KX-T7665UA (serv.man3) - Panasonic PBX Service Manual (repair manual). Page 12

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This circuit performs the setting of the call path/vol. adjustment in each call mode by IC4.
The audio gain of each call is determined by download data from PBX.

• IC2 sends each audio data to IC3 by PCM interface.
• IC2 receives each audio data from IC3 by PCM interface.

1. Handset Call

Transmitting signal is inputted from handset microphone and amplified by IC4 built-in Amp 

→ A/D conversion →Gain adjust-

ment by IC4 

→inputted to IC3 and IC2 in PCM data →then sent to PBX.

Receiving signal is inputted from PBX 

→IC2→IC3→D/A conversion after the input to IC4 in PCM data→Gain adjustment by

DSP of IC4

→Pin 26 of IC4→ outputted from Q51 to handset.

2. Speakerphone Call

The speakerphone mode can only provide a one-way communication path.
IC3 compares transmitting signal and receiving signal during speakerphone mode, then provide a one-way communication
path.

Transmitting signal is inputted from SP phone mic. and amplified by IC4 built-in Amp

→ A/D conversion→inputted to IC3 in

PCM data

→Gain adjustment by IC3→inputted to IC2→ then sent to PBX.

Receiving signal is inputted from PBX is 

→IC2→Gain adjustment by IC3 →inputted to IC4 in PCM data→D/A conversion→pin

24 of IC4

→pin 27 of IC4→pin 28 of IC4→pin 29 of IC4→ outputted from pin 3 and 4 of IC4 to speaker.