International projectionist (July-Dec 1934)

10 INTERNATIONAL PROJECTIONIST July 1934 than 3 or 5 volts for the 7-mm. and 8-mm. carbons, respectively. These are the outside limits, for in order to utilize the total ranges the mechanism would have to be adjusted so that the average position of the carbons would be exactly at the center of the permissible variation. It must also be borne in mind that if the arc length varies in either direction much beyond the limits of good operation, the current and voltage become erratic and swing through a considerable range. If the feeding mechanism is controlled by either the current or the voltage the above action causes a sudden change in the rate of feeding that is not desirable. Feed Adjustments It is not practicable to adjust the feeding mechanism so that it will operate exactly at the center of the permissible range, nor can it be expected that other conditions might remain sufficiently constant to keep it exactly in that position. It is therefore necessary that the mechanism be designed to feed the carbons within a variation much less than the theoretically allowable limits. The narrower the range, the easier it is for the projectionist to maintain the lamp adjustment within the limits of satisfactory arc operation, and maintain a uniform intensity of screen illumination. These narrow limits for maintaining the arc length presented a real problem, but through close cooperation between the National Carbon Company and the manufacturers of projection equipment the desired results have been accomplished. In the paper1 previously cited, it was mentioned that the high-intensity a-c. arc can be operated in series with a suitable resistance unit from the power line, but that for practical reasons a resistance is never used. A transformer and reactor are used instead. The transformer gives an electrical efficiency of 90 per cent or more, a figure that can not be equaled when ballast resistance is used. The reactor is usually the leakage reactance of the windings of a "high-reactance" transformer. It is desirable that the reactance be kept comparatively low, in order to maintain a reasonable power factor. On the other hand, it must be high enough to assure sufficient stability so that the arc may not be extinguished by ordinary drafts at the longest desirable arc length. Tests have shown that a 40-per cent reactance will afford sufficient stability to the arc. In other words, if the no-load voltage on the secondary of the transformer is about 40 per cent higher than the load voltage, there will be sufficient stability of the arc for ordinary applications. Additional reactance would improve the factor of safety, but above a no-load voltage of about twice the load voltage, the effect would not be noticeable. A lower reactance could be used and the results might be satisfactory in most cases, but any advantage achieved in reducing the reactance would not be worth the risk of an "outage". Another important factor in the design of a transformer for use with the high-intensity a-c. arc is the possible variation of line voltage on different installations. The transformer should be provided with suitable taps, or other means, so that it can be adjusted for the average line voltage of the theatre in which it is installed. If the line voltage at the theatre should vary appreciably, convenient means should be provided for the projectionist to change the transformer taps and so regulate the secondary voltage. Standardization in Studio and Theatre Aim of S.M.P.E. Sound Committee Abstracts from Report of the Committee to Spring, 1934, Meeting Fig. 7. 8-mm. a-c. high-intensity carbons: 80 amj>eres, 26 volts; medium arc length, arc disturbed by external forces. IN A communication addressed to the Chairman of the Committee by President Goldsmith the Committee was asked '* . . . to formulate standards of sound recording and reproduction (audio-frequency characteristics) of such a type that the producing studios and the theatre circuits can all agree to accept them at a reasonably early date after the standards shall have been agreed upon. "The present state of sound recording and reproducing indicates that the matter is definitely urgent. There is an unnecessary amount of deviation in releases from the various studios, and it is obvious that the full advantages of improved methods of reproduction can not be realized under the present conditions. Such standardization is the most important problem facing the Committee." In order to attack the aforementioned problems, it was thought advisable to establish two major sub-committees of the Sound Committee, one of the subcommittees to be representative of the East Coast and the other of the West Coast . . . On April 12th, the East Coast SubCommittee met and the discussions and conclusions that were reached were, it is believed, of particular significance to the Society and to the motion picture industry in general. The first question considered was, "Is it agreed that frequency characteristics measured in current or power are a measure of quality?" In answering the question, the Committee agreed that "in a linear, flutterless, noiseless system, frequency range measured in current or power is one factor that determines quality." It was agreed that for the purpose of study by the Committee, the sound system should be div ided into not less than four sections: (1) Acoustics of the stage and characteristics of the microphone. (2) From the output of (1) to and including the release print. (3) From the release print to the input of the loud speaker. (4) From the loud speaker to the ear. One of the most difficult tasks was to determine a starting point for the discussion. It was the consensus of opinion, however, that a standard for determining frequency characteristics should be esi tablished. It was agreed that frequency characteristics should be measured in terms of calibrated prints of frequency film, this print corresponding to release prints. Special Print Available A print is being prepared, to be independently calibrated in the Bell Telephone Laboratories and in the laboratories of RCA Victor Co., which, when completed, will be kept in the offices of the Society as a reference standard. Data will be available in the S. M. P. E. office regarding the measuring circuits employed in calibrating the film, and the methods of making comparisons with sub-standards. This film will be available to studio personnel for use in calibrating secondary standards. An attempt will be made, by the time of the next meeting, to obtain data on the frequency characteristics being used in the several recording studios in the East. These data will serve as a basis of discussion of present practices and methods followed in determining the characteristics now employed. It is the aim of the Sound Committee to lay a foundation for one, two, or even five years' work, if necessary; and to formulate a plan so that at the comple