Extension of the Dynamic Range of Wireless Microphones
The dynamic range of the human ear is far superior to the quality 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 without considerable distortion. Wireless transmission of the microphone signal is carried out by frequency-modulated systems with peak deviation of 75 kHz, and achieves S/N ratios up to 70 dB (CCIR-weighted). Even at low-sound levels, the output noise of the equipment is about 6 dB higher than the microphone noise. At higher sound levels, the dynamic range of the microphone exceeds by far that of the wireless equipment. The idea to take a noise reduction circuit could for a long time not be realized. Existing circuits could not be used because of their physical size and power comsumption. A new IC permits the realization of a high-quality transmitter with 10 mW r.f. output at 9 V and 14 mA, operating down to 6 V. With the corresponding receiver circuit, the dynamic range could be extended to 82 dB. The system offers further advantages at low-level r.f. input, e.m.i. suppression and switching noise.
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