KX-DT321UA - Panasonic PBX Service Manual (repair manual). Page 13

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This circuit performs the setting of call paths and volume adjustment in each mode of operation, via IC4.
Audio gain settings are downloaded serially from IC6, which obtains its information from the PBX.

• IC4 sends PCM audio data (Rx) to IC3 (Digital speakerphone chip) via a PCM interface.
• IC4 receives PCM audio data (Tx) from IC3 via a PCM interface.
• DTMF-like tone (from key-press, etc) is input to IC4 in analogue form.

1. Handset call:

Transmit signal is input from handset microphone. Pin 57 of IC6 drives analogue switch (IC9) to connect handset microphone
to the signal path. The signal is amplified by an internal amplifier within IC4 

→ A/D conversion within IC4 → Gain adjustment

(within IC4) 

→ sent to PBX via IC3 and IC2 as PCM data.

Receive signal arrives from PBX 

→ IC2 → IC3 → D/A conversion from PCM data by IC4→ Gain adjustment by DSP of IC4 →

amplified by 1st amp of IC11 

→  output from L52 to the earpiece of the handset.

2. Speakerphone call:

When in speakerphone mode, only a one-way communication path is provided at any given time. IC3 compares the transmit
and receive PCM signals during speakerphone mode, and then provides a one-way communication path, dependant on which
end has the greatest audio content.
Transmit signal is input from the Speaker-phone microphone into IC4. The signal is amplified by an internal amplifier within
IC4 

→ Gain adjustment within IC4→ sent to PBX via IC3 and IC2 as PCM data.

Receive signal arrives from PBX 

→ IC2 → Gain adjustment by IC3 → D/A conversion from PCM data by IC4 → pin 24 of IC4

→ pin 27 of IC4 → pin 28 of IC4 → pin29 of IC4 → output from pin 3 and 4 of IC4 to directly drive the speaker.

3. Headset call:

Transmit signal is input from headset microphone. Pin 57 of IC6 drives analogue switch (IC9) to connect headset microphone
to the signal path. The signal is amplified by an internal amplifier within IC4 

→ A/D conversion within IC4 → Gain adjustment

(within IC4) 

→ sent to PBX via IC3 and IC2 as PCM data.

Receive signal arrives from PBX 

→ IC2 → IC3 → D/A conversion from PCM data by IC4→ Gain adjustment by DSP of IC4 →

amplified by  2nd amp of IC11 

→ output  from L54 to headset.

Speech is prevented from being received by handset by IC6 pin 58 output, which drives base of Q51 (ON) which applies mute
to the handset receive path (when handset is on-hook during call using the headset).