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FILM TRANSMISSION EQUIPMENT 107 projected picture. Bias lighting is used in much the same way as grid bias in a vacuum tube and changes the sensitivity of the tube to prevent grid current in the amplifier. (These controls are not generally adjusted during transmission.) While a program is on the air, a technician, known as the shading technician, sits watching a monitor on which is shown the picture from the camera. He has similar controls to those used by the studio camera control operator; in addition, he has four shading controls. These are two forms of vertical shading —saw tooth and parabolic. The names refer to the shape of the wave forms produced. The former corrects top or bottom shadows or flare and the latter similar faults in the middle of the picture. The horizontal shading controls are similar except that they correct for right- or left-hand side and center shadows and flare. While this job does not call for a lot of engineering knowledge but merely the ability to make rapid decisions and to act on them at once, it is one of the most important during the time that a film is on the air. A skilled shading technician can make all the differ- ence between a good and a poor film show. Almost every scene in a film has a different light value and causes different spurious emissions from the previous scene; therefore, the shading man has to be wide awake all the time to correct the picture continu- ously. An added feature of the film camera is the fact that it is possible to reverse film electronically. This means that if a negative film is the only one available it can still be used by setting the polarity switch on the top left-hand side of the camera to "nega- tive." This will now produce positive pictures over the television system. In the "positive" position, it operates normally. For special effects, a reversed, negative picture results from positive film and the switch in the "negative" position. The illustration of the RCA film camera shows the projector I working directly into the camera, but this does not mean that there has to be one camera for every film projector or slide pro- jector. RCA has introduced a device called a Multiplexer which makes it possible for two film projectors and one slide projector