International photographer (Jan-Dec 1935)

Six The INTERNATIONAL PHOTOGRAPHER June, 1935 A Study of the Michrophone Motion Picture Sound Recording The Condenser; Transmitter, Etc. Chapter XIX DiAPHRAc-.* «iCfl Ring, COMNtCTINl, WiRfs BACK SUPPORT CARBON BLOCKi CARBON GRANULES RUBBER RING, SCHEMATIC SYM80U FOR Sino-i-E Button carbon tran6mittir, '€ f~ OUT PUT c Fie. ("IC«o PHONE 1 (Figures No. 1 and No. 2, herewith appearing, are drawn in illustration of the text of Chapter A fill , May, 1935, issue International Photographer.) This chapter continues the discussion that was begun last month of the microphones used in motion picture sound recording. That first chapter on microphones described the single-button and double-button carbon microphones, or transmitters. We will resume now with a study of the functioning of the double-button transmitter. But first it must be repeated that neither of the carbon button transmitters is now employed for sound recording, their use being confined to the telephones and the public address s>stem that provide intercommunication in the sound recording installation. Operation of the Double-Button Microphone The diaphragm of the double-button carbon microphone is always connected to a tap brought out at the exact electrical center of the primary winding of the microphone transformer, as shown in Figure 2 ; and the carbon electrodes are connected to the two ends of the winding. This places the resistance of each button across one-half of the transformer winding, thereby creating a balanced circuit that helps greatly to reduce the distortion produced by even harmonics. A sound wave causes the microphone diaphragm to move backward and forward, thus alternately decreasing the pressure on the carbon granules in one chamber while increasing the pressure on the granules in the other chamber. Due to the accompanying variations in the resistances of the chambers, this alternately causes the current through one side of the transmitter and one side of the split winding of the transformer to decrease and the current through the other side of the transmitter and the other side of the transformer winding to increase. It is as though each side of the transmitter was a rheostat, the resistance of which is varied by the sound picked up. These current changes in the transformer primary do not oppose, but aid each other and add together, inducing a voltage in the secondary winding that is twice as great as it would be if only one cell of the transmitter were used. As long as the resistances on each side of the microphone diaphragm remain constant and equal (as when no sound is being received), the steady battery current, and any fluctuation in that steady current, will not induce a voltage in the secondary of the transformer. This is because a steady current does not produce a flux in the transformer core, due to the fact that the currents flowing through the two halves of the primary winding are equal and in opposition, and so balance each other outThis prevents magnetic saturation of the core by the steady d-c. current, which could readily happen in the microphone circuit shown in Figure 1 if the transformer is not properly designed. The value of the steady current flowing through the transmitter at all times is regulated, in the case of the double-button transmitter, by a series rheostat or potentiometer in the center lead of the transformer primary circuit. Usually this button current is adjusted to a value of about twenty-five milliamperes per button ; but often with certain microphones satisfactory results can be obtained with currents as low as ten or fifteen milliamperes. A lower current is preferable because it is less likely to cause burning of the carbon granules due to arcing, and it is better for the delicate contact surfaces on the diaphragm. As the microphone ages with use, its resistance increases, with a resultant loss of sensitiveness. As we have seen, the double-button transmitter is much less sensitive than the single-button transmitter, due mainly to the stretching of the diaphragm, and so at least one stage of audio-frequency amplification is necessary to bring the speech current produced by a double-button transmitter up to a useful value. The Condenser Microphone The condenser microphone, which is composed of a eound pick-up device called a condenser transmitter and a suitable amplifier, is a much later evolution than either of the two types of carbon transmitters. It was first described by Mr. E. C. Wente, who developed it, in an article entitled "The Electrostatic Transmitter," published in the Physical Review for July, 1917. It produces speech current of the highest quality and is perhaps the most perfect microphone in use at the present time. The condenser microphone has several disadvantages, CINEX TESTING MACHINES CINEX POLISHING MACHINES BARSAM-TOLLAR MECHANICAL WKS. 7239 Santa Monica Blvd. Phone GRanite 9707 Hollywood, California Please mention The International Photographer when corresponding with advertisers.