Projection engineering (Jan-Dec 1931)

Page 12 PROJECTION ENGINEERING box in which the film is exposed to light shining through a lens system in one end. However, the two machines differ in that the film moves uniformly and continuously past the lens system in the recording machine, whereas in the camera the film is drawn down intermittently a frame at a time past the lens. The same type of film magazine is employed now generally on both machines, and they are both equipped with footage counters that indicate the number of feet of film run through the machine. ' The mechanical construction of a film recording machine is evident from the accompanying photograph. The rectangular metal box is the chamber in which the film is exposed, and the two drum-shaped containers are the magazines in which the film is held before and after exposure. The film travels down from the loaded magazine through the recording machine and on into the other magazine. On the end of the exposure chamber opposite the driving motor is the lens system and the light valve assembly. Farther out on this same end of the machine is the lamp house in which the lamp that furnishes the light of constant intensity is located. The footage indicators are the two knob and pointer arrangements on the front of the exposure chamber. The whole recording machine and motor assembly is built on a special concrete foundation. Just beneath the center of the machine is suspended a square metal box carrying the photoelectric cell amplifier, the function of which was discussed in a preceding article. The large panel on the wall back of each recording machine is the control board on which are mounted the battery and speech circuit controls and meters and the other equipment that is associated with the recording machine. There are usually two film recording machines attached to each recording channel. The film magazines are removable to permit loading and unloading the film, and the door on the front of the exposure chamber is easily opened to allow the threading of the machine and the punching of the film for the start mark. The film magazine from which the film is taken is known as the feed magazine and the other one is termed the take-up magazine. It is necessary that the angular velocity with which the film is carried past the lens system that focuses the image of the light valve onto the film be as uniform as it is physically possible to make it. In the photograph showing a loaded film recording machine, it will be observed that there are two large wheels around which the film is carried inside of the exposure chamber. These wheels are known as the sprockets. It is while passing over the left-hand sprocket, which engages just twenty of the sprocket holes in the edge of the film, that the film is exposed by the light passing through the light valve. These sprockets are both driven by the motor shaft through a worm and worm-wheel, but whereas the right-hand sprocket is driven directly by the shaft of the worm-wheel, the left-hand. sprocket is driven through a special mechanical filter. This mechanical filter is a form of heavy flywheel that is connected directly to the sprocket shaft and through a filter arrangement of springs to the shaft carrying the worm-wheel drive. Mechc Filt er Courtesy Hal Roach Studios Loaded film recording machine. The purpose of this mechanical filter is to iron out any variations that may occur in the speed of the driving motor and to compensate for any imperfections in the driving gears. The heavy flywheel built directly on the motor shaft likewise helps to steady the speed of the motor. All of this mechanical filtering has for its object the elimination of flutter in the sound track, which is produced by any intermittent variations in the angular velocity of the film as it is carried past the point of exposure. The speed at which the film is pulled through the recording machine is precisely ninety feet a minute, corresponding to a driving motor speed of I2CO r.p.m., and in the ideal recording machine, the film would pass the exposing light beam at this speed with absolute uniformity and steadiness in rate of travel. The uniform rotation of the left-hand sprocket wheel is not affected by variations in the rotation of the right-hand, or feed, sprocket, due to the two loops of film by which the sprockets are connected, because any variations in the rotation of the feed sprocket from normal will be absorbed by these two film loops and have no effect on the angular velocity of the left-hand sprocket. This right-hand feed sprocket draws the film out of one magazine and feeds it down to the other take-up magazine. If a steady sine wave representing any audio frequency, such as iooo cycles per second, is recorded and there is any flutter present in the recording machine, the recorded sound when it is projected will vary up and down in tone. The same thing happens to any sound that is recorded in a machine in which there is flutter present, but it is more noticeable with a steady tone because of the pureness of the note. The Assembly Directly ahead of the lens system attached to the front end of the exposure chamber, and through which the modulated light shines on the film and exposes it, is the magnetizing winding and the light valve assembly. Still farther out pn this end of the recording machine is the metal lamp house in which the lamp that provides the light for exposing the film is located. Between the lamp and the light valve is a condensing lens system that serves to concentrate the light from the lamp onto the opening in the frame of the light valve. The mounting of the lamp is adjustable so that it can be moved around to the position that permits the condensing lens to focus the light most sharply on the hole in the light valve frame. This is one of the adjustments that must be made . every morning by the men who operate the film recording machines. An eighteen-ampere projection lamp with a heavy flat ribbon filament less than an inch long is used. It is lighted from a twelve-volt lamp battery through an adjustable rheostat. The lamp and light valve are so arranged that the plane of the ribbon filament of the lamp is focused on the plane of the ribbons in the light valve. The metal housing that may be seen over this lamp is for the purpose of shielding the recording operator's eyes from the very bright light of the lamp. Since the speed with which the film moves past the lens system in the recording machine is constant and unalteiable, the degree of exposure of the sound track is regulated by varying the intensity of the light from this lamp. In order that a sound track of the correct average density will be obtained, it is necessary that just the right amount of current be supplied to the lamp. There is a vernier rheostat on the main rheostat that controls this lamp, and the two of them, with the ammeter that indicates the amount of current drawn by the lamp, are mounted on the large control panel on the wall. The arrangement of the battery circuits