The motion picture projectionist (Nov 1931-Jan 1933)

December, 1931 Motion Picture Projectionist 17 Sound Head Optical Systems By R. J. Stier t No single item of the sound reproducing equipment is guarded with more jealous care than the sound head optical system. Projectionists have repeatedly been warned by the sound equipment manufacturers not to tamper with it — that its adjustment is critical and a task for the sound engineer alone. In the following article Mr. Stier explains the sound head optical system and describes the important function it performs in sound-on-film reproduction. — The Editor. OUITE contrary to general opinion, there is nothing of mystery about the sound head optical system as it is used in the average theatre reproducing equipment. It is true that contractual obligations enjoin the theatre owner from authorizing or permitting the projectionist to make adjustments on optical systems he feels are out of proper adjustment. These obligations are included for the universally recognized reasons that the highly trained service engineer can, most advantageously, unerringly locate the causes of troubles which appear to sound as though they were occasioned by faulty optical system adjustment and correct them to the best interest of all concerned. Special Training Required In addition to specialized technical training, optical system adjustment normally requires the use of a special test film, meters (or equivalent apparatus) and keen discriminatory judgment. As a concrete illustration: faulty optical system adjustment causes poor quality sound distinguishable to the trained ear by the lack of "brilliance" or "sibilance" in the upper frequency range. Under some conditions a faulty amplifier tube which, because of age has lost certain of its characteristic qualities, will cause a similar sound output to result. Obviously, adjustment of the optical system would be no cure for this particular trouble and would, in fact, aggravate it. The purpose of this article, then, is to explain the operation of the optical system and to describe means of adjustment in order that some of the mysteries surrounding this piece of apparatus may be removed. This information is offered to enhance the projectionist's general fund of information only and in no wise conflicts with the instructions in force at his theatre. Since such adjustments are delicately balanced, and since a misadjustment of the order of 0.001 inch may cause the difference between sat t Engineering Dept, RCA Photophone, Inc. isfactory and unsatisfactory sound, the wisest course is easily apparent; let the well-equipped service engineer keep this equipment in adjustment. While, popularly, only the "lens barrel" is spoken of as the optical system for purposes of completeness this discussion will include the source of light, i. e., the exciter lamp and the photocell. Under these conditions, then, the complete optical system consists of the source of light, the lens barrel, and the photocell. Purpose of Exciter Lamp The function of the exciter lamp is that of furnishing a steady, intense source of light. In its ideal form, the exciter lamp filament would be simply a straight line, horizontal, parallel to the plane of the film, and perpendicular to the direction of film travel. Since a lamp filament consisting of a straight piece of wire would not yield sufficient light for the purpose, and, in addition, would sag considerably when heated to incandescence, thus introducing a variation in focus, the filament of the exciter lamp used today generally consists of a very closely wound tightly coiled tungsten wire. The axis of the helix formed by the filament wire is in the horizontal plane and the lamp is so positioned as to meet the requirements mentioned above, namely, it is parallel to the plane of the film and perpendicular to the direction of film travel. Because of its heaviness and relatively low operating temperature the exciter lamp has a reasonably long life and is an extremely brilliant source of light. In the case of reproducing equipments fully alternating current operated and having no rectifiers to supply the exciter lamp, the filaments of the exciter lamps are made considerably heavier and are operated at higher current values. The thermal inertia of the lamp is, in these cases, increased and the light emitted by the lamp does not vary with rapid fluctuations of exciter lamp terminal voltage with the result that undesirable "hum" is greatly reduced. Components of Lens System The "lens barrel" proper contains two condenser lens assemblies and a slit. The slit consists of a pair of knife edges micrometrically set in position. They must be absolutely parallel and must be separated by precisely the distance required by the design of the optical system. If the distance separating the knife edges is too great, even though they be parallel, the over-all response of the reproducing equipment will vary widely from that desired. Widening the slit has the disastrous effect of reducing the high frequency response of the reproducer; too great a reduction of the distance between the knife edges of the slit actually improves the response characteristic of the equipment but does so at the sacrifice of gain or an increase in amplification required. In general, a narrower slit possesses a more desirable response characteristic but also requires more amplification to produce suitable sound in the auditorium than does a wider slit, which possesses a less desirable response characteristic, but which requires much less amplification to produce sound of the same intensity in the auditorium. In this matter, as in all other engineering problems, a suitable compromise is effected between the extremes of ideal response with its resulting impractical amplification requirements and the modest amplification requirements of a wide slit having a totally unsatisfactory response. Width of Slit Slits in commercial usage today are of varying width. However, nearly all, in combination with their condensing lens assemblies, result in a light beam whose width is approximately 0.001 inch at the point at which the light beam strikes the sound track of the film. It will be seen readily that a slit having a width of 0.005 inch used with a lens having a reduction ratio of 5 to 1 will result in the same width light beam as will a slit having a width of 0.0015 inch used with a l/2 to 1 reduction lens — all other conditions being equal. In addition, the use of the wider slit and greater reduction power lens offers one advantage of adjustment not possessed by the smaller slit and lens assembly which is that any given variation from perfection in adjustment affects, on a percentage basis, the resultant width of the light beam at the film to a lesser degree. To understand the necessity for a narrow slit (or light beam) consider, for a moment, sound as it is recorded on the film. Standard film speed for sound pictures is ninety feet per minute. If that figure be reduced to terms of feet per second the answer will be found to be 1.5; or eighteen inches per second. Now, if sound be recorded on this film continuously for one second it will always fill eighteen inches of length on the sound track. This is true regardless of the frequency (or number of complete vibrations per (Continued on page 33)