Radio broadcast .. (1922-30)

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Thousands now all over America use the Claroceptor for clearer A. C. recep- tion. Get one right away—at radio parts dealers. Write for descriptive folder. £7.50 complete with cord and plug CLAROCEPTOR Built by ADVANCE ELECTRIC CO. 1260 W. Second St. Los Angeles, Calif. JOBBERS and DEALERS, GET OUR PROPOSITION VOLUME CONTROL IN BROADCASTING (Continued from page 261) audible, and, as amplifiers are used on all circuits, it is readily understood that cross- talk becomes a serious factor in transmis- sion over telephone wires. Experience has shown that the cross- talk level on a circuit is something of the order of minus thirty-five decibels. To express this differently it might be said that if an amplifier having a gain of thirty-five decibels were placed across a dead circuit and its output connected to a loud speaker or pair of headphones, a jumble of cross- talk from adjacent circuits would be clearly audible. This means, of course, that if we permit the energy level of the program being transmitted over the circuit under discus- sion to fall to a level of minus thirty-five decibels the j unable of noise and cross- talk in the circuit will be as loud as the program being transmitted at that partic- ular instant. It is necessary, therefore, to maintain a ratio in favor of the program transmitted. Inasmuch as this cross-talk level varies considerably with open wire circuits, de- pending on weather conditions, the margin between the lowest levels of the transmitted program and the cross-talk must be kept wide enough to prevent the cross-talk being heard by the radio listener. Cross- talk does not always consist of jumbled conversation and music but very often is induction produced by adjacent power lines, resulting in what is commonly called "line hum." Total Volume Range Experience has taught us that if we are to transmit a program over a telephone circuit in a satisfactory manner that we must not permit its minimum energy level to fall below minus twenty-five decibels. Of course, there are cable circuits where this minimum can be dropped to a minus thirty and sometimes minus thirty-five, but this latter condition exists chiefly in cities where underground lead-covered cables are used exclusively. We, therefore, find that the volume variation permissible over a telephone circuit is from plus six to minus twenty-five DB, a volume variation of approximately thirty decibels. However, the full range of 30 DB cannot be utilized as it is not possible to operate at the max- imum level plus six decibels in practice. To prevent too high a level being trans- mitted on the telephone circuits, the repeating amplifiers equipped with vacuum tubes will not pass energy levels in excess of plus six decibels peak level. Volume in excess of this merely over- loads the vacuum tubes, producing har- monics and distortion. Inasmuch as several hundred of these telephone repeaters are used in the networks of the National Broadcasting Company, a margin of safety must be allowed at the top to prevent overloading on repeaters whose gain setting is such that they may more than compensate the loss introduced by the telephone line. This condition is not brought about by the repeater changing its gain but by the telephone circuit changing its characteristic due to the weather condi- tions. For instance, when the circuit is lined up prior to a program it may be bright daylight with an intense sun shin- ing on the open wire lines, causing the resistance of telephone circuits to be at a maximum and, therefore, the transmission loss the highest. After sundown, the temperature of these wire lines is reduced materially and some of the transmission loss is removed, bringing up the input levels to all subsequent repeaters on that circuit, thus increasing the output level to a point in excess of the allowable maxi- mum. Of course these conditions are watched closely and attempts at correc- tion are made while the programs are in progress. Practical Limits Experience has taught us that it is necessary to leave a margin between the absolute maximum and the practical operating maximum. Therefore, our pro- grams are transmitted with a maximum energy level of plus two decibels, reducing the possible volume variation that can be transmitted to twenty-seven decibels. In transmitting a full symphony or- chestra whose volume variation is ap- proximately sixty decibels between its minimum pianissimos and maximum fortes, it is absolutely necessary for an experienced control man to reduce the maximum energy level to plus two de- cibels and raise the minimum energy level to minus twenty-five decibels. This can be done successfully if the volume control man, whose training makes it possible for him to visualize these problems, is assisted by a musical director who can interpret for him the score of the symphonic music several bars in advance of the playing in order that he may smoothly control with accuracy the gain settings of his amplifier. If this man did not function, the radio audience would listen to a badly over- loaded and distorted transmission on the fortissimos and would be unable to hear the minimum pianissimos because they would be over-ridden by cross-talk. This explanation is not written in any criticism of these limitations placed on radio broad- casting by the wire lines, but it is a compli- ment to the engineers who have made the present facilities as satisfactory as they are for the handling of radio programs. At the present time every effort is being bent to improve these line facilities not only with respect to volume variation that can be handled, but also to widen the band to transmit more bass and more harmonics. Characteristic Curves Several curves showing the line charac- teristics of network circuits will be found accompanying this article and an explana- tion of the data they contain may be of interest. The four curves in Fig. 1 show the audio- frequency characteristic of transmission through WEAF, Bellmore. The output curve shows frequency characteristic of the radio transmitter alone which is even from 30 to 10,000 cycles within plus or minus 1 DB. This, of course, covers a range of frequencies considerably greater than the average receiver and loud speaker will reproduce. The second curve shows the charac- teristic of the telephone line .from New York to Bellmore, including the line amplifier at New York and the speech amplifier at Bellmore. It will be noted that there is a slight drop at the lower end which is of no consequence and at the upper end of the curves it will be noted that the telephone line cuts off sharply at 8000 cycles. The telephone line from 711 Fifth Avenue to Bellmore is a cable cir- cuit of latest design and unlike the usual run of broadcast circuits, is passing fre- quencies from 30 to 8000 cycles. By ref- erence to one of the other curves showing the typical network circuit, a comparison of the two line characteristics may be noted. The third curve is the addition of the first two curves, showing the radio trans- mitter and line together. The bottom curve shows the overall characteristic of the transmission system from the microphone to the air, including all speech input equipment at 711 Fifth Avenue; considering that this includes the entire broadcasting system, a variation of 6 DB is still acceptable to the average radio receiver and the average ear, so it 292 • • RADIO BROADCAST FOR MARCH