Projection engineering (Jan-Dec 1931)

Page 14 PROJECTION ENGINEERING lect them as the beam is swung up and down under the deflecting action of the electrostatic fields. The rate of deflection per second will determine the number of times each commutator plate will be connected and disconnected within that time. For example, if the picture frequency transmitted is 21 frames per second, a rate of deflection in the commutator of 7 times per second will paint each transmitted frame a different color (or shades of that color) and the superimposition of the different frames at the receiver will reproduce all the natural colors of the subject being televised. A minimum of 12 frames per second is necessary to produce the illusion or continuity upon the optic nerves of the human eye. Twenty frames give even better results and a greater number of frames (about 32) would result in steadier, non-jumping pictures. Unfortunately we are again limited by the number of frequencies permissible within certain limits now allowed to television broadcasting stations. However, for all practical purposes the 20 frame per second value is quite satisfactory. When each frame is to be painted separately the problem is different and the difficulties also increase. To avoid the colors from jumping or moving beyond the object it would be necessary to repeat the same color at least 20 times a second, and as three colors are used it would be necessary to transmit 60 frames per second. This would be impossible to accomplish with mechanical devices, but certain new sys tems of electrical scanning now being experimented with will probably overcome this difficulty. Supposing, however, that 21 frames a second are transmitted. Each color will be repeated seven times in a second and in that case if the commutator has three contact plates the frequency of deflection will be 7. The mode of operation of this system is as follows : The photoelectric cells (5) receive the light reflected from the object (6) ; this light varies in intensity with the color or shade or brilliancy of the object. The photocells are divided into three different groups and each group is made up of cells having different characteristics. The commutator connects successively each group of cells to the transmitter circuit and the voltage variations given by those cells are amplified and transmitted. For example, a part of the object which is red will reflect a certain amount of light which will be reflected upon all cells, but only cells having a red color filter will allow this light variation to pass. The process of color selection is then automatically performed by the color filters and the different cells of various characteristics. The problem of assorting these colors is accomplished by the commutator. Supposing that at any given time the group of cells receiving more blue light is connected at the transmitter and that at exactly the same time another commutator working in synchronism connects a contact plate (9) connected to a light source (10) with a blue filter (11), then that color will be projected by lens (12) upon a lens disc (13) and then upon the screen (14) with exactly the same brilliancy or shade of color as it was transmitted. The same process is repeated over and over again. Each frame is painted one color or shade of that color and all the 21 frames superimposed one upon another blend and reproduce all the natural colors of the subject being televised. For the sake of simplicity mechanical scanners are shown, but the scanningcan also be done electrically within the light sources and thus eliminate all mechanical motions. Fig. 3 shows a new and very interesting form of "electronic" commutator. A large electromagnet winding is connected to a source of direct current. A special neon tube containing one grid made of metallic screen and of three or more collector plates, is placed in the center of the magnet so that the top plate is exactly in the center of the field. The electromagnet repelling electrons within that tube from all sides, focuses them to the center of the field, the top plate glows and current flows through the circuit: By placing an extra field within that field, a saw-tooth oscillator for example, the focal point of the lines of force is displaced up and down and causes the electrons to move from one plate to another, thus effecting a commutating action. This device can work at extremely high speeds or rate of commutation. Innovation to stage production in sound system for Norman Bel Ceddes' "Hamlet" ANOTHER innovation for legitimate stage productions was contributed by Electrical Research Products, Inc., with the opening in New York of Norman Bel Geddes' "Hamlet" at the Broadhurst Theatre, on November 5. Shakespeare's century-old lines were enhanced by modern science in the form of music and sound effects coming from the loudspeakers of a Western Electric music reproduction system. Special recordings have been made of a type similar to those used for electrical transcription programs broadcast over the radio. Twin turntables, operating at 33 1/3 revolutions per minute, are so arranged that there is no pause in switching from one record to the next, and also provide for superimposing one sound effect upon another. This superimposing facility is used to particularly marked advantage in the ghost scene, during which the weird sound of the wind at midnight continues through a brief musical selection which heralds the appearance of the ghost. The sound from the phonograph records is reproduced through five loudspeakers located at various positions back-stage and controlled by a switching system which permits selective or simultaneous operation. Thus, in the mob scene by switching from one speaker to another in sequence the sound gives the illusion of a crowd surrounding and approaching the citadel, and the weak, far-off murmurs and cries develop to a great climax and loud cheers when the gate is finally broken down. The prelude, overture and entire musical accompaniment to the show is reproduced over the system — there being no orchestra or other conventional music used in connection with the play. The reviews and audience comments in Philadelphia, where the Bel Geddes production had its tryout, were enthu siastic about the entertainment value contributed by the most unusual music and sound effects. "Hamlet" marks the fourth recent major stage production in which Western Electric sound equipment has been used to marked advantage. The first was the "Miracle of Verdun," the Guild production where synchronized music and sound were used to heighten stage realism. The George White "Scandals" at the Apollo Theatre has a public address system installed to reinforce the voices of the actors and singers, thus making rear seats as desirable as the front row from a hearing standpoint. In the new Earl Carroll Theatre where the "Vanities" is meeting with such success, a complete system of general reinforcement, including facilities for curtain call, announcements, manager's call system and reproduction of the stage program throughout the dressing rooms, is an important added service.