Radio Broadcast (Nov 1926-Apr 1927)

184 RADIO BROADCAST DECEMBER, iv_ the audio frequency range normally transmitted to avoid resonance effects. The diaphragm is usually from two to three inches in diameter, which gives a capacity of about 400 micromicrofarads with close spacing between diaphragm and back-plate. This relatively low capacity limits the length of the cable between the transmitter and the input stage of the amplifier system to under 20 feet, and even then it is necessary to devise a special low capacity conductor, since inherently the instrument is a high impedance device, liable to bypassing of the higher sound frequencies if shunted by any considerable capacities. In one form, the transmitter is incorporated in one unit with the first tube, to get around this difficulty. In studio pick-up, however, there is little objection to the more usual arrangement of a compact two-stage amplifier placed on the floor, with a 12-foot length of low-capacity cable running to the transmitter, which is mounted on a concert stand. The vibratory system of the condenser transmitter is essentially the same as that of a high quality carbon transmitter. The latter requires a flow of direct current, in which audio variations are produced through the changes in resistance consequent on the vibration of the diaphragm. Analogously, the condenser transmitter operates with a constant polarizing voltage, which may be as high as 500 volts, but to reduce insulation difficulties, is more commonly set at about 200 volts, supplied by the amplifier plate battery through a suitable resistance. The vibration of the diaphragm of the condenser, when affected by sound waves, varies the capacity of the instrument by about one-hundredth of one per cent., which is enough to produce a slight audio ripple on the grid of the first tube. Fig. 2 shows how the polarizing voltage is connected to the transmitter and the audio output tapped off capacitively. The instrument has a tendency to be two or more times as sensitive at very low and very high frequencies than in the middle range from 1000 to 5000 cycles. This may be corrected in the associated amplifier. The sensitivity of a condenser transmitter is given by Wente as 0.35 millivolt per dyne of force exerted by the air wave impinging on each square centimeter of the diaphragm. A high quality carbon transmitter will produce over 5.0 millivolts for one dyne per square centimeter of sound pressure, across a 200-ohm load. The carbon transmitter is therefore much more sensitive, since it produces more voltage per unit of air pressure across a low impedance than the condenser across a high impedance. Putting it in terms of telephone levels, we may say that a condenser transmitter, with an output of a fraction of 1 microwatt, is 60 TU's down. A high quality carbon transmitter of the usual sensitiveness is only about 30 TU's down. A gain of 30 TU's means about two stages of high quality amplification. We note, therefore, that the condenser is two stages below the pushpull carbon, while the latter is still two stages below zero level, which may be taken as the average commercial telephone power, involving a power of 0.01 watt. The relatively low quality commercial telephone transmitter is from four to five stages better than a condenser in power output. Unfortunately, it does not provide the quality of output required in broadcasting. Wente intended the condenser transmitter mainly for reliable measurements in the field of sound, and it continues to be used for this purpose in such highly fruitful measurements as those of Fletcher and Wegel on the sensitivity of the ear, Crandall and Mackenzie on energy distribution in speech, etc. Its more immediate use in broadcasting (a "practical" application, as short-sighted persons would say) provides material for scientific controversy among the more luxurious broadcasters. It is a favorite topic for luncheon arguments among the metropolitan broadcast engineers, second only to analyses of the shortcomings of announcers. The condenser, with its associated amplifier, so placed and padded that it does not pick up microphonically on its own hook, with the best of tubes, and the transmitter itself kept clean and dry, gives a beautiful acoustic output with a practically silent background. The latest and best carbon microphones do substantially the same thing, but expert laboratory maintenance and a large stock to choose from must be available. Some of the early models of condensers were unsuited for broadcast operation, and the troubles to which they gave rise, noised about {noised is an unconsciously chosen appropriate word) among the technical brethren, gave the instrument a bad reputation, which, as is usual in such cases, tends to cling to it beyond the proper time. Regarding this, I offer in testimony one condenser transmitter which has given excellent quality without the least disturbance for nine months, although knocked over twice by the studio staff. On the other hand, I should not like to be left without a few good carbons around the station; one sleeps better that way. If Mr. Harry Sadenwater, the champion of condensers among broadcast operating engineers, and Mr. O. B. Hanson, whom 1 nominate for the same position on behalf of the carbon 373-W and its successors, should care to stage a public debate in Carnegie Hall, 1 shall be glad to receive a free ticket and to cheer at the ringside. Confidentially, however, 1 shall continue to flatter both manufacturers with purchase orders, no matter who wins. DR. L. W. AUSTIN, OF THE BUREAU OF STANDARDS Doctor Austin is chief of the laboratory for special radio transmission research and the illustration shows him at work in his laboratory, making observations with his double-axis receiving loop which is used in the study of transmission characteristics of radio waves