Abstract: Radio receivers which switches the received voice signal according to a supervisory signal used for supervising the condition of the propagation path through which the voice signal is transmitted are known. However, if the reception level of the radio wave received by a conventional radio receiver is low, the level of the supervisory signal becomes lower while the level of the noise accompanying the supervisory signal becomes higher, whereby the noise acts like the supervisory signal so as to provide a voice signal containing a lot of noise to the user of the radio receiver. To solve this problem, the radio receiver according to the present invention compares the level of the supervisory signal and the level of the noise accompanying the supervisory signal, so as to control the outputting of the voice signal according to the comparison result.

Abstract: An echo canceling apparatus has two microphones, both receiving an acoustic signal including a near-end speech component and an echo component. A first adaptive filter cancels the echo component in the output of the first microphone, obtaining a first residual signal. A second adaptive filter cancels the echo component in the output of the second microphone, obtaining a second residual signal. The two residual signals are added together with a timing delay compensating for the delay between their near-end speech components. In the resulting sum signal, the near-end speech component is enhanced, while the echo component, and a near-end background noise component, are relatively weakened.

Abstract: A frequency-modulated signal transmitter and frequency modulation method are provided wherewith the linearity of an FM modulated signal is secured even when the level of frequency modulation after FM modulation is low. A level-subtraction circuit 24 is provided between a speech processing circuit 20 and a frequency modulator circuit 22 for subtracting the signal level of the speech-processed signal D2 from the speech processing circuit. The level-subtraction circuit comprises a level-subtraction stage 28, to which the speech-processed signal is input, that outputs a level-subtracted voice signal to the FM modulator circuit, and an average FM noise level setting stage 26 for setting the average level (±Esn) of the FM noise generated in the frequency modulator circuit and outputting that average FM noise level (±Esn) to the level-subtraction stage.

Abstract: A digital modulator capable of obtaining a high FM modulation index and reducing the quantization noise without increasing the sampling frequency of an A/D converter of input voice. A phase variation .phi.(Ts) of the digital data (Ts is the sampling period) produced on the basis of the output from the A/D converter is divided by a predetermined integer N (N>1) by a divider, and is fed to an integrator. The integrator integrates the divided phase variation .phi.(Ts)/N for every time interval Ts/N, thereby producing a phase component .phi.(t) of the complex envelope of the modulation signal. The phase component .phi.(t) is resolved into cosine and sine components at the time interval Ts/N. The cosine and sine signals are D/A converted into analog signals which are quadrature modulated by a quadrature modulator.

Abstract: A digital modulator capable of obtaining a high FM modulation index and reducing the quantization noise without increasing the sampling frequency of an A/D converter of input voice. A phase variation .phi.(Ts) of the digital data (Ts is the sampling period) produced on the basis of the output from the A/D converter is divided by a predetermined integer N (N>1) by a divider, and is fed to an integrator. The integrator integrates the divided phase variation .phi.(Ts)/N for every time interval Ts/N, thereby producing a phase component .phi.(t) of the complex envelope of the modulation signal. The phase component .phi.(t) is resolved into cosine and sine components at the time interval Ts/N. The cosine and sine signals are D/A converted into analog signals which are quadrature modulated by a quadrature modulator.

Abstract: An adaptive filter estimates the noise component in a primary audio signal by convolving a secondary audio signal with a set of coefficients. The estimated noise component is subtracted from the primary audio signal to produce an output signal. During steady-state operation, the adaptive filter coefficients are updated as to minimize the difference between the primary audio signal and the estimated noise component. Steady-state operation is identified automatically by monitoring the power level of the primary or secondary audio signal, or the power level of the output signal. Coefficient updating is suspended when the monitored power level rises from a steady state to an unsteady state, and is resumed when the monitored power level returns to its previous steady-state level, or settles into a new steady state.

Abstract: A high frequency filter for frequencies higher than the VHF band comprising of a closed conductive housing (53), a pair of input and/or output means (54) like an antenna provided at both the extreme ends (53-5, 53-6) of said housing (53), a plurality of resonators (51-1 through 51-5, and 51a-1 through 51a-5) arranged on a straight line between said antennas (54), each of said resonators having an elongated inner conductor (51a-1 through 51a-5) with a circular cross section, and an elongated rectangular dielectric body (51-1 through 51-5) surrounding said inner conductor, one end of each of said resonators being fixed at the single plane (53-1) of the housing (53) and the other end of each of said resonators being free standing. The length of said inner conductor and the dielectric body is substantially 1/4 wavelength, and the duration (52-1 through 52-4 ) between two resonators is determined according to the specified coupling coefficient for the desired characteristics of the filter.