International projectionist (Jan-Dec 1947)

with that of obtaining a meter reading with a pointer that sticks by tapping gently on the case in order to break up the static friction. The range of frequencies that can be covered depends largely upon the velocity of the medium past the recording heads. For, as in film recording, a higher velocity results in longer recorded wavelengths, with an accompanying improvement in scanning and high-frequency output. Recordings are usually made with constant current through the recording coil, which subjects the recording medium to a constant magnetomotive force. This corresponds to the constant amplitude method used in variable-area film recording. Frequency Response In reproduction, however, the magnetic head develops a voltage proportional to the time rate of change of flux through the coil. As a result, the output voltage for a constant amplitude magnetic recording varies directly with frequency, increasing as the frequency increases. The increase is not maintained with the higher frequencies largely because of the loss in magnetization due to the high reluctance of the return path through air after the medium leaves the recording head, and due to the demagnetizatizing effect of the leakage flux at the recording pole piece. As a result, a typical frequency response curve for a tape speed of 8 inches a second shows an increase in response with frequency up to about 1500 cycles, and then a rapid decrease. In order to obtain a flat frequency response, electrical compensation in the playback system is required. Recording and reproduction is usually accomplished with the same head although separate heads may be used. The head may consist of one or two separate pole pieces, or it may be the present popular "ring" type which contacts^ the medium along one side and has one or more air gaps, across one of which re 2, j <f & 6 J a 9 jooo 2. Frequency In Cycles per Seoond FIGURE 2. Unequalized frequency response for tape at 8 inches per second. + -*■ cording or reproduction takes place. Work hardened steel wire of low carbon content, commonly called music wire (because of its use for piano strings) was one of the first mediums used for magnetic recording. It still is in use today, due to its low cost, availability, and the fact that acceptable results can be obtained. The permanent magnet qualities of carbon steel are poor compared with tungsten, chrome or cobalt steels, and as a result, a magnet length many times the diameter or crosssectional dimensions is necessary to obtain a strong magnet. Good permanent magnet materials are usually hard and brittle, and much work has been done to obtain a material with the desired magnetic properties which can be drawn or rolled to the desired small cross-section and will be flexible enough to permit bending about the small pulleys and reels used in recorders. Tungsten and chromium, which improve the permanent magnet qualities of steel, have been successfully used in FIGURE 1. (a): Illustrating the analogous magnet arrangement in a wire recorded at two different frequencies; (b) ring type reproducer; (c) flux variation in the wire. commercial tapes. A recently developed alloy called "vicalloy" has given excellent results. Stainless steel wire of improved permanent qualities has become available and recently a soft brass wire plated with a nickel cobalt alloy has been introduced for magnetic recording purposes. Tape in which a dispersion of fine magnetic particles in a suitable binder is coated on a base of either paper or plastic material has been developed in this country and in Europe, simultaneously but independently. Low noise and useful output voltage at high frequencies (for a given linear speed) is claimed for this new product. Various Applications Since magnetic recording allows immediate playback, produces no shavings, requires little operating skill, and permits reuse of the medium, it is an ideal system for home recording. There is no waste of medium due to an unsatisfactory or unwanted recording — the wire or tape is simply recorded over again. These features, immediate playback and reuse, make it attractive commercially for original recordings to be later rerecorded to the final medium as required for printing and duplicating, or where the recording does not have to be kept permanently. The recording may be monitored directly with a playback head located adjacent to the recording head and only a fraction of a second behind the recording. Tape may be cut and spliced or portions blanked out or rerecorded. In these respects it is much like film except that the sound track is not visible: it must be played in order to determine whether or not a recording exists. INTERNATIONAL PROJECTIONIST January 1947