| 摘要 |
In this incoming audio detection circuit for a speaker telephone, an impedance element is connected between the output of the microphone amplifier and the telephone line terminals. Speech signals present at each end of the impedance element are respectively rectified but not filtered. The resultant rectified but unfiltered signals are compared. If only outgoing speech is present, the rectified signal derived from the microphone amplifier end of the impendance element will be of greater magnitude than, and will have a greater phase angle extent than the rectified signal derived from the telephone line end of the impedance element. A comparator senses this condition and maintains the speaker telephone in the outgoing audio mode. In the presence of incoming speech, instants will occur when the magnitude of the rectified signal derived from the telephone line exceeds that of the rectified signal derived from the microphone amplifier. In response to this condition, the comparator causes the speaker telephone immediately to switch to the mode in which incoming speech is reproduced by a loudspeaker.
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