British Kinematography (1948)

199 the microphone is given by Q = Cv in the appropriate units. Having once supplied this charge, the battery and resistor could be disconnected, leaving Q a constant, when variation of capacitance C would produce variations of voltage which are inversely proportional. In practice, the polarising resistor and battery are left connected in order 1 to make good any leakage. The microphone then functions as a source of • alternating voltage in series with a capacitor, and is resistance loaded. It is therefore " frequency conscious," having a falling output at low frequencies. Specimen microphones of early type were found to have a capacitance of about 250 pF. ; with a 7J megohm resistor the attenuation at 50 c/s. would ; therefore be about 6 db. If this attentuation is not to be exceeded, the latest designs having a capacitance of the order of 40 pF. should be used with a resistor of at least 40 megohms. These figures ignore the effect of the " breather " or vent in the microphone itself, which imposes an additional shunt leakage path (usually of very high resistance), and also any D.C. isolating network between microphone and valve. We therefore find that the miniature trend in microphone design calls for a very high resistance input circuit to the first audio valve, while the : requirement of low noise level calls for a low resistance input circuit. Any compromise cannot fully satisfy both conditions, and this tends to restrict further improvement in microphone design. One method of dealing with this problem is to adopt a cathode follower circuit, in which the grid resistor is arranged to perform the function of the microphone polarising resistor. Its shunting effect on the microphone will i be many times its D.C. resistance, due to negative feedback. It is preferable to feed the cathode follower into a buffer valve before passing the output to a long line, but a short length of cable is permissible between the cathode follower and the buffer stage for boom working. Advantages of 4< Phasitron " Circuit The " Phasitron " makes a bolder approach to the problem, and although there is not yet a large selection of quiet valves available, the grid resistance problem is dispensed with, and noise from resistances is negligible. It makes possible a new approach to microphone design, for item (c) above indicates that increased efficiency will be ^obtained by further reduction in microphone capacitance. This can be achieved in existing designs by restricting the size of the fixed electrode to a small area opposite the centre of the diaphragm where movement of the latter is greatest. Indeed, the body of the microphone may be made entirely of low-loss insulation on which the tuning coil could be wound, the small microphone electrodes being at one end, and a small iron-dust trimming core at the other. At present we are only using a minute fraction of the capabilities mentioned in (d), and this re-design of the microphone element as an integral H.F. tuned circuit and modulator should enable a far greater part of the linear characteristic to be utilised at normal sound levels, with a corresponding increase in sensitivity and reduction in noise level. The radio link possibilities, too, are capable of valuable service in film production. Admittedly, studio operation calls for microphone positions which can best be provided by a boom, on which a cable presents no difficulty, but newsreel, location, and guide-track work is all carried on under conditions where cables are a real handicap. Our experiments in this field lead us to believe that the short range radiating microphone with its local batteries and small folded dipole will be compact and inconspicuous enough to be used not only wherever an ordinary stand microphone is used, but also in positions where a microphone pick-up