International projectionist (Jan 1941-Dec 1942)

Effect of Static on Sound Systems METHODS commonly employed in theatres to combat "man-made static" interference can best be understood if the nature of the troublesome impulses is analyzed. They are, usually, radio signals, even when not originating in any kind of radio device. Diathermy machines, which produce so much interference of this kind, are essentially short-wave radio transmitters. Many of them could be heard hundreds of miles away with no other change than connecting their output to a transmitting antenna. It is easily understandable that they can influence vacuum tube apparatus at short distances. Less generally realized, perhaps, is the fact that any automobile motor is a kind of radio transmitter, putting out a signal at about twenty meters. The radio-frequency generator in the car is, simply, the spark plug. The spark is not a mere D.C. discharge, leaping steadily and smoothly across a gap because of its high voltage. No spark is a simple flow of D.C. Early wireless transmitters, before the days of radio, used spark-gap generators as their source of radio-frequency current. Actual automobile spark plugs were used by radio amateurs up to about 1920 and affected code transmissions over quite a few miles distance. Passing automobiles are not likely to trouble the average theatre sound system, simply because the spark and all wires leading to it are so thoroughly shielded by the motor block and car body. Much more troublesome are sparking contacts in flashers and other advertising displays associated with the front of the theatre. When current sparks across an airgap it effects an erratic contact, a highspeed, make-and-break contact, generating a high-frequency field. However, a spark never produces a pure or single frequency. The greater part of its energy will be concentrated on or about a frequency determined by the inductance and the capacitance of the circuits carrying the sparking current. The oldtime spark wireless transmitters were tuned to their assigned frequencies by adjusting inductance and capacitance intentionally added to their circuits. Sparktype diathermy machines are tuned that way today. In the case of sparking in flashing sign contactors, generator brushes or other machinery, the bulk of the spark's high-frequency energy will be bunched at a frequency determined by accidental, unavoidable inductance and capacitance of the wiring. The oscillating discharge is never "sharp": it spreads out over a wide range of neighboring frequencies. Hence, if a sound system is nearby and at all subject to picking up this form of disturbance, there will likely be some sensitive wiring, somewhere associated with that system, with accidental inductance and capacitance capable of responding to one of the many frequencies the spark discharge puts out. An erratic, irregular high-frequency current will then be induced in such wiring. Since sound equipment is not designed for high frequencies in the sense of radio frequencies — millions of cycles per second — the h.f. induced in the sound wiring will not be amplified or heard in the speakers, but the irregularities or modulation thereof, if of audio frequencies, will come through. The usual result is a rasping, tearing, erratic kind of noise. This last point should, perhaps, be emphasized. Sound, as every projectionist knows, consists of frequencies from 15 to 15,000 cycles, more or less, and can be represented by electric currents of the same frequency range. Yet sound currents are received by radios tuned to hundreds of thousands or millions of cycles. The radio-frequency current is the "carrier" on which the audio or sound current is superimposed; that is, the audio or sound component constitutes an irregularity in the high-frequency radio current. This irregularity is separated from the radio current in the receiving set, further amplified, and applied to the radio speaker as ordinary sound A.C. That is exactly what happens when a theatre system picks up a spark discharge as noise: the high-frequency discharge generated by the spark is the carrier, and the irregularities accompanying it are separated from it in the sound system, amplified, and heard as noise. It is plain from the foregoing that the remedy is to keep out the carrier. This is accomplished as a rule by shielding and grounding. Where these methods fail, as they sometimes do, two different remedies remain. One is to stop the trouble at its source, such as by fixing sparking contactors so they won't spark. Another method of stopping the discharge is by connecting a condenser across the contacts: or better still, by connecting each contact to ground through a condenser. This addition, where the condensers are of relatively large capacitance, absorbs and thus suppresses the high frequency generated. Where the source of the sparking cannot be located, or where it is outside the premises and not under theatre control, another resource is to locate the "receiving" circuit in the sound system and de-tune it so thoroughly that it will no longer respond to the interference. This is usually done by connecting radiofrequency choke coils of appropriate inductance in series with the offending circuit of the sound system. N.T.S. Head Scores Neglect During "Sellers' Market" IN RESPONSE to a request for comment on the current "sellers' market," the result of the great demand on materials and manufacturing facilities by national defense needs, Walter Green, president of National Theatre Supply Co., issued the appended statement, which should prove of considerable interest not only to projectionists, exhibitors and supply dealers in general, but also to those manufacturers of equipment who unwisely refrain from promotion efforts at this time and contribute as little as possible to the efficient operation of their equipments in the field. Mr. Green's statement follows: "One of the shrewdest comments on human nature in business was made recently by a well-known sales executive when he said : 'The basic symptom of a sellers' market is a mounting neglect to cultivate customers.' "Never must it be said that we at National are guilty of such neglect. It is our duty now, more than ever before, to build for the future, to visualize the day when there will be no so-called 'sellers' market', either real or imagined. "Customers have long memories. Suppliers who have given them the service to which they are entitled will be remembered and will profit accordingly. The others will fall by the wayside. This is the time to give customers an extra measure of personal service. These are the days when we can cement our present friendships and create new ones. "National can and will show exhibitors how to get the best out of their present equipment, if new equipment is not immediately available. National can suggest an equipment check-up that will help avoid shut-downs at crucial moments. By being fore-sighted for our customers and for those we want as customers, we can render the kind of service that will pay dividends, not only now, but three, four or five years from now." AUGUST 194 1 15