Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, June, 1930 Page 19 quired. However, the power output of two tubes in parallel is not equal to twice the output of a single tube. Empirical determinations have shown an increase of about 30 to 40 per cent. Furthermore, the use of tubes in parallel does not permit a signal voltage in excess of that permissible with one tube. 20. Push-pull arrangements afford advantages not found in parallel systems. In the first place, it is permissible to apply a greater value of signal voltage, approximately twice that permissible with one tube. Second the output is greater than that available with two tubes in parallel. Third, the operating characteristic of the tube is altered so that greater freedom from distortion is secured. Fourth, pushpull systems minimize hum due to potential variations in the grid-filament circuit due to the hum in the filament supply or induced into the grid circuit. 21. The association between the vacuum tube and the device connected into the tube plate circuit is very important, so much so that it is the difference between good quality reproduction and poor reproduction. Hence haphazard) replacement of defective coupling units is impossible. When one unit is removed, the replacement should be identical. 22. With respect to the plate voltage applied to vacuum tubes, it is necessary to remember that a voltage drop takes place across the coupling unit located in the plate circuit. This means that the voltage at the tube plate is less than the voltage at the source of supply. Due to the current carrying capacity of the coupling units, plate voltage increase is definitely limited, since each increase in plate voltage increases the plate current and flow through the coupling unit. Excessive current flow will injure the plate coupling unit, regardless of its type. 23. Do not remove tubes from series filament installations unless the proper precautions are exercised to prevent overloading of the other tube filaments. The same is true of tubes in parallel connected across transformer windings. Forms of Coupling The choice of any one type of coupling unit is largely a matter of like and dislike. Also of the required frequency characteristic. In this connection we can say very little since the amplifier designer is the one who decides upon the type of coupling employed in the amplifier. With respect to trouble, however, all amplifiers are alike. Not that the same trouble will be found in all types of amplifiers since different types of coupling units are afflicted with different forms of trouble, but rather that the basic factors associated with trouble are present in all amplifiers. To secure satisfactory operation with all amplifiers, certain requirements must be fulfilled. A defective condition means that one of the requirements has not been fulfilled. The conditions are : 1. Input signal voltage within operating limits of vacuum tubes in the system. 2. That the plate potential at the tube plate be correct. 3. That the grid potential at the tube grid be correct. 4. That the filament potential at the tube filament be correct. 5. That the coupling units be in perfect condition. 6. That circuit wiring be correct. 7. That connections in the circuit be perfect. 8. That all the amplifier components be in good condition. 9. That the filament supply be in good condition. 10. That the grid supply be in good condition. 11. That the plate voltage supply be in good condition. 12. That the tubes be the type suited for the receiver. 13. That the tubes be in good condition. A cursory analysis of the above is sufficient to show that each of the subjects is allied with the other and also associated with certain phenomena. To expedite matters, we believe that it is better to give basic reasons for the various requirements, thereby permitting application of the data to the various types of amplifiers. We say without fear of contradiction that with respect to trouble all amplifiers are alike. That is to say, the conditions of overload are applicable to all types ; the effect of an open is the same in all amplifiers ; the effect of a short circuit is the same in all amplifiers, etc. Hence, why dwell upon the individual types? The conclusion of this useful article ivill appear in the July issue of Projection Engineering. A SOUND TELEVISION SERVICE PROPOSED Construction of a broadcasting station in New York .which would be used entirely for broadcasting sound in conjunction with a television station synchronized with it, is sought by Radio Pictures, Inc., of New York, in an application filed April 5 with the Federal Radio Commission. The application reads as follows : "Construction permit for construction of a new station for the purpose of broadcasting sound in conjunction with television signals which are broadcast from a television station at the same location which is now licensed. VOLUME CONTROL In the July issue of Projection Engineering will appear an account of the application of volume control to public-address systems. There have been many inquiries for detail information on this timely subject. The article is by E. W. D'Arcy, chief engineer of the DArcy Laboratories. THE CHIEF PROBLEMS OF TELEVISION (Concluded from page 17) than 40,000 cycles and in radio communication it is extremely difficult to transmit wide signal bands. It is, therefore, obvious that either the picture must be much smaller or must be divided so that it may be sent by a large number of channels. Transforming the Impulses Now assume that the signal has been transmitted undistorted to a distant point. The electrical signals must be changed into light signals. The same difficulty at the receiving end is encountered as at the sending end. Here again the light signals are not strong enough. It is possible to imagine an optical system by which this might be accomplished, but it would not be practical to have a system of lenses and lights revolving at the speed of the disc. It is necessary to develop a method of controlling the intensity of the light source very rapidly so that it can be viewed directly. Sender and Receiver Must Keep Step Another element upon whose solution success depends is synchronization. The recreation of the image is possible only when the elements fall in the right positions at the right instants. No element of the image can be out of place by a considerable fraction of the size of an element. To obtain synchronization is to have the speed of the revolving discs at both receiving and sending ends rotating at the same speed. After the same speed is reached it is sometimes difficult to retain it, due to fluctuations in the line voltage at either the receiving or sending end. To summarize, the four chief problems the engineers in the field of television are at present endeavoring to solve are : First, converting light signals into electrical signals ; second, the transmission of these electrical signals ; third, re-converting the electrical signals into light signals ; and fourth, synchronizing the scanning discs at the two ends of the circuit. WARNER BROTHERS ACQUIRE ALL BRUNSWICK RECORDING Warner Brothers recently acquired all the radio, phonograph and phonograph recording interests of the Brunswick-Balke-Collender Company for $12,500,000. The company retains its billiard table interests. Negotiations were completed by Harry M. Warner and Robin Bensinger, president of Brunswick-BalkeCollender. With the interests acquired by Warner Brothers go patents used in connection with them.