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It is composed of bell detection circuit, pulse dial circuit, line amplifier, sidetone circuits.

1. Circuit Operation

The signal waveform for each point is indicated below. The signal (low level section) input to pin 10 of BBIC IC4 on the digital
board.
TEL LINE 

→ PC101 (1, 2 → 4) → IC4 (10)

Normally (ON-HOOK condition), LINE RELAY (RL101) and Opt Isorator (PC102) is OFF. While OFF-HOOK, RL101 and PC102
turns ON. This LINE RELAY is controlled by pin 83 of IC4 through the Q101. PC102 is controlled by pin 12 of IC4 through the
Q104.
ON-HOOK:
IC4 (83) Low Level 

→ Q101 OFF → RL101 OFF 

OFF-HOOK:
IC4 (83) High Level 

→ Q101 ON → RL101 ON 

The pulse dial is generated by operating the transistor PC102 while OFF-HOOK (RL101 ON) condition.
Make state:
IC4 (83) High Level 

→ Q101 ON → RL101 ON

Break state:
IC4 (83) Low Level 

→ Q101 OFF → RL101 OFF

1. Circuit Operation

The reception signal output from the line transformer T101 is input to pin (2) of IC101 via C130, R138 and then the signal is
amplified at pin (1) of IC101 and sent to the reception system at through the LPF.

1. Function

This circuit detects the signal (cuts the current) output from the converter when the other party finishes ICM recording and

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goes into the ON-HOOK status.
TAD detects this signal and disconnects the line. When the TAD is operating, pin (4) of PC105 becomes a low level.
While detecting the CPC signal, pin (4) of PC105 becomes a high level. When the CPC signal is detected, the TAD opreation
stops and the line is disconnected.

1. Function

This circuit is used to detect that the telephone connected in parallel to the same line is OFF-hook while the unit picks up the
line. If this detection circuit is activated when TAM is being delivered, the delivery stops and the circuit is automatically
released.

2. Circuit Operation

If the line is picked up, C107 is charged by following the path shown below.
D105 (+) 

→ R109 → D106 → C107

When the electric potential difference between the base and emitter of Q105 becomes less than about 0.3V, Q105 and Q106
and PC103 turn off, then the IC4 pin 14 becomes a high level.
In this condition, if a telephone connected in parallel goes into OFF-hook status, the base of Q105 becomes low.
On the other hand, the emitter of Q105 goes down slowly because the capacitor (C107) is charged. Q105 turns on when the 
electric potential difference between the base and emitter of Q105 becomes more than 0.6 V while being charged.
When Q105 turns on, Q106 and PC103 also turn on, then the IC4 pin 14 becomes a low level.

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The unit receives the calling number from the central office.
RCID contains numbers only.
When a telephone line is captured after the bell rings, 500Hz of ACK signal is sent to the line and the dual tone signal is output.
The calling number is recognized by the digit number corresponding to the dual tone.

The outline of RCID TX/RX is shown below.

1. Receiving Signal

Receiving signal route is same as “ICM Record signal”. 

2. ACK signal

ACK signal route is same as “DTMF for TEL LINE signal“.

Refer to Check Sheet for Signal Route (P.124) for signal route.

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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.

The speakerphone can only provide a one-way communication path.
In order 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 function is necessary to control the flow of the outgoing and incoming signals.
This switching function is contained in IC4 and consists of voice detector, TX attenuator, RX attenuator, comparator and
attenuator control. The function analyzes whether the TX (Transmit) or the RX (receiver) signal is louder, and then it processed
the signal such that the louder signal is given precedence. The vice detector provides the attenuator control corresponding to
the TX signal. The comparator receives a TX and RX signals 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.

Base Unit and Cordless Handset mainly consist of RF Module and DECT BBIC. 
Base Unit and Cordless Handset transmit/receive voice signal and data signal through the antenna on carrier frequency.

The voice signal input from the TEL LINE interface goes to RF Module (IC28) through DECT BBIC (IC4).
The voice signal passes through the analog part of IC4 where it is amplified and converted to a digital audio stream signal. The
burst switch controller processes this stream performing encryption and scrambling, adding the various other fields to produce
the GAP (Generic Access Profile) standard DECT frame, assigning to a time slot and channel etc.
In IC28, the carrier frequency is changing, and frequency modulated RF signal is generated and amplified, and radiated from
antenna. Cordless Handset detects the voice signal or data signal in the circuit same as the following explanation of Receiver
Block.

The signal of 19.2 MHz band (18.81792 MHz ~ 18.97344 MHz) which is input from antenna is input to IC28.
In IC28, the signal of 19.2 MHz band is down converted to 864 kHz signal and demodulated, and goes to IC4 as GAP (Generic
Access Profile) standard DECT frames. It passes through the decoding section burst switch controller where it separates out the
frame information and performs de-encryption and de-scrambling as required. It then goes to the DSP section where it is turned
back into analog audio. This is amplified by the analog front end, and goes to the TEL LINE Interface.