British Kinematography (1950)

December, 1950 gould, dalby and birch : magnetic ^WwH4?S "01 W 183 Capabilities of Magnetic Sound. It is possible to-day to obtain results with magnetic recording which compare most favourably with any other system of recording, including optical. For example, at 35 mm. film speed the frequency response can be kept level between 50 and 10,000 c/s. ; at 16 mm. film speed between 100 and 5,000 c/s., and the 8 mm. film can achieve something in the region of a top response of 3,000 c/s.3 One great advantage of magnetic recording, as compared to optical recording, is a much higher signal-to-noise ratio. Scratches, dirt and dust have no appreciable effect upon magnetic reproduction. An accepted figure of volume range on noiseless optical recording is 50 db. The magnetic recording field claims up to 60db. and some systems as high as 70db., given a good magnetic coating, used in conjunction with good magnet designs, and good circuiting. The great disadvantage of the system as it concerns the film technician is that the magnetic recording is not visible. It can, of course, be made visible by the simple expedient of allowing metal dust to adhere to the magnetic striations,4 or the track can be reproduced on a cathode-ray tube, but no method exists to my knowledge which gives an editor the same facility of reading the track as he can with an optical recording. Design Requirements. The various links in the chain of a magnetic recording system are the wire, tape or coated film, the electro-magnet, the mechanism which feeds the wire, tape or film across the magnet pole pieces, and the amplifier system. Time need not be spent in much consideration of the mechanical aspect. Obviously the design must be similar to that used in optical recording, in that the magnetic carrier must be pulled across the flux gap of the magnet at a constant speed, its movement free from flutter. The amplifier design differs somewhat from optical recording systems in that severe frequency correction, depending upon the type of magnetic coating, is necessary. In playback it is necessary to consider that the voltage output of the magnet increases with frequency, which calls for further correction. Nevertheless, there is nothing in the design of recording or playback amplifiers which the electronic engineer cannot take in his stride. The electronic circuit used for recording must also include an oscillator, which provides a current at a frequency well above the audible range, for bias and erasing. Other than in the film industry, the carrier takes the form of a J-inch wide plastic tape, such as is used on the machine which has been used to reproduce the earlier part of my talk. A plastic tape is not very suitable for synchronous sound recording, by virtue of the fact that it is pulled through the mechanism by means of a pinch capstan drive ; a smear of grease will create variable slip. Stretching of the tape is also an ever-present risk. It is, therefore, more practicable to use a sprocket-holed carrier where perfect synchronism is required, and magnetic powders can be coated on to any film base. Existing arrangements entail the use of a 35 mm. base on which several magnetic tracks can be recorded. Some development has been attempted whereby the J-inch tape can be kept in synchronism in spite of slip and stretch,5 but these resemble generally the same methods used in the past to keep a gramophone record in synchronism. As 35-mm. is the professional standard, it would appear to be wise to concentrate on a 35-mm. carrier for magnetic coating. Sound for the Non-Professional. Before the development of magnetic tape, the average layman had no simple means of recording sound. He could, of course, have purchased a fairly expensive gramophone disc-cutting equipment, but few people had the ability to use it. The machine that has been heard to-night can be bought at a reasonable price, and little skill is needed to make sound recordings of fair quality