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REPRODUCTION IN THE THEATRE 399
printing and developing of the sound track on the film or badly worn disc records, too low a charging potential for the photoelectric cells, too low filament currents or plate voltages on the amplifiers, insufficient magnetizing current on the receivers and other similar causes.
The extraneous noises which usually tend to be introduced by mechanical vibrations and interference from the power supply circuits, have been reduced to a minimum by the use of shielding and of both mechanical and electric filters in the design of the apparatus. Very little extraneous noise will be introduced in the film machine, provided the sprockets are kept in proper alignment and the film gate is kept clean.
Poor maintenance may result in development of bad contacts, faulty tubes and other similar troubles. These troubles usually result in causing one or more of the above types of distortion to be introduced into the reproduced sound.
PORTABLE SOUND PROJECTOR
Fig. 13 is a photograph of the Western Electric 202A projector, a portable equipment.
This projector has a maximum throw of sixty feet. The maximum picture size is seven feet by eight feet. The apparatus is suitable for audiences up to 800.
In the upper left hand corner of Fig. 13 may be seen the 1000 watt incandescent projecting lamp, with its mirror and condensing lens. Directly to the right is the projector with the orifice through which the beam is projected discernible under and adjacent to the lifting handle on the outside of the case. Directly under this is located the photoelectric cell and between this cell and the center of the case is located the sound gate, lens assembly and exciting lamp. Only one magazine is used. The two reels are placed on the same shaft with a spacer between them, the take up reel in the inner position. The film is threaded from the outside reel up through the outer feed sprocket in a large loop which passes over the top of the projector down through the light gate over the intermittent sprockett, then over the inner feed sprocket, down through the sound gate, over the sound sprocket and into the take up reel. It thus leaves the magazine and describes a loop through the apparatus back to the other reel in the same magazine. The preliminary amplifier