The motion picture projectionist (Nov 1931-Jan 1933)

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December, 1931 Motion Picture Projectionist 27 quires only black and white, instead of varying shades of grayness, it appears to be simple in handling. However, both the variable area film and the variable density film are seriously affected by the necessity of using a ray of light which, while small, is still of appreciable thickness. If a ray of light of imperceptible thickness could be used and satisfactory exposure obtained, many of the troubles in photographic development and in loss of the higher frequencies would be eliminated. Several rather elaborate papers have been written on the limitations of the two types of recording, but there still appears to be considerable theoretical and practical work to be done. Noise Reduction One of the most interesting recent developments in film recording is the so-called "noiseless" recording, which should preferably be known as "noise reduction." Operators are well acquainted with the "hiss" produced by the ordinary film when there is no sound recorded on it. This "hiss" or ground noise is due to small irregularities in the density of the print. If Fig. 14. The Light Valve Yoke the density of the positive were absolutely uniform there would be no ground noise. However, up to the present, it has been found impossible in the commercial handling of film to eliminate absolutely these irregularities which apparently are due largely to very fine dirt (dirt being, as usual, defined as matter out of place). Fortunately these irregularities are reduced in their effect as the area of the white part of the film is made smaller and as the density of the sound track is made higher — likewise, the noise is less perceptible when the sound is made louder, since it is masked by the sound itself. On the basis of these facts, noise reduction methods have been evolved for both types of recording. Application to Variable Density Recording In the variable density method, the purpose is to have the average density of the positive film as great as practicable at all times — that is, when there is no sound on the film the density of the sound track should be maximum. When the sounds are relatively weak, the average density is somewhat reduced and when the sounds are the loudest which can be recorded the average density becomes the same as it would be were noiseless recording not being used. In order to do this it is necessary that the density of the negative be just the opposite — that is, during the quiet intervals, the negative sound track will be practically transparent, whereas, during the loud periods the average density will be the same as with noiseless recording. The method of producing this effect is to place upon the light valve ribbons a direct current bias which causes them to come very close together and^ also superimpose upon this direct current bias a varying and opposing bias which is proportional to the loudness of the sounds being recorded. The current for the superimposed bias is produced by tapping off a part of the sound current at the bridging bus (Fig. 5, Part 2), amplifying this part and then rectifying it. This rectified part is then superimposed upon the direct current bias in such a way as to reduce that bias, thus increasing the average opening between the ribbons until in the extreme case that opening is the same as it would be were noiseless recording not being used. Its Application to Variable Area Recording Two methods of producing noise reduction on variable area film have been used. The first method is by placing upon the oscillograph ribbons a biasing current which acts in a manner similar to that of the light valve ribbons described above. This biasing current moves the average position of the oscillograph mirror close to one edge of the sound track for weak currents and keeps it near the center of the sound track for strong currents. The other method is to introduce a smaller shutter whose position is varied by means of a similar rectified current in such a manner as to follow closely the envelope of the variable area record, thus blocking out the light from that part of the normally exposed area not penetrated by the valleys in the negative. This blocked-off area becomes in the positive a black area which reduces the amount of clear film in the sound track and correspondingly reduces the amount of background noise. The background noise on disc records is largely due to the abrasive filler used in the disc record itself. DIAGRAM OF APPARATUS FOR VARIABLE AREA SOUND RECORDING Much work has been done towards finding a new material which while satisfactory in other respects will eliminate this noise. The work has not yet been completely successful, but it appears that when combined with work in other directions it will in the near future be possible to produce discs having the background noise very much reduced. (To be continued) BOOK REVIEW TALKING PICTURES by Bernard Brown, B.Sc. (Eng.). 12 chapters, 302 pages, table of contents and index. Illustrated. Published by Isaac Pitman & Sons, New York and London. Price, $3.00. DREPARED and printed in Eng1 land, this attractively bound volume contains, to quote its title page, "A practical and popular account of the principles of construction and operation of the apparatus used in making and showing sound films." To the American reader this subtitle is somewhat misleading, inasmuch as the subject matter is not restricted to the application of sound-on-film methods but also embraces a description of the principles of disc recording and reproduction. A chapter on the subject of theatre acoustics has likewise thoughtfully been included in the volume. Although the book has manifestly been written primarily for the British projectionist, a good portion of the text and many of the pictures with which it is profusely illustrated being devoted to the application of sound apparatus to foreign projection equipment, the volume contains much of interest for the American reader. The subject matter is presented in a logical and interesting manner indicating a comprehensive and intimate knowledge of the field and painstaking preparation of the text for publication. The volume should prove a useful addition to any projectionist's library. Fig. 15 N. Y. and Chicago S. M. P. E. Sections Elect Officers Officers for both the New York and Chicago Sections of the Society of Motion Picture Engineers have been elected for the ensuing year. In the New York Section, P. H. Evans of Warner Brothers has been elected Chairman; Donald E. Hyndman, Eastman Kodak Company — Secretary-Treasurer; and the Managers elected are M. C. Batsel, RCA Photophone, and J. L. Spence of the Akeley Camera Company. For the Chicago Section, the officers are: R. Fawn Mitchell — Chairman; Burton W. Depue — Secretary; and Robert P. Burns and Oscar B. Depue — Governors.