Motion Picture Herald (1953)

As part of the “new techniques,” stereophonic sound is steadily increasing the use of magnetic recording. Here is how magnetic differs from the optical method. Advantages of Magnetic Sound LUBRICATION t, POLISHING COAT ^ RON OXIDE LAYER I BONDING COAT/ FILM BASE FIGURE I — Representation of one kind of magnetic striping on film. In this type the magnetic material is applied in liquid form to a bonding coating on the film base. REMOVABLE CELLOPHANE BACKING-^ IRON OXIDE LAYER \ THERMOPLASTIC V\ ADHESIVE ' / FILM BASE FIGURE 2 — How magnetic stripe is applied in the laminated type of construction, in which the magnetic material (iron oxide) is first applied to a strip of plastic material. MAGNETIC FLUX INDUCED INTO MAGNETIC LAYER' SIGNAL FROM RECORDER /RECORDING HEAD ' MAGNETIC OXIDE FILM MOTION FIGURE 3 — The sound current flows through a coil of the recorder, producing a variable magnetic flux in the coil core, and the flux passes through layer of iron oxide on the film. The drawing shows path of magnetic flux in the oxide and recorder coil core. MAGNETIC FLUX REMAINING IN OXIDE LAYER FIGURE 4 — Representation of the film after it has left recorder, with each section of track given its specific magnetic energy. Flux flows partly in the oxide, partly in air outside the oxide layer. AFTER PASSING OVER RECORDING HEAD £ £ £ MAGNETIC MAGNETIC FLUX INDUCED INTO REPRODUCER HEAD FROM MAGNETIC OXIDE LAYER / SIGNAL TO preamplifier REPRODUCER HEAD MAGNETIC OXIDE >• FIGURE 5— Indicating distribution of magnetic flux in iron oxide layer and coil core of the pickup head during reproduction. By CIO GAGLIARDI magnetic recording on motion picture film is now being adopted by the film industry at a high rate. To many theatre people this type of recording and sound reproduction may be new and strange; be that as it may, magnetic recording itself is not really a new invention. As far back as 1898, Valdemar Poulsen built a magnetic recording instrument, which he called the “Telegraphone.” He found it possible to magnetize a steel wire to different degrees along its length and then to play it back translating these varying magnetic fields into corresponding electrical impulses. 1 he invention lay dormant until about 1930, when the “Magnetophone” was produced in Germany. This machine used an oxide coated tape in place of the steel wire ; however, the quality of reproduction was rather poor, the noise level being very high, but the instrument served adequately as a dictating machine. In this country the use of magnetic recording on wire or tape was not considered attractive until the beginning of the Second World War. Under the impetus of war activity, development progressed very rapidly in the manufacture of ferric oxide coated tapes, of recording and reproducing mechanisms, and of amplifiers. At first the quality of equipment limited the use of this medium to speech records only, but as successive improvements were made, magnetic recording quickly displaced the standard disk records in the radio industry and is now seriously invading the field of film optical recording. The recording material principally used in magnetic recording is a form of iron oxide ground to an extremely fine powder and held together by suitable binder and solvent. The iron oxide, which has magnetic properties similar to iron, must be distributed through the mixture with an even density, and then the mixture must be applied to the carrying base (film, plastic, or paper) in an even layer so that its magnetic permeability remains constant for the full length of the film. The standard thickness of the oxide coating is approximately six ten-thousanths of an inch (0.0006”), and the oxide stripes can be made from 15 mils wide up to the full width of the film base. This form of application will provide a very thin and very flexible ribbon of magnetic material upon which the signal can be magnetized. Figure 1 shows the construction of one type of magnetic striping on film. A bonding coat is applied to the film base, then the magnetic material in liquid form is applied from a constant feed hopper to the bonding coat. After passing through the necessary drying devices, the surface of the stripe is lubricated and buffed to maximum smoothness. Figure 2 shows the construction of magnetic striping using the lamination process. The iron oxide is first deposited on a cellophane backing strip, then the oxide is covered on the opposite side with a thermoplastic adhesive. The magnetic stripe is rolled on to the motion picture film and heated for a permanent bond, then the cellophane backing is removed. This process uses no liquid solvents and should not endanger the film in any way. These methods of application will provide stripes of magnetic material along one side of the film ready for the signal to be recorded. Now let us see how this signal is applied, stored and reproduced. THE MAGNETIC TRACK We all know that iron substances may be readily magnetized by locating them in a magnetic field, and that these iron particles will retain their magnetism for a long time if they are not disturbed by other magnetic flux. This particular quality makes it possible to record and store different formations of magnetic signal in the thin layer of iron oxide contained in the magnetic stripes on motion picture film. For recording purposes, the film with its oxide stripes, or complete oxide surface, ( Continued on page 64) 4 MOTION PICTURE HERALD, NOVEMBER 7, 1953