International projectionist (Jan-Dec 1947)

VOLUME XXII JANUARY 1947 NUMBER 1 Magnetic Recording By H. E. ROYS Radio Corporation of America MAGNETIC recording has the inherent advantage that only a rearrangement of the magnetic elements occurs during recording, a rearrangement which is not visible but which remains essentially unchanged until subjected to a stronger magnetic force, so that processing (needed by film or wax disk) is not required and therefore immediate playback or reuse of the medium is possible. For these reasons magnetic recording appears attractive for applications where immediate playback is desirable, or where reuse of the medium for other recordings is permissible. The wire or tape during the recording process is magnetified by a small electromagnet called the "recording head," and the magnetization is in accordance with the amplitude and frequency of the recording current. Usually the direction of magnetization is parallel with the line of travel of the medium — "longitudinal magnetization" — although in some cases where a steel tape is used, the direction of magnetization is directly through the tape — "perpendicular magnetization." The net result of the longitudinal method may be thought of as a string of tiny magnets laid end to end, with like poles adjacent, where the length of each magnet depends upon the speed with which the medium passes over the recording magnet as well as upon the frequency of the signal being recorded. The strength of each magnet, although depending upon the composition of the medium and other factors, is a direct function of the magnitude of the signal current. To reproduce this strip of magnets and convert the magnetic energy back into electrical energy, it is passed over another electromagnet called the "reproducing head" Here the varying flux through the coil due to the travel of the tiny permanent magnets across it induces a voltage which is like the original signal voltage. To obliterate any previous recording and asure uniform results, the medium is first conditioned by passing it over an "erasing head," which is located in front of the recording head. The wire or tape is subjected either to a strong unidirectional magnetizing force which saturates it, or to a varying force which leaves it unmagnetized. The frequency of the varying force is well above the audible range (20 to 30 kilocycles or higher), and the use of such a high frequency has resulted in the procedure being called the "high-frequency" method of recording; whereas the use of direct current is known as the "d.c." method. At the recording head, a magnetic "biasing" force is applied in addition to the magnetic force of the recorded signal. The correct bias in magnetic Magnetic recording, in existence since 1901 when the inventor, Vaidemar Poulsen, first used it for recording wireless telegraph signals, has received much publicity recently, and many statements have been made as to its merits and possible replacement of othei methods of recording. Here is the what, how and why of this novel process. recording is just as important as the correct grid bias for the amplifier tubes in a film recording amplifier. If d.c. erasing is used, a d.c. biasing force of opposite polarity, but much less in magnitude, is required; and if high-frequency erasing is used, a varying magnetic biasing force of less magnitude and usually of the same frequency is applied. The high-frequency method of recording is widely used today because of the improvements resulting in less noise and distortion. The d.c. method was the first to be used, although the high-frequency method has been known for a long time, as patent information shows that Carpenter and Carlson used it before 1921. A simple explanation of the action of the high-frequency bias sometimes used is that the varying force agitates the magnetic elements so that they are more easily acted upon and oriented into a position consistent with the direction and magnitude of the force executed by the relatively slowly varying signal magnetomotive force. For a mechanical analogue, this action might be compared INTERNATIONAL PROJECTIONIST • January 1947