Projection engineering (Sept 1929-Nov 1930)

Projection, Engineering, October, 1929 Page 17 How and Why the Fader Technical Data on the Design, Characteristics and Application of Typical Faders By Horatio W . Lamson* WE assume that all of our readers are familiar with the effective motion-picture technique, the fade-out, whereby the close of a scene is dissolved byrestricting the field of view towards the center of the picture frame, and, at the same time, reducing the intensity of illumination. This is usually accomplished in the studio by slowly closing one or two iris shutters while the camera is in operation, or the fade-out may be produced in the projection booth by the manipulation of an iris shutter on the projector. With the advent of synchronous and nonsynchronous sound accompaniment for motion pictures, the need arose for an analogous acoustical fading-out device, the fader. The purpose of this instrument1 is twofold. The acoustical condition of a theatre, due to its dimensions, architectural features, and especially the F F Jl » .. — .. .. vvw 1 1 ii ii h R5 Fig. 2. Circuit of the simplest form of fader. size of the audience, varies considerably from time to time, so that the fader serves first of all as a convenient means for controlling the volume level of the reproduced sound in order to achieve the most natural and pleasing results. The proper monitoring of a sound presentation to meet the existing conditions contributes greatly to the success of the program. The fader also provides for reducing the output of an expiring film or disc sound track to zero and subsequently building up the level of the new sound track to the proper value. An abrupt change between the sound tracks at full volume levels gives rise to undesirable transients in the electrical systems. By the use of the fader the change from one to the other can be made in such small steps that it is not perceived by the audience, and transients are minimized even though the transfer is made as quickly as possible. To accomplish these pur Fig. 3. Circuit arrangement used when a transformer is interposed between the pickup and the amplifier poses the fader is built in the form of a two-sided or bilateral attenuator. We propose to discuss the relative merits of various types of faders. Voltage Divider Type The simplest scheme is shown in Fig. 2, Pi and P2 represent two pickups (for film or disc tracks), and s is a movable contact which slides along the resistances R and connects to the grid of an amplifier tube. Here, because the tube draws no energy, the fader operates as a simple voltage divider having a total resistance of 2R. The load R in which each pickup is terminated should have a value approximating the impedance of the pickup if it is desired to match impedances. The adjustable resistance in all of these faders may be constructed in one of two forms : as a continuously-adjustable slide-wire or as a step-by-step attenuator in which a contact-stud type of switch adjusts the attenuation in predetermined discrete increments to give any desired calibration scale, which, however, is usually made uniform in decibels. The slide-wire type may likewise have a scale calibrated uniformly in decibels if the form upon which the wire is wound be cut in the proper shape. The calibration of the arrangement shown in Fig. 2 is simple, since no power is drawn from the output. If r be the resistance between s and the filament, then the number N of decibels attenuation for any particular scale position of s is given by r # = 20 log—. (1) R It is often desirable to interpose a transformer between the pickup and the amplifier. In this case the fader arrangement shown in Fig. 3 might be used. Here the values of R should be quite large so that the currents drawn from the secondary coils of the transformers Ti and T2 will not be of sufficient magnitude to injure their frequency-response characteristics. The calibration equation is, of course, the same as that for the arrangement of Fig. 2. Current Attenuator The faders function in both Figs. 2 and 3 purely as voltage dividers. When these methods are used, it is highly desirable that the fader be made an integral part of the amplifier so that the lead connecting s to the grid of the tube may be shielded and kept as short as possible. Such a procedure is feasible in non-synchronized equipment, but it is not so desirable with * Engineering Dept., General Radio Company. 1 Horatio W. Lamson, "How and Why the Talkies," General Radio Experimenter, III, December, 1928 and January, 1929, and Projection Engineering, Sept., 1929. Fig. 1. At the left is the fader proper: at the right is the "dummy" or auxiliary control. In practice the two are mechanically coupled.