Projection engineering (Jan 1932-Mar 1933)

Page 22 Standards for electrical reproduction and recording of sound ALTHOUGH projectionists are quite generally familiar with the application of the transmission unit, known as the decibel, to the discussion of the electrical reproduction and recording of sound, there is no doubt that they are often perplexed as to whether to include the photographic characteristics of the film in such discussions. For a given input to the microphone at the recording studio, and knowing the electrical characteristics of the microphone, amplifiers, light valves, and loudspeakers, it should be easily possible to calculate the output of the loudspeakers, basing the entire calculation on the transmission characteristics of the electrical circuit. However, the system is not a complete electrical transmission system, but is broken approximately at its middle by the motion picture film. The problem is, how to include the photographic characteristics of the film in the discussion. Fortunately, the photographic characteristics of film are logarithmic in nature, exactly as are the transmission characteristics of electrical circuits. The density of an exposed light-sensitive emulsion is a logarithmic function of S. M. P. E. Committee recommends aperture dimensions the quantity of light to which it has been exposed. Again, fortunately, the current output of a photoelectric cell bears a linear relation to the light incident upon it; and, finally, the luminous output of a light valve or glow-lamp is a linear function of the electric power operating the one or the other. So, the phenomena found in motion picture systems operate according to two laws, one of which is linear, the other of which is logarithmic. By converting the logarithmic functions into decibels, it is possible to consider them as a group, and the photographic characteristics of the film may be regarded in terms of equivalent electrical characteristics. The transparency (or transmission) of motion picture film is a linear function of its exposure, and its ability to transmit light is a linear function of its transparency. But the density of motion picture film is a logarithmic function of the transparency, so that in order to define the characteristics of the film in decibels we have to consider the logarithm of the transparency, or simply the density. V. C. Hall in the March issue of the Journal of the Society of Motion Picture Engineers, explains very clearly the manner of including the characteristics of the film in making calculations of the performance of the entire system. If the transparency of the film at two points is Tj and T2, respectively, and PROJECTION ENGINEERING the. corresponding densities are Dj and D2, the change of response of the system as the film passes from one point to the other is calculated in decibels either by 20 log (T2 — Ti) or simply by 20 (Di — D2). By employing densities, therefore, instead of transparencies, the calculation is greatly simplified. Much time has been spent by the standards committee of the S. M. P. E. during the past several months, on the problem of establishing dimensional standards for projector apertures. When sound was introduced into the picture, a portion of the frame of the film was taken up by the sound track, so that the photographer had to restrict the scene he was photographing conformably to the reduced area of the film which was available for photographic purposes. Likewise, the projectionist, in order to preserve on the screen a pleasing shape of picture, was forced to mask the top and bottom of the projector aperture, and so was unable to project to the screen the part of the action included in the masked portions of the film. On account of these difficulties the S. M. P. E. standards committee recently recommended that the width of the aperture be increased from the present width of 0.800 inch to 0.825 inch, changing the height by only 0.010 inch in order to assure a pleasing shape of picture in theatres employing large angles of projection. On May 9 to 12 inclusive, the Spring, 1932, Convention of the S. M. P. E. will be held at Washington, D. C, with headquarters at the Wardman Park Hotel. An extremely interesting program of papers is being planned, and arrangements are being made to hold an attractive exhibition of newly developed motion picture apparatus. Coming at the time of the Washington Bi-Centennial, there will be much to attract attendance to the convention, and everyone is cordially invited to attend. AAA Answers to Projectionists' Examination Questions It is a good sign to see so many projectionists taking the time to study the questions appearing in the successive issues of PROJECTION ENGINEERING. Some sets of answers are not correctly worked out, but in all replies sent in there is evidence of a desire to learn. The presentation of a series of technical questions monthly gives each projectionist reader an opportunity to test his knowledge. Sets of answers received to questions 21 to 25, which appeared in the December, 1931, issue, include those sent in by C. L Merrell, Selmer, Tenn.; Robert A. Passmore, Valdosta, Ga.; Frank Griesel Regina, Sask.; Albert Bolze, New Glasgow, N. S., Canada; Kirk Buchak, Minneapolis, Minn.; Wilbur N. Leamon, Marion, Ind.; W. Hoy, Calgary, Alberta.