KX-DT333RU - 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 DSP built in IC2.
The audio gain of each call is determined by download data from PBX.

• IC2 sends each audio data to IC6 by PCM interface.
• IC2 receives each audio data from IC6 by PCM interface.

1. Handset Call

Transmitting signal is inputted from handset microphone and amplified by IC6 built-in Amp

→ A/D conversion→inputted to IC2

in PCM data

→Gain adjustment by DSP of IC2→then sent to PBX.

Transmitting signal inputted from handset microphone is controlled by transmission switching SW IC8 as below.
IC1-pin 52 L : Handset Call
(IC1-pin 52 H : Headset Call)
Receiving signal inputted from PBX is 

→Gain adjustment by DSP of IC2→D/A conversion after the input to IC6 in PCM

data

→IC6 built-in Amp output→outputted from IC2 built-in Amp to handset.

2. Headset Call

Transmitting signal is inputted from headset microphone and amplified by IC6 built-in Amp

→ A/D conversion→inputted to IC2

in PCM data

→Gain adjustment by DSP of IC2→then sent to PBX.

Transmitting signal inputted from headset microphone is controlled by transmission switching SW IC8 as below.
IC1-pin 12 H : Headset Call
(IC1-pin 12 L : Handset Call)
Receiving signal inputted from PBX is 

→Gain adjustment by DSP of IC2→D/A conversion after the input to IC6 in PCM

data

→IC6 built-in Amp output→outputted from Q6 (transistor for buffer) to headset.

3. Speakerphone Call

Speakerphone call realizes all double-call by DSP(echo canceller) built in IC2. For a call in the most appropriate condition,
DSP checks voice level of the calling party, acousticity in a room, voice level of the other party and state of the line of contact,
and cancels echo in transmitting and receiving at the start of calling.  It operates in a half double-call state in a few seconds for
the check.

Transmitting signal is inputted from SP phone mic. and amplified by IC6 built-in Amp

→ A/D conversion→inputted to IC2 in

PCM data

→Gain adjustment by DSP of IC2, then sent to PBX through echo canceller.

Receiving signal inputted from PBX is 

→Gain adjustment by DSP through echo canceller of IC2 →inputted to IC6 in PCM

data

→D/A conversion→IC6 built-in Amp output→outputted from IC7(Amp) to speaker.

Power Amp IC7 is controlled as follows;
IC1-pin 13 L

→IC7-ON

IC1-pin 13 H

→IC7-OFF