| 摘要 |
The invention relates to a door intercom system which can be dialled and activated via a telephone station of an in-house telephone system and which comprises at least a power supply component, an amplification circuit arrangement which transmits messages according to the two-way voice communication principle and which is DC-isolated from the in-house telephone system, and a door station equipped with a microphone and loudspeaker. In order to implement fault-free DC-isolation and, while reliably preventing acoustic feedback from the door station and values exceeding an optimum level, to enable calls to be made which are controlled by the telephone station and which allow damped two-way communication, at least one transformer is provided which feeds a speech amplifier and is supplied by a microphone amplifier. Inversely controllable attenuation stages with overlapping action, which damp the microphone amplifier when a call is made from the in-house telephone system and which damp the speech amplifier in the absence of the speech AF of the in-house telephone system, are allocated in each case to these amplifiers.
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