This hands-free telephone prevents acoustic feedback between its speaker and microphone by attenuating either the transmission or receive path under the control of a corresponding pair of counters (computers), each counter sensing the signal divided and digitized from its corresponding path. An increase in counted pulses indicates feedback and causes a counter to more quickly reach a predetermined counter state (count), to trigger attenuation (damping) before it can be reset (restored) to zero by the other counter.
In the invented speaker system, a passive radiator 401 is mounted on a top board 403a of baffle 403 in a direction opposite to a speaker unit 402. The speaker unit 402 is mounted on the opening of a cylinder 403b protruding from top board 403a at a place inner from the end. The remaining part of the opening is closed with a sub-baffle 406, to form a front closed cavity 404 and a back closed cavity 405. In this way, both the passive radiator 401 and the speaker unit 402 are fixed to the top board 403a whose rigidity being the highest; which reduces the unwanted vibration.
An A/D converter converts an analog signal to a digital signal. A plurality of cascade-connected notch filters include a first notch filter which is connected to the output of the A/D converter. A D/A converter is connected to the output of the last stage notch filter for converting a digital signal to an analog signal. The output of the last stage notch filter is connected to the input of a fast Fourier transform unit for analyzing the frequency. Analysis results of the fast Fourier transform unit are supplied to a detector. A coefficient having the same center frequency as that of a peak frequency outputted from the detector is selected from a coefficient memory and it is transferred to a second coefficient memory. Thus, the frequencies of the notch filters are set to eliminate howling.
Audio signals are digitized and an FFT is conducted on samples of the digitized signals to produce corresponding frequency spectrums. These spectrums are analyzed, such as by determining one or more peak frequency magnitudes which are 33 dB greater than harmonics or subharmonics of the frequency in a plurality of several successive spectrums, to detect resonating feedback frequencies. The offending frequency is then filtered in the time domain, either in the digitized form or analog form, to eliminate the feedback.
A sound system microphone equalization apparatus and method comprising a plurality of filters programmable by a microprocessor. User interfaces permit adjustment for each microphone channel of the parameters of the three notch filters, and four tone control filters in the filter network.
