Kabushiki Kaisha Toshiba

Takuya Kanazumi

Osamu Kurokawa

Ken Nakamura

Akira Ishikura

Buntaro Sawa

Yoshihiro Nomura

Masakuni Hyodo

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.

Handsfree unit

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.

Telephone with hands-free operation

Growing community of inventors

Tokyo, Japan

Takuya Kanazumi