Dynamic volume expander varying as a function of ambient noise level

摘要

To provide for response of audio reproduction equipment with the full dynamic level range, for example of music, intended by a composer, although radio signals transmitted have been compressed to reduce the signal-noise level, an expander circuit (20a, 20b, 20c) is activated based on ambient noise level conditions as sensed, for example, by a microphone (12) which determines the ambient noise level. If the ambient noise level, for example within certain frequency bands, as selected by filters (32a, 32b, 32c) is low, so that the full dynamic range (difference between low volume and high volume passages) can be reproduced, a range expander (20a, 20b, 20c) is controlled, for example within selected frequency bands, to reproduce the respective frequencies, or the overall signal, with a dynamic range greater than that of the received compressed signal and, preferably, corresponding to the dynamic range of the original signal which was then compressed for transmission. To prevent a microphone from responding not only to ambient noise but also to the reproduced program, the reproduced program level can be sensed, for example, by an amplitude detector (14a) and, when the reproduced program level is low, briefly entirely interrupt the program level while activating the microphone to sense then ambient noise level and, if the sensed ambient noise level is low, thus permit control of the dynamic range expansion circuitry to expand the dynamic range of the reproduced signal to improve listening conditions.