Projection engineering (Jan 1932-Mar 1933)

Page 20 RCA Photophone remote level indicator THE Type PB-30 remote level indicator is a device used in conjunction with a sound-on-film recorder for the purpose of visually indicating, at some point remote from the recorder, the volume level of sounds being recorded. It consists essentially of a small recording optical system mounted in a small case of rectangular shape. Basically, the unit operates as does any photophone sound-on-film recorder except that the light beam is finally directed against a white screen for visual monitoring instead of being directed through an optical lens barrel assembly to a film. This remote level indicator consists of a lamp and socket, a condenser lens, an aperture plate, a "dry" galvanometer, two mirrors, an adjustable mirror mounting, a fixed mirror mounting, a white screen, a fixed resistor, a variable resistor, a base casting, a terminal block and a removable cover. The rating of the lamp is 4 volts, ^4 ampere. The fixed resistor mentioned is connected in series with the lamp, and is of such a value that the lamp will draw its rated current when the terminals are connected to a 6 volt source. The lamp socket is adjustable so that the lamp filament may be readily centered upon the optical axis of the condensing lens. The condensing lens is located in a brass tube mounted between the lamp socket and the galvanometer, and serves to focus the image of the lamp filament upon the galvanometer mirror. In the brass tube with the condensing lens is the aperture plate, the aperture being rectangular in shape. The galvanometer is much smaller than the galvanometers standard with Types PR-3 and PR-4 recorders, and is similar in general construction to the galvanometer used in the Type PR-7 recorder (Types PM-15 and PM-19 newsreel equipment). The galvanometer is air damped, no oil chamber being used as is the case in the Types PR-3 and PR-4 recorders. The galvanometer is totally inclosed in a dust-proof case. The light beam from the lamp reaches the galvanometer mirror through a small window containing a lens. The light reflected by the galvanometer mirror emerges through the same lens, but at an angle from the incident beam, and strikes the adjustable mirror. From the adjustable mirror the light beam passes to the fixed mirror and is reflected to the viewing screen, upon which the image of the aperture is focussed by the galvanometer lens. The two mirrors are used to obtain a long throw from the galvanometer in a small space, since the longer this throw the greater is the apparent sensitivity of the device due to the resulting magnification of the galvanometer deflection as it appears on the screen. The mirror is adjusted in angular setting by means of a worm-gear to provide the necessary centering of the aperture image on the viewing screen. A hole (one inch by one-half inch) in the front of the cover permits observation of the viewing screen during normal operation. The cover also contains a partition which surrounds the lamp (except for the space between the lamp and the condensing lens) and prevents reflected light from interfering with the sharpness of the aperture image on the screen. The cover is clamped to the base casting by two thumb-nuts which must be unscrewed to permit its removal. A variable resistor is connected in series with the galvanometer coil to provide a sensitivity or calibrating control. This resistor is located under the base casting and its shaft appears in front of the galvanometer in Fig. 1. Four terminals are provided on the rear of the base casting for the connection of the lamp filament current supply and the sound current input to the galvanometer. The overall length of the remote level indicator with the cover in place is 6}4 inches, and the width and height are both 3^2 inches. PROJECTION ENGINEERING Electrical Connections The filament current supply for the lamp may be obtained from any source of 6 volts, direct current, or from any other direct current supply provided suitable resistors are connected in series with the line to maintain a potential of 6 volts across the lamp terminals of the remote level indicator at a current of 0.75 ampere. For use with the Type PA-30 amplifier the galvanometer terminals will be connected to the low impedance winding of a line coupling transformer, the other winding of which will be connected in parallel with the recorder across the 500-ohm sound output line from the main recording amplifier. For use with the Type PA-47 amplifier, the same coupling transformer will be used, but its high impedance winding must be connected across the output of the power amplifier unit which supplies the sound currents to the monitor speaker. For use with the Type PA-2 amplifier and the Type PR-1 recorder no coupling transformer is required, the galvanometer of the remote level indicator will be wired in series with the recorder galvanometer, and the thermogalvanometer volume indicator on the amplifier must be disconnected. In this case the sensitivity control is connected in parallel with the galvanometer instead of being in series as shown in Fig. 2. Adjustments Only three adjustments are required to place the remote level indicator in the proper operating condition. These adjustments are very simple and once made require no repetition except when it is necessary to replace the lamp or, possibly, the galvanometer. Tne adjustments are : the adjustment of the lamp and socket, the adjustment for zero modulation, and the adjustment for full modulation of the light beam. Characteristics The resistance and frequency response characteristics of the galvanometer are very closely similar to the galvanometers used in all RCA Photophone soundon-film recorders. The long throw of the light beam, resulting from the use Fig. 1. The Type PB-30 remote level indicator. Center — Front view with cover removed to show location of parts. Right — Cover of the unit shown in the center view. Left — Rear view showing location of terminals. GALVANOMETER