New broadcast microphone

The condenser microphone has always been regarded as giving the highest quality but has suffered from the disadvantage of low sensitivity and somewhat high background noise due to leakage and other faults inherent in the first-stage amplifiers. The International Standard Electric Corporation have now developed a new circuit for use with the condenser microphone which reduces greatly the background noise. The capacity changes of the microphone are used to alter the tuning of a circuit loosely coupled to an oscillating circuit just out of tune with it. The resulting modulated carrier wave is then rectified. This circuit eliminates the use of the 15-megohm resistances needed in the original circuit, and so avoids the thermal agitation noises due to this cause. There is an improvement of from 14 to 24 decibels in the signal-noise ratio, and the output level with an average speaker 5 ft. from the microphone is minus 20 decibels. The frequency characteristic between about 30 and 8000 cycles is withinplusmn1.5 decibels, while above 8000 there is a small rise. No background noise can be detected under normal studio conditions, and the microphone does not overload under conditions causing blasting in the usual types of studio microphone.