Abstract: A mobile radio communication apparatus having a communication function adaptable to both an analog mode and a digital mode is controlled by a control unit, when it receives from a radio base station a message signal representing that the digital mode is provided. When the strength of the message signal in the analog mode received by a receiving unit from the radio base station is at a predetermined level or higher, a mode of communication services is shifted into the digital mode. When the strength is lower than the predetermined level, the standby status in the analog mode is maintained. Therefore, the mode of communication services is not alternated between the analog mode and the digital mode, with the result that both an outgoing call and an incoming call can be performed successfully even in an unstable service area where a transmitted signal cannot reach the radio base station.

Abstract: The present invention provides a cheap handsfree unit which can prevent the howling phenomenon and reduce the input sound from the loud-speaker into the microphone in both the analog and digital modes. A reception side comprises a signal level adjusting circuit constituted by a switch circuit and attenuators, and a transmission side comprises a signal level adjusting circuit constituted by a switch circuit and attenuators, to introduce the attenuation of 15 dB or 30 dB in response to the analog or digital mode. A voice switch is constituted so that the analog reception signal and the analog transmission signal are rectified respectively by rectifiers, the levels of both signals are compared by a comparator, and either switch or switch having a higher input signal level becomes conductive as a result of the comparison. The attenuation is introduced on the basis of the communication mode, only to either the analog reception signal or analog transmission signal having a smaller signal intensity.

Abstract: A hands-free circuit includes a transmit noise level detector which detects a transmit noise level and a first comparator which compares the transmit noise level with a reference level. If the transmit noise level is higher than the reference level, the first comparator provides a control signal indicating a noisy state has been detected. In response to the control signal, a transmit audio level is attenuated. Further the attenuated transmit audio level is compared with a receive audio level by a second comparator which provides an output signal indicating either a transmit-mode or a receive-mode. In response to the output of the second comparator, one audio signal is attenuated.