much promise for the industry.
In 1941 the Armour Research Foundation became interested in wire recording, and much credit must be given to them for the extensive campaign they launched to acquaint the public with its possibilities. Many people probably first heard of magnetic recording as a result of these promotional activities.
The story of magnetic recording in the postwar world has still to be written-is, as a matter of fact, being written now. With the resumption of civilian radio and phonograph production, many companies have signified their intention of entering the magnetic-recording field, and a considerable amount of equipment for home-recording purposes is now being produced. It is still much too early to evaluate the public's reaction. In the last chapter of this book an attempt is made to foresee the outcome, but such an attempt belongs rightfully in the realm of prophecy.
* The name Soundmirror has subsequently been used as a trade name for various types of magnetic recorders manufactured by The Brush Development Company.
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U.S. Patent No 873,078, Dec. 10, 1907: Pederson and Poulsen (Longitudinal Recording Head).
U.S. Patent No. 873,083, Dec. 10, 1907: Pederson and Poulsen (D-C Biasing).
U.S. Patent No. 873,084, Dec. 10, 1907: Poulsen (Magnetic Recordings on Smooth Discs or Cylinders).
U.S. Patent No. 900,304, Oct. 6, 1908: Pederson and Poulsen (Solenoidal Head).
U.S. Patent No. 1,640,881, Aug., 1927: W. L. Carison, et al (A-C Biasing).
U.S. Patent No. 1,653,467, Dec. 20, 1927: J. A. O'Neill (Powdered Recording Media).
German Patent No. 500,900, Jan. 31, 1928: Pfleumer (Powdered Recording Media).
U.S. Patent No. 1,758,531, May 13, 1930: Pfanhauser (Powdered Recording Media).
To Chapter 6
- Begun, S. J. Magnetic Recording New York: Rinehart & Company, 1949.
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