Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

40 April 5, 1930 EXHIBITORS HERALD-WORLD within the coil varies and the direction of this force will depend upon the direction in which the lines are cut. FILM REPRODUCER With the optical or film record a different type of pickup apparatus is required. Any sound, as you know, may be defined by pitch and loudness. These characteristics, pitch and loudness in a film record (Western Electric System) may be identified respectively by the number of parallel bands per unit length and the density of each band. In other words, the relation of these lines is an index of the pitch of the sound and the density of these lines represents the sound intensity. There is another type of film record which involves a somewhat different principle and is known as a variable area film record. The details of this method of sound recording will be discussed in other papers. However, this record is reproduced in identically the same way as the constant width variable density record first discussed. In transforming the film record into electric energy the essential elements required for this transformation consist of an exciting lamp, a lens system and a photoelectric cell. Fig. 3 is a schematic diagram of the sound head, parts of which will be described later in detail. It is evident from the relative location of apparatus as shown in Fig. 3 that it is not feasible to print the film sound record directly beside the picture to which it applies. The sound track is printed 14% inches or 19 frames in advance. This allows some slack between the sprocket which carries the picture with an intermittent motion before the picture projection lens and the sprocket which must carry the sound record with a uniform motion in front of the photoelectric cell. Special precautions are necessary to prevent vibrations and speed fluctuations, due to either a varying supply voltage or a varying load, from affecting the uniformity of rotation of the sound sprocket. The speed of the driving motor is automatically OCALI TE Soniiid Screen Vocalite combines a distinct clear transmission of sound with sharp brilliant pictures. It enables better projection because of its mechanical, scientific and optical features, the result of painstaking study on the part of the laboratory staff of the Beaded Screen Co. IT IS 100% PERFECT FOR LIGHT, SOUND, COLOR AND THIRD DIMENSION Descriptive Literature on Application BEADED SCEEEN COUP 448 WtST 37™ STREET N.Y. FACTORY ROOSEVELT, NEW YORK FIGURE 2 controlled as described in this paper under the heading Maintaining Synchronism. A mechanical device is also interposed between the sound sprocket and the rest of the moving equipment of the projector so that no abrupt change of speed will be transmitted to the sound sprocket. Fig. 4 shows the exciting lamp and the lens relative to the film plane. The light from the exciting lamp is focused onto the film in a very narrow beam, one mill in width. The reason for the very narrow beam of light is that the reproduced frequency is a function of the speed of the film and the width of the light beam falling on the film. In order to meet the requirements for high frequency reproduction, it is necessary that this beam be so small that it may project independently each of the narrow bands into the window of the photoelectric cell which is located beyond the film. The photoelectric cell on which the light falls after passing through the sound record is shown in Fig. 5. Film reproduction is made possible through the use of this photoelectric cell or one having similar characteristics, that is, a cell capable of emitting elecrons at a rate proportional to the incident light FIGURE 5 within certain predetermined limits. This cell consists of two electrodes, one a photoactive metal and the other the sole function of which is that of an electric conductor. The photoactive metal most used for the purpose of sound reproduction is potassium. However, other alkali me:als have been used. A polarizing voltage is placed across the terminals of the photoelectric cell through such a high resistance that in operation there is obtained from the cell a voltage across this resistance which is proportional to the incident light. This cell may be though of simply as a re EXCITING LAMP FILM PLANE SLIT WIDE * 3/16' LG -APPROX. 4 1/4 LIGHT BEAM .001“ *;SSo, WIDE < 1/8 LONG AT riLM PLANE LENS TUBE DIAGRAM FIGURE 4 sistance which varies directly with the quantity of light falling on the cell. If the emulsion on the sound track be dense naturally not a great deal of light will pass through it. But if the emulsion is less opaque more light will go through and the sound will be greater. The principle is similar when variable area sound track is used. When the serrations have a wide swing extending well across the sound track the volume will be high; a small swing indicates low volume. The photoelectric cell circuit is shown in Fig. 6. In a high impedance circuit such as this, local interference, sometimes termed static, is readily picked up and it not guarded against will produce serious distortion in reproduction. Since the energy level is so small, induced current may be appreciable in comparison to the sound currents themselves. In addition, there are other electrical effects which may create some distortion. Because of the low level of this energy, it would be dangerous to transmit it any great distance before it has been amplified. Therefore, an amplifier (called a PEC amplifier) is placed immediately adjacent to the photoelectric cell circuit to amplify the energy to a level at which it can be safely transmitted. This amplifier increases the photoelectric cell output approximately 50 decibels or an energy ratio of 1 to 100,000. The photoelectric cell and amplifier are encased in a heavy metal box which is fastened to the frame of the projector and the frame is carefully grounded. Further precautions are taken to insure against mechanical shock bv carefully suspending the tubes of the amplifiers. The output of this amplifier is approximately the same as that of the magnetic reproducer used in disc reproduction. This will permit the remainder of the reproducing system to be used interchangeably between film and disc pickup. As shown in Fig. / this change is facilitated by means of a transfer switch. The photoelectric cell amplifier is not considered necessary when cells of lower impedance and higher energy output are used. REPRODUCTION IN THE THEATRE, by S. K. Wolf. Paper based upon lecture-demonstration before School in Fundamentals of Sound Recording and Reproduction conducted by the Academy of Motion Picture Arts and Sciences, Hollywood, 1929. (P-2 Wo.) This is the ninth paper in the scries. Addendum PORTABLE SOUND PROJECTOR prepared for Academy Tcchanical Digest by R. A. Grist of Electrical Research Products, Inc., staff. [To Be Concluded Next Week]