Speaker-Phone Circuit - Panasonic KX-T7565X Service Manual

(2) RECEPTION SIGNAL PATH
The digital input signal from the telephone line is changed to an analog signal by IC3 and sent
to the receiver via the following path:
(3) CIRCUIT DIAGRAM FOR TRANSMISSION / RECEPTION SIGNAL PATH.

8.8. SPEAKER-PHONE CIRCUIT

(1) FUNCTION
This circuit controls the automatic switching of the transmitted and received signals to and
from the telephone line, when the unit is used in the hands-free mode.
(2) CIRCUIT OPERATION
The speakerphone can only provide a one-way communication path. / In other words, it can
either transmit an outgoing signal or receive an incoming signal. / This switching circuit is
contained in IC8 and consists of a Voice Detector, Tx Attenuator, Rx Attenuator, Comparator
and Attenuator Control. The circuit analyzes whether the Tx (transmit) or Rx (receive) signal is
louder, and then it processes the signals so that the louder signal is given precedence. / The
Voice Detector provides a DC input to the Attenuator Control corresponding to the Tx signal.
The Comparator receives a Tx and Rx signal, and supplies DC input to the Attenuator Control
corresponding to the Rx signal. The Attenuator Control provides a control signal to the Tx and
Rx Attenuator to switch the appropriate signals ON and OFF. The Attenuator Control also
detects the level of the volume control to automatically adjust the volume for changing ambient
conditions.
(3) CONTROL SIGNAL PATH
Control signals for transmission and reception are input to IC8 via the following path:
(Transmission Control Signal Path)
(Reception Control Signal Path)
(4) TRANSMISSION / RECEPTION SWITCHING
The comparison result between Tx and Rx output is a DC level at IC7 Pin 23.
21