British Kinematography (1949)

46 is also equipped with a special head for wiping or erasing the recorded sound, thus making it possible to re-use previously recorded film. Fiom Fig. 13 it will be seen that the recording head and the playback head are both mounted so that they ride on the flywheel drum round which the film passes ; the drum is connected to a shaft carrying a flywheel, which is consequently driven by the film itself — an arrangement which long experience has shown ensures a constant and highly uniform film speed. The cover of the recording head has been removed to illustrate the magnet, which is normally enclosed in a screening can. The film passes the reproducing head only a fraction of a second after it has passed the recording head, and the former can be used for playback, re-recording or monitoring purposes. Fig. 14 illustrates details of the recording and reproducing heads and of the bridge on which they are mounted. The cover has been removed from the recording head and the position of the bridge reversed to that shown in Fig. 13 in order to illustrate the means by which the heads can be aligned in proper position. Comparison with Photographic Recording It is now time to make a brief comparison between photographic sound film and magnetic sound film, and I will therefore enumerate some of the many advantages and one of the few disadvantages of the latter. (1) Magnetic sound film requires no developing and printing before it can be played back, which can be done immediately, for example through the monitor amplifier, thus giving the assurance that what is heard from the monitor amplifier is on the film. As a consequence of the elimination of film processing, photographic types of distortion are absent. Moreover, since the magnetic sound film stock can be handled in daylight no dark rooms are required for it. (2) The magnetic sound records can be erased and the magnetic film stock used over again. (3) It is possible to erase a part only of the magnetic sound track and to dub in new sound in its place. On the other hand, there is a certain wear and tear of the heads, which have a normal life of about 300 hours, after which time they need replacement. Summing up, therefore, there is a number of reasons why magnetic sound film could be introduced with advantage into the film studios : (1) The frequency response can be made at least equal to that of the present photographic film. (2) The signal-to-noise ratio is very appreciably better. (3) The magnetic coating can be introduced on the same carrier as the photographic coating, having approximately the same physical dimensions. (4) The present speed of 35mm. film of 18in. per second and the same film driving mechanism can be used. (5) Photographic types of distortion are absent. (6) The cost of developing and printing can be avoided until the final release negative is made. (7) The small size of the magnetic recording and reproducing heads does not necessitate bulky equipment, and in many cases existing equipment can be easily adapted to accommodate them. One can therefore assume that there should not be too many difficulties in the way of introducing magnetic sound into the film studios, as it is so easy to handle and can be so readily adapted to the whole of the existing technique of mixing, dubbing and cutting, etc., to produce a final composite master sound record. Magnetic Sound only in the Studio All these advantages can be used to the best purpose in the studios, but