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Extension of the Dynamic Range of Wireless Microphones
The dynamic range of the human ear is far superior to that of existing technical sound equipment. The equivalent noise level (CCIR 468-1) of the best microphones is about 20 dB. Peak levels of 120 dB and more are picked up with appreciable distortion. The microphone signal is transmitted by wireless frequency-modulated systems with a peak deviation of 75 kHz, and signal-to-noise ratios of up to 70 dB (CCIR-weighted) are achieved. At low sound-pressure levels the output noise of the equipment is about 6 dB greater than the microphone noise. At higher sound levels the dynamic range of the microphone exceeds by far that of the wireless equipment. Heretofore the idea of using a noise-reduction circuit was not practical. Existing circuits were considered unsuitable because of their physical size and power consumption. A new compandor integrated circuit makes possible a high-quality transmitter having the 10-mW radio-frequency output at 9V and 14mA, and operating down to 6V. The external circuitry for this integrated circuit has been improved so that it can be put to better use in wireless microphones. The dynamic range is extended to 82 dB. The system offers further advantages at low radio-frequency inputs, including suppression of electromagnetic interference.
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