Projection engineering (Sept 1929-Nov 1930)

Page 10 Projection Engineering, May, 1930 Frequency Cuoe/icTieisTic Caere Photo Tube ^MPLiF/ee Foe D/pect P/c<up Equipment " \n \ P 0 ■$ .s 4 — h 5 > \ > C < § § i 1 FeevuENcyjH Cycles Pee Second Fig. 8. Important to obtain a gain frequency ch a racteristic flat from 15 cycles to 50 kc. essentially flat from 15 cycles to 30 kilocycles has been attained. At first we fed these lines out of paralleled output tubes but encountered the classic difficulties of the system. We have had considerable success with special low impedance tubes designed for the purpose, the operating characteristics of which are as follows : Ep— 100 v. Es 10 v. Fig. 9. Simple geometric figure for transmission test Ef— 2.5 v. Ip — 72. ma. M — .55 Rp— 710 Gm— 775 We have also utilized "grid-positive" tubes in this connection with the operating elements so chosen as to give a low output impedance. These tubes are purposely allowed to draw grid current and are current operated rather than voltage operated as in the ordinary case. Fig. 10. Evidence of distortion. In outside operations over a distance of several miles from a studio where permanent setup is possible, we have run into a great deal of trouble which we were able to correct by utilizing specially strung separated lines of large cross section with appropriate delay correction networks. Impedances Matched We have been able to reduce reflection with attendant losses and distortion of the picture by carefully matching the line impedances and correction of the delay characteristics. Fig. 11 shows the type of network inserted in the lines to correct for phase distortion. The entire program as transmitted, together with the outgoing speech for synchronizing over a second station, is controlled by a program director, an announcer, one operator who sets up the film pickup machines and operates the main amplifier, and another who controls the carrier. Fig. 12 shows the layout schematically. Periodic runs over these lines serve to keep their frequency characteristics up to normal requirements. The transmitter proper is similar to any broadcast transmitter of similar capacity except for the fact that the interstage circuits are coupled so as to obtain substantially even modulation up to 30 kc. input. Special modulator circuits are employed with chokes so wound as to affect but slightly modulation at the higher frequencies, and are so designed as to be capable of complete modulation of the carrier when desired. I have shown the circuit employed in coupling the last two stages and the 600-ohm transmission line feeding the antenna in Fig. 13. There are two basic classes of amplifiers ; those which draw grid current and those which must never be permitted to do so. In circuits as we know them today we maintain the grids of a-f. amplifiers always slightly negative to avoid harmonic distortion which is fatal to quality. Our precautions are, however, equally fatal to economy. In our r-f. transmission circuits, though, we employ amplifiers which are fed at an extremely high voltage level. We bias the grids of these tubes to cut off (of plate current) and beyond, and even then drive the grids positive during a fraction of each cycle. While this method is productive of harmonics of the fundamental, it is also highly efficient. In our audio amplifiers we would be unable to filter out and destroy this harmonic distortion, but in r-f. circuits the harmonic frequencies are widely removed from the fundamental and quite amenable to discipline. If the interstage transfer takes place across a capacitance the reactance may be made so low at harmonic frequencies as to effect a virtual short circuit. Grid Circuit Damped The fact that these amplifiers draw grid current gives rise to a further distorting effect in that the grid-filament impedance is substantially lowered during a portion of each cycle and precautions must be taken to prevent this change being reacted back into the plate circuit of the preceding stage. To this end, the grid circuits are damped by resistances of such value that the grid-filament impedances will always remain high in comparison. In a circuit employing 100 per cent, modulation we rate the carrier power at one-fourth the peak power under modulation. The r-f. stages must be so designed as to handle these increases. The main advances needed for better transmission are along the following lines : o. Photoelectric cells having a high output with little frequency discrimination, no background noise, Fig. 11. Type of network to correct phase distortion. and a uniform sensitivity over the color spectrum. o. A light source, even in distribution and steady in intensity and with emission over the entire spectrum. c. A scanning method of extreme ac curacy and mechanical simplicity. d. Increased economy of the trans mission band through better understanding of transmission circuits. So far we are able to transmit moving pictures of a standard character and good detail with surprising realism. Certainly some of the subjects, where the conditions of lighting are favorable, are easily comparable with the motion picture of two decades ago. We transmit regularly motionpicture films of prominent personages, vaudeville teams, singers, etc., together with synchronized speech which is fed to a second transmitter over a 20 mile leased line. We transmit