| 摘要 |
PROBLEM TO BE SOLVED: To suppress sidetones which cause unpleasant sensation and improve the speech quality by suppressing the wraparound value of transmitting signals by making the transmitting and receiving routes of a slave unit pass via the speech transmitting/receiving route of a two-way call control LSI when the slave unit talks via a line wire. SOLUTION: A mode switchable speech transmitting/receiving amplifier gain control circuit a26 of a two-way call control LSI a23 has the hand-free and slave unit calling modes and properly switches the loss value to be put into a speech transmitting amplifier a24 and a speech receiving amplifier a25 of the LSI a23 in each mode. Then the level is lowered by about 6 dB for the LSI a23 contained in a master unit A to suppress the sound volume of a wraparound signal and to reduce the sense of echoes. If the LSI a23 has a speech transmitting signal higher than a prescribed level, the circuit a26 functions to put a loss of 20 dB into the amplifier a25. Thus, sensitivity of a microphone a9 is improved, and the route gain is increased. As a result, the acoustic connection is facilitated between a loudspeaker a8 and the microphone a9, and howling is prevented.
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