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16.8.1. 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.
16.8.2. 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 at a given
time, but cannot do both simultaneously. Therefore, a switching circuit is necessary to control
the flow of the outgoing and incoming signals.
This switching circuit is contained in IC601 and consists of a Voice Detector, Tx Attenuator, Rx
Attenuator, Comparator and Attenuator Control. The circuit analyzes whether the Tx (transmit)
or the Rx (receive) signal is louder, and then it processes the signals such 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 a Rx signal, and supplies a DC input to the Attenuator
Control corresponding to the Rx signal. The Attenuator Control provides a control signal to the
Tx and the Rx Attenuator to switch the appropriate signals on and off. The Attenuator Control
also detects the level of the volume control to automatically adjust for changing ambient
conditions.
16.8.2.1. Transmission signal path:
The input signal from the microphone is sent through the circuit via the following path:
- Mic
Pin 9 of IC601
IC601
Q605
Pin 12 (Line 2) of IC402.
16.8.2.2. Reception signal path:
Signals receive from the telephone line are outputted at the speaker via the following path:
- Tel Line
Pin 6 (Line1) or Pin 5 (Line 2) of IC402
C626
R626
IC601
Pin 28 of IC601
Speaker.
NOTE:
( )*: In case of Line 2
16.8.2.3. Transmission/reception switching
The comparison result between Tx and Rx outputs as a DC level of IC601 of Pin 26.
Tx level is high ........ Pin 26 = Pin 21 — 6 mV
Rx level is high ........ Pin 26 = Pin 21 — 150 mV
Pin 10 of IC601
C406
R456
(Q603)*
C622
Pin 20 of IC601
Pin 3 of IC601
Pin 8 of IC402
R618
C612
Pin 16 of IC601
38
Pin 4 of
Pin 11 (Line 1) or
Pin 9 of IC402
/ Pin 29 of
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