International projectionist (Jan-Dec 1949)

tortion, each of which has been given a descriptive appellation. Symptoms: Wows, flutter, gargle, and whiskers. If the variations are either irregular or of any frequency up to 5 c.p.s., the effect is known as "wows"; il of some frequency between 5 and 30 c.p.s., it is called flutter; between 30 and 200 c.p.s., "gargle" ; and if over 200 c.p.s., it is known as "whiskers." "Gargle" cannot be heard in the bass tones, and "whiskers" are heard only as a raspy quality imparted to the high notes: for this reason "whiskers" may be wrongly interpreted by the untrained listener as non-linear distortion. Causes: Wows may be caused by grime-caked grease in rotary stabilizer bearing races, or by insufficient pressure — roller tension on the scanning drum. Wows and flutter are often caused by insufficient gate tension in the old-style soundheads — often well-nigh incurable in those antiquated models having an "impedance roller" interposed between sound sprocket and gate. "Gargle" and "whiskers" are caused by insufficient gate tension and worn sound-sprocket teeth in old-fashioned soundheads; also by a fluttering piece of lint accidentally lodged in the sound gate. Note that all forms of travel distortion may be produced by bent or sprung sound-sprocket shafts in certain obsolete soundheads ! Extraneous Noise Sources 6. Extraneous Noises. These are classified as types of distortion, although actually they are not. Extraneous noises result from the superimposition of un-, wanted externally-generated signals upon the recorded signal. When the film is at fault through scratches — an excessively large number of splices, coarse silver grains, etc. — little can be done about it. Cutting off the higher frequencies of sound by means of a variable tone control eliminates some of the hissing surface noise, but it also spoils the "brilliance" of the recording. Offending splices may be blooped over with Movietone ink; but the bloops, in order to be effective, must be at least one inch in length and very smoothly graduated. Inexpertly-made bloops create more noise than do bare splices. Frame-line noise and sprocket-hole "motor-boating," if not due to misalignment in printing, indicate that adjustment of the soundhead lateral film guide is needed. Power-line hum (120 d.v. if 60-cycle current, 100 d.v. if 50-cycle) may find its way into the sound system because of a burned-out rectifier tube, a defective power-amplifier tube, inductive pickup by amplifier circuits from an unshielded power-supplying transformer, inductive or capacitative pickup by photocell leads or speaker cables, short-circuited filter chokes or open-circuited filter condensers, or light from the projection room falling upon the photocells. Hum from nearby high-voltage neon signs is easily picked up. A strong clattering 96 d.v. "hum" sometimes accompanied by a faint 60 or 120-cycle hum or a high-pitched squeal, is caused by light from the projection head leaking into the photocell — usually by reflection from some object like the glass in the projector port. Recurring clicks are caused by film splices, by film marred by "sprocketing," by marquee flashers, and by sundry electrical devices in the theatre or surrounding buildings. Electrical shielding of the disturbing device is indicated. The 'Small-Shot' Effect A curious type of clicking noise warrants special mention because it has occasioned much needless worry. The crackles sometimes heard when a sound system is warming up are usually due, not to loose connections or something on the verge of burning out, but to the Schroteffekt (German for "small-shot effect"). This phenomenon, common to nearly all types of amplifier and radio tubes, has been explained on the basis of statistical variations in the number of electrons per second emitted by the tube cathode. One need not become alarmed by such sputtering and popping noises. They cannot be helped and they do no harm. Squeals and whistles in the sound are usually due to microphonic tubes. Such tubes, while they may be otherwise good, HENRYS. BE ARDSLEY— Owner, Chief Theatre, Oberlin, Kansas — says: "After nineteen years of RCA Service we still consider it our best assurance of continued satisfactory sound presentation." To get the benefits of RCA Service — write: RCA SERVICE COMPANY, INC., Radio Corporation of America, Camden, N. J. have elements which were loosened in manufacture or by rough handling. The loose elements within the tube begin to vibrate and the vibrations, by varying the distance between the elements, cause the plate current to be affected proportionately. The slight variations in the current are amplified to a singing howl. But squeals and howls may also be caused by wornout tubes with low electronic emission. All tubes should be checked periodically. How Many? Was this copy dog-eared when it came to you? How many men read it ahead of you? You would receive a clean, fresh copy if you had a personal subscription — and you wouldn't have to wait — you would be first to read it. Use coupon below. INTERNATIONAL PROJECTIONIST. 19 West 44 St.. New York 18. N. Y. D 1 year — 12 issues — $2.50 D 2 years— 24 issues — $4.00 Foreign and Canada: Add 50c per year. Enter my subscription for Name Address City State 30 INTERNATIONAL PROJECTIONIST • JULY 1949