Radio Broadcast (May 1928-Apr 1929)

DECEMBER, 1928 BROADCAST STANDARDIZATION 103 input to the amplifiers from film and disc sound records. The illustrations were excellent. Richardson talks on paper, so to speak, which is fine in fostering informality but uses up a lot of words when carried too far. The August 25 issue of the same magazine carried a description of the RCA Photophone system, under the title of "How RCA Photophone Times Synchronism." Except for the title and a few mistakes in the text, the article was informing enough. The combined bibliography and review printed above will give some idea of the variety of publications in which articles on or pertaining to sound pictures are to be found. It includes only those periodicals in which such material appears more or less regularly. Broadcast Standardisation THE National Electrical Manufacturers' Association (NEMA) has a transmitter Section which deliberates occasionally on the subject of what broadcast transmitters should be like and in what terms it is valid to talk about them. The last meeting, in June, 1928, discussed a number of technical subjects especially pertinent in view of present developments. The following methods for adherence to assigned frequencies by means of automatic master oscillator control are specified: a. Quartz crystal b. Standard clock with harmonic amplifier c. Tuning fork d. Magnetic striction bar In any case it is specified that the master oscillator is to be arranged to be independent of external changes in humidity, temperature, barometric pressure, or loading. Under the allied subject of frequency monitoring the Section adopted as a standard the use of an oscillating or heterodyne frequency meter whose frequency is held constant by one of the methods above, and so constructed that it can be shipped periodically to a primary standardizing laboratory. In rating the coverage of a broadcasting station the population contained within the area over which the field strength is 5000 microvolts per meter, or more, is considered basic. Beyond this, under favorable transmission and reception conditions, it is permissible to add the population within a circular area having a radius four times the mean radius of the basic area. In determining the distances corresponding to the 5000 microvolts per meter field strength, measurements are to be made during the daytime on not less than 10 radii spaced at approximately equal angles around the station. AH this is, of course, empirical, but it is certainly effective in bringing down estimates of broadcast coverage from the blue sky to the solid earth. Applying the method to a specific case, we may use the Radio Field Strength Contour Map of Washington D. C, and Vicinity, presented as Fig. 9 in the paper by Bown and Gillett: "Distribution of Radio Waves from Broadcasting Stations over City Districts," (Proceedings I. R. E., Vol. 12, No. 4, August, 1924). This map was based on measurements made on the old wcap 500-watt transmitter, which is no longer in existence. The contour lines in the case of Washington are quite close to circles, the transmitting conditions being favorable for urban conditions (few high buildings, and a general distribution of low buildings and open spaces). The 5 millivolt per meter contour is a circle with a radius of about 14 miles around the transmitter. The population within this circle would have been the basic population © Fairchild Aerial Camera Corp. AN OLD MAP SHOWING RADIO FIELD-STRENGTH CONTOURS OF WEAF SUPERIMPOSED ON AN AERIAL PHOTOGRAPH OF NEW YORK CITY served by wcap. Under favorable conditions wcap would have been credited with the population within a radius of 56 miles (four times the mean radius of the basic area, in this case fourteen miles). The half-tone on these pages shows the radio field strength contours for weaf superimposed on an aerial photograph of New York City. In rating the audio-frequency characteristics of a broadcasting station the NEMA Transmitter Section prescribed the following method: The number of octaves transmitted above 800 cycles (the mean speech frequency) and those transmitted below 800 cycles, with a deviation not to exceed plus or minus 1 TU, measured from the microphone input terminals to the rectified antenna output, shall be counted, and the smaller of these two numbers multiplied by two. The resulting number shall stand as the audio-frequency characteristic rating of the station. On this basis a transmitter with a frequency characteristic flat within 1 TU between 100 and 7000 cycles would receive a rating of 6, since it transmits 3 octaves both above and below 800 cycles. If it only went up to 4000 cycles its rating would drop to 4, since it would be based on the two octaves above 800. Even if it went down as low as 50 cycles it would receive no extra credit, since the method of rating requires a balance between the ability to transmit high and low notes. About the highest rating within reach is 8, entailing flat transmission up to 12,800 cycles on the high end, and 50 cycles on the low. Apparently no one can get credit for going down below 50 cycles. If loud speakers are improved this point might be criticized, and likewise the 1 TU tolerance is open to question, since it cannot be detected by ear. A 3 TU tolerance might be preferable in practice. The general method, however, seems excellent. Under "Modulation Capability" the committee specifies a single-frequency sine-wave audio input, to the maximum degree of modulation possible without "noticeable distortion," the analysis being on the basis of rectified radio-frequency output. For the purpose of supervising modulation the Section specifies the use of a standard volume indicator, on the scale of which the following relative limits are to be allowed: Constant testing tone Music peaks Piano peaks Speech 30 divisions 30 20 15-20 These values correspond to standard practice in chain broadcasting. The piano is more sensitive to slight overloading and so is given more margin, while speech is kept down to a value where announcements will not break into the music with obtrusive loudness. Regarding microphone set-ups for broadcasting the committee decided that, in "view of the present relatively undeveloped state of this portion of the art," this subject should be tabled. This was no doubt a prudent move, since as things stand there are as many microphone set-ups for a given aggregation of musicians as there are musicians, announcers, engineers, musical directors, acousticians, program managers, commercial sponsors, studio supervisors, and advertising experts in the room, every man is sure he is right, and nobody can prove anything one way or the other. Under "Standard Reference ('Zero') Level for Broadcasting Use," the NEMA group laid down the following specification: "It shall be standard for broadcasting use, to regard the term 'reference level' ('Zero level') as referring to a power of 10 milliwatts, corresponding to a current of 4.17 milliamperes flowing through a resistance of 600 ohms, or 2.47 volts across 600 ohms." This definition should put a stop to the endless wrangling about "zero level" which has been going on among the broadcasters. Transmitter name plates, says the NEMA, should contain the following data: (a) Power rating in kilowatts; (b) Radio-frequency range over which the set will deliver full power; (c) Characteristics of the antenna for which the set is designed. That the power rating is to be in terms of power delivered to the antenna should have been specified, since in many countries power to the plates of the radio-frequency tubes is the basis of rating. If the Transmitter Section continues its work on this plane it will become one of the most infl uential agencies in this branch of the industry.