International projectionist (Jan-Dec 1937)

December 1937 INTERNATIONAL PROJECTIONIST 17 to mask out all light except that which has passed through the track. The photographic method just described is not applicable to this type of head. The "TIME II „KCIA Mow March "TIME e/tCl A »0/rr¥ RCA variable area track mask is adjusted by listening for hum, either in the projection room or through an observer in the auditorium; or by observing the mask through a magnifier while a piece of film is in position and the machine is turned over slowly. The magnifier will show whether an image of a sprocket hole edge or of a framing line is projected on the mask in such position as to overlap the slit through which light reaches the cell. The mask is then adjusted laterally until the only light penetrating the slit is that which comes from the sound track. Focal Placement of the Film 2. Since light focussing arrangements of some type are an inescapable necessity, it follows that the sound head must piovide means for keeping the sound track at the point of focus. A common piojection room experience demonstrates the extent to which this adjustment is critical. Some test films are so made that they can be threaded and operated with the emulsion side of the film facing either way. When threaded the wrong way, the sound track is removed from proper focal placement by the thickness of the celluloid. The loss resulting from this small maladjustment is conspicuous in output meter readings. Yet the displacement amounts only the film thickness, 6/l,000th inch. In some heads, the film is held in focal adjustment by being made to pass through a sound gate, where polished metal shoes under spring tension press against it. More recent design causes the film to slide over a curved surface, which may be either stationary or rotating. The sprockets, guide rollers or pad rollers of such heads are so placed that the moving film is held firmly against the curved surface, and therefore in focal position. With either design, wear of the parts ov buckling of the film permits the sound track to shift periodically into and out of focal placement. In a sound gate, this may result from wear of the shoes, or weakening of the tension of the springs behind them; in either case the film is held less tightly. Wear of the guide, pad or idler rollers that press the film against a curved surface has the same effect. A buckled (corrugated) film cannot be kept in perfect focal alignment by any mechanism. Since a departure from perfect focal adjustment shows up readily in sound volume as read by an output meter, it follows that when the sound track shifts FILM GUIDE LATERAL OUIOE ROLLER ASSEMBLY PHOTOCEa SHIELD periodically into and out of focal placement, volume in the theatre will rise and fall in synchronism. This effect, taking place at a rate of several hun EXCITE.R. LAMP ADJUSTING NUT SOUND ' HOLD BACK TAKE-OFF DRUM SPROCKET I , ADJUSTING NUT UNIT LATCH Interior of operating side of modern RCA soundhead, showing principal features THIS THEATR -rf Pate/Mti&'&c THIS THEAT -d ?a&ti3lte*$A W. E. variable density track dred (or several dozen) times per second, produces an annoying type of distortion known as flutter. The need for periodic inspection of all parts of the head concerned, and their prompt replacement when they show signs of wear, is obvious. Control of Film Motion 3. Another and more common form of flutter results from irregularity in the motion of the film through the beam of light. Controls for this motion are built into every head. Nearly all heads move the film in addition to controlling it, but there is one type in widespread use which has no sprockets at all. Film comes down to it from the intermittent sprocket, loops around the sound head controls, and returns to the take-up sprocket of the projector; thence back through the sound head to the lower magazine. In this form of head, steadiness of film motion is insured by the action of the essential control, a rotating drum against which the film presses; in all others the action of a rotating drum or other control cooperates with sound head sprockets in keeping the film pace steady. If the speed of the film varies, the striations or peaks of the sound track will pass through the light at speeds other than intended. Since the rate at which the striations or peaks pass through the light govern the frequency of sound, it is plain that when the film moves faster than it should the pitch of all sounds is raised; when the film moves too slowly the pitch is lowered. When the rate of speed changes periodically, a few dozen or a few hundred times per section, all sound will undergo a rhythmic rise and fall in pitch. This is the most common type of sound flutter, extremely annoying to the audi