Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1929 Page 11 similar in action to any of the variety of electrical phonograph reproducers on the market. It contains, however, a number of refinements which improve its fidelity over a wider range of frequency and volume. Change-Over and Fading The picture film and sound record are, of course, started together and both have the same playing time. If the subject be a multi-reel feature, as the end of the record is approached a duplicate projection machine and sound table are loaded with the succeeding film and sound record and, at a given cue in the picture, are ■j ■■■■■ Hi s MOCK BEMStM M 1 1 OUT IN OUT IN OUT ID R 01 15 15 \ / \9~ I M m. rr e 5 '." -9 — 7 ■'v K 0 5 ^H H An interesting type of fader manufactured by the Ward Leonard Electric Co. set in motion. Immediately thereafter the transition or "change-over" is made. This is done on the screen by closing the shutter of the expiring projector and instantly opening the shutter of the new machine. The change-over between the sound records is accomplished by a device known as a fader, by which the intensity of the response coming from the expiring record is reduced to zero and, subsequently, the response from the new record is brought up to any desired level. This transition may be accomplished gradually or rapidly, as is desired, and, like the change-over on the screen, is rarely perceptible to the layman in the audience. The Amplifiers The electrical impulses coming from the fader are then passed through a series of special amplifiers where they are enormously intensified. As an illustration let us consider the Bell Laboratories' system. This consists of three units. The first is a three-stage resistance-coupled amplifier employing low-power tubes energized by direct current. The second unit consists of a push-pull stage of medium-power tubes heated by alternating current, while the third unit consists of a push-pull stage using high-power tubes energized by alternating current. Plate potentials for all tubes are obtained from rectified alternating current. Small theatres require only the first two-units, while large theatres take one or possibly two of the third type units to obtain sufficient volume of sound for the auditorium without overloading. The three units have an over-all gain of 80 db, that is, they are capable of multiplying the energy of the reproducer one hundred million fold. The Loudspeakers The output of the last amplifier is brought to the output control panel where the energy is subdivided among several loudspeakers or horns, usually four in number. Two of these are placed in the orchestra pit and directed toward the balconies, while the other two are located behind the upper edge of the screen and directed downwards towards the rear floor seats. A small monitoring horn is placed in the projection booth for the A new type fader made by the General Radio Co. for General Talking Pictures Corp. convenience of the operators. A special type of screen, reflecting light well but transparent to sound, is desirable. The long exponential horns having an effective length of about fourteen feet are capable of receiving up to five watts of electrical energy and converting from 30 to 50 per cent, of it into sound. Provision is made on the output control panel for varying the amounts of power delivered to the individual horns. When the equipment is first installed in a theatre, a careful study is made and these controls are adjusted in such a manner that the directive properties of the horns tend to focus the sound so as to produce the illusion that it is emanating from the center of the screen. An acoustical treatment of the theatre by the use of reflecting and absorbing surfaces is often desirable to enhance this effect. Once made, Ihese adjustments are rarely changed. f»>*tp MM RY ROOM £ General layout of equipment comprising the Western Electric Sound Projector System, or ERPI.