High Frequency Magnetophon
Magnetic Sound Recorders
James Z. Menard Captain, Signal Corps
Army Branch, FIAT (US)
Quoting from document page 5 pp, not equal to the PDF numbering, AOB)
During the past few years the Germans were very active in the field of magnetic sound recording. Lorenz produced limited number of wire recorders for dictaphone use, but most significant work was done by the Allgemeine Elektrizitäts Gesellschaft (AEG) and the Reichsrundfunk Gesellschaft (State Broadcasting Service) (als known as RRG, AOB) who cooperated to develop and perfect a system of magnetic recording using plastic tape coated or impregnated with microscopic particles of magnetic material. A number of models of recorders were produced using this process and fairly detailed survey of them may be found in the Technical Liaison Division Intelligence Report SRM-1. "German Sound Recording", which may be obtained from Intelligence Branch, Office of the Chief Signal Officer, War Department, Washington 25, D.C. The mostly widely used military recorder, the Tonschreiber b, is described in considerable detail in the Enemy Equipment Intelligence Service report No. EEIS-11-12, which is also available from the above agency. While the early models of these recorders were not outstanding in performance, the development of supersonic eraising and recordingprocess, coupled with considerable improvement in the recording tape, has resulted in production of recent commercial models which give extremely high quality performance, as was indicated in the first report mentioned above.
Early American work
The first reference which has been located on this subject is a proposal in 1918 by Leonard F. Fuller of Palo Alto, California, who suggested the use of radio frequency erasing, with direct current premagnetization, for the Telephone wire recorder developed by Poulsen.
In 1921, Carlson and Carpenter of Washington, D.C. described a system for recording on steel wire or tape in the presence of a supersonic signal to agitate the carrier during the recording process. No reference was made to methods for erasing and the system was proposed as a modification of the Telegraphone.
In 1929, Carl W. Rhodenham of Hollywood, California, described a method of using alternating current of audio or higher frequency to erase a magnetic recording.
In 1931, James H. Alverson of Los Angeles, California, proposed a system for magnetic recording using a radio frequency carrier to saturate the wire in absence of audio, and the audio signal was used to suppress the carrier producing a recording. No erasure was thought necessary with this system.
None of these proposals bore fruit in a successful commercial equipment. Some of them appear unsound in theory and actual practical application, but they do show that some early thought had been given to this field of recording.
While several firms and persons in Germany participated in development of magnetic recording systems, the only significant work known to have been done on high frequency erasing and recording systems was performed by personnel of the Reichsrundfunk Gesellschaft (RRG). This organization concerned on improving the performance of the Magnetophon sound recorders produced by Allgemeine Elektrizitäts Gesellschaft (AEG) of Berlin, and their research resulted in development and perfection of a high frequency recording system which was incorporated into recent equipment of AEG. The high frequency system is not believed to have been applied in Germany to any magnetic recorder than the Magnetophon series using plastic tape.
The development of this recording technique is credited to Dr. Hans Joachim Braunmühl and Dr. Walter Weber of Berlin. Their proposal for this system was made in 1940 and the system found widespread application in broadcast service after time. Despite the supply difficulty, by the end of the war, most of the broadcasting stations in Germany had used the Magnetophon to replace other types of recordings, and it provided recording performance not surpassed by any other method, with certain advances not offered by other systems.
Significant Features of Magnetophon Recording System
Recording is done on a plastic tape about 0.04 millimeter thick and 6 millimeters wide, which is impragnated or coated. with microscopic particles of magnetic materials. This unique recording material has been the subject of considerable study, and the successful development is a large degree responsible for the excellent performance of the recording system. A later section of this report discusses the design factors, physical properties and manufacturing process of the tape.
Erasing, Recording, and Playback Heads
The high frequency models of the Magnetophon are equipped with three armatures, or heads, which perform the function of erasing, recording and playing back. These heads, which are similar in appearance, differ only in gap width and winding impedance. They are arranged so that all three are in contact with the tape, and work on a longitudinal magnetisation principle. Fig. 1 shows a sketch of the recording and playback heads in relation to the tape. The material and dimensions of these heads have been carefully studied to secure the lowest possible tape speed compatible with the desired volume and frequency range. Detailed specifications of the heads will be found in a later section of this report.
Erasing and Recording Process
The significant feature of the process are the use of high-frequency erasure which leaves the tape in a magnetically neutral state, and the use of a certain value of high-frequency signal during the recording process, to minimize the incremental magnetization background noise and decrease distorsion due to the nonlinearity in the magnetization curve of the tape. This process had been subject of considerable study, and is responcible for the great improvement in recording performance. It will be discussed in greater detail in a later section of the report.
The smooth performance of the Magnetophon recorders is the result of well designed mechanical system. It includes features and construction which appear elaborate by American standards, but which contribute to convienent and trouble-free operation in continuous service. Constant tape speed, even tape tension, quick rewind, switching surge suppression, and instantaneous braked stopping are refinements provided by use of separate motors for tape reel-off, braking, tape pulling and tape take-up, with solenoid brake operation and interference suppression filters. Particulars of the mechanical system are given in detail in a later section of this report.
Performance Achieved by the System
For commercial use the tape is prepared in 1,000 meter rolls, which are some 10 to 12 inches in diameter (meant is tape width, AOB), 6 millimeters thick (actually 6.5 mm, AOB), and weigh about the same as two twelve inch records. One such roll gives a playing time of 22 minutes in the standard Magnetophon, which has a tape speed of 77 centimeters (about 30 inches) a second.
With proper equalization in the recording and playback process, a tape speed of 77 centimeters per second allows a frequency range whose upper limit is 10 kilocycles (down 2 db) and the lower limit is governed only by the associated electrical circuits. Actual overal response measurements of typical systems, shown in Figure 18 indicate virtually flat characteristics between 30 and 9000 cycles, including the associated speaker equipment. Personnel of the Rundfunk laboratories state that with present tape quality, it is possible to extend the range to at least 12 kilocycles at present tape speed, and this is done with amplifiers of new design, which have not yet gone into production.
A dynamic range of 60 to 75 decibels or even 80 decibels has been achieved in this equipment. Actual values are dependent on the tape characteristics, hum pickup, and to a surprising extent, on the quality of the preamplifier used for playback. The Germans have not perfected a suitable low noise high-gain audio amplifier tube, and despite the use of direct current heating, low power operation, and high input transformers step-up, the tube noise and hum originating in the first stage has been a limiting factor for them.
The tape drive system is of such design and quality that there is no wow, flutter or other variation detectable to the most critical listener. Film speed is determined by the synchronous motor with separate motors, to pay out and take up the tape. Measurements show that variation due to tape stretch and other factors under normal operating conditions amounts to less than 1%.
General Operating and Handling Qualities
Magnetic recording as practiced in the Magnetophon, is simple, rapid, and almost foolproof. A recording operation consists of checking the erasing and recording current, setting the audio volume level, threading the tape, and pushing the start button. Momentary over-modulation produces only a slight increase in distortion as curvature of the magnetization curve encountered. This is a gradual effect, and there is no phenomena comparable to the groove cross-over or light-valve clashes encountered in mechanical or photographic systems. A recording is ready instantly and may be monitored during the process. An unsatisfactory portion is remedied by erasing and re-recording the defective sections, or by making the correction and later, during editing cutting out the incorrect section. The tape can be played an infinite number of times without detectable deterioration, may be stored for years, and can be erased and re-recorded as often as desired. Dust causes no los of quality or increase of noise. Programs can be made of cuts from several recordings, spliced with only scissors and cement. From the standpoint of economy, operating flexibility and convenience, this system offers advantage not found in any other process of recording.
Particulars of Significant Features
The use of this type of recording material was proposed in 1928 by Fritz Pfleumer in Dresden, Germany. Development of such a recorder was undertaken jointly by AEG, who worked on mechanical and electrical portions, and I.G. Farben, who worked on the tape. Three principal types of tape construction have been used.
The first to see widespread comercial commercial application was the "C" tape, which was a cellulose-acetate backing carrying an active layer of 0.61 to 0.02 millimeter thick, composed of 90% Magnetit (Ferric Oxide) of grain size 0.002 millimeter and 10% adhesive. This tape tended to become brittle after long storage, and occasional samples seemed to "shed" a little of active surface. Noise level was fairly high due to non-uniformity of the active layer.
The next advance was the "L" tape, which used a body of "Luvitherm", a variety of polyvinylchloride, in which the Magnetit was directly impregnated (??, AOB). This produced a very uniform material with excellent mechanical properties, which does not appear to deteriorate with age like the "C" tapes. Background noise with this tape is considerably reduced and it gives excellent results, particularly with the high-frequency systems.
Just before the end of the war, intensive research is stated to have culminated in development of a type "LG" tape, which had a "Luvitherm" backing with a carefully controlled, particularly uniform active surface of Magnetit with adhesives. Studies of particle size, permeability and layer dimensions led to the developement of a layer size and composition which give optimum performance. It is stated that this new tape gives recording characteristics somewhat superior to the "L" tape and retains the excellent handling and life characteristics of the latter, with some increase in mechanical strenght. It is stated that the "L" tape is much the easiest to produce in quantity, but that mass production problems for manuafcture of the "LG" tape had been solved when the war forced termination of activity. Manufacturing process for these tapes are covered in detail in Technical Industrial Intelligence Committe Report No. 59 of June August 1945, by C.W. Hansel, J.H. De Bell, W.C. Coggin, and W.E. Gloor.
Erasing is accomplished by energizing the previously described head with between 120 and 150 milliamperes at a frequency in region of 35 to 40 kilocycles. The ferquency is not critical. It must be high enough to cause several reversals of polarity while the tape is moving a distance equal to the erasing gap-width, and the upper limit is determent by the hysteresis lossess and the difficulty of feeding the coil. Sufficient energy for erasing can easily be obtained from a single self-excited beam tetrode oscillator comparable in power to the 6V6. It is German practice to feed the eraser through a condeser in series resonance. Some circuits couple from the oscillator to the head through a step-down transformer. Others couple directly from the oscillator circuit.
In the high-frequency erasure process the tape, moving past the head, enters first an increasing high-frequency field, which magnetize it alternately to saturation in both directions, and then passes through a decreasing field which magnetize it in diminishing swings, until it passes out of the field in a neutral condition.
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