9
KX-TS880MXB/KX-TS880MXW
4.2.3.
Speakerphone Circuit
4.2.3.1.
Function
The 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.
4.2.3.2.
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 simul-
taneously. 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 processed 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 sig-
nal.
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.
1.
Transmission signal path:
The input signal from the microphone is sent through the circuit via the following path: MIC
→
Pin 9 of IC601
→
Pin 10 of
IC601
→
Pin 3 of IC601
→
Pin 4 of IC601
→
R601
→
C602
→
L107
→
Q107
→
Tel line.
2.
Reception signal path:
Signals receive from the telephone line are outputted at the speaker via the following path: Tel line
→
Q401
→
C404
→
R600
→
C603
→
Pin 27 of IC601
→
Pin 26 of IC601
→
Pin 19 of IC601
→
Pin 15 of IC601
→
Speaker.
3.
Transmission/Reception switching
The comparison result between TX and RX outputs as a DC level of Pin 25 of IC601. TX level is high ...... Pin 25 = Pin 21 -
6mV RX level is high ...... Pin 25 = Pin 21 - 150mVComparator output is connected to the attenuator control inside of IC601.
4.
Voice detector
The output of the mic amp (Pin 10 of IC601) is supplied to Pin 13 of IC601 as a control signal for the voice detector.
5.
Attenuator control
The attenuator control detects the setting of the volume control through Pin 24 of IC601 to automatically adjust for changing
ambient conditions.
