KX-NT400RU - Panasonic PBX Service Manual (repair manual). Page 11

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This circuit performs the setting of the call path/vol. adjustment in each call mode by a built-in DSP of VoIP(IC1).The audio gain
of each call is determined by download data from PBX or by internal data from flash memory(IC2).
In case of the extension call with NT300 or NT400, The telephone call becomes a characteristic in accordance with G722.

1. Handset Call

Transmitting signal is inputted from handset microphone, and amplified by IC24 

→

Inputted to IC1-pin112 

→ A/D conversion and Gain adjustment by a built-in DSP of IC1 → then sent to PBX.

Transmitting signal inputted from handset microphone is controlled by transmission switching SW IC29 as follows; 
IC29-pin5 L: Handset Call
 (IC29-pin5 H: Headset Call)
Transmitting signal inputted from PBX 

→ Gain adjustment by a built-in DSP

 Receiving signal inputted from PBX 

→ Gain adjustment by a built-in DSP of IC1 → D/A conversion and outputted from IC1-

pin115. 

→ Amplified by IC20, and outputted to handset.

2. Headset Call

Transmitting signal is inputted from headset microphone, and amplified by IC24 

→

Inputted to IC1-pin112 

→ A/D conversion and Gain adjustment by a built-in DSP of IC1 → then sent to PBX

Transmitting signal inputted from headset microphone is controlled by transmission switching SW IC29 as follows;
IC29-pin5 H: Headset Call
 (IC29-pin5 L: Handset Call)

Receiving signal inputted from PBX 

→Gain adjustment by a built-in DSP of IC1 → D/A

conversion and outputted from IC1-pin115. 

→ Amplified by IC20, and outputted to headset.

3. Speakerphone Call

Speakerphone call realize all double-call by a built-in DSP(echo-canceller)of IC1. For a call In the most appropriate condition,
DSP checks voice level of the calling party, acoustics 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 microphone and amplified by IC23 

→ A/D conversion and Gain adjustment by a built-in

DSP of IC1, then sent to PBX through echo canceller.

Receiving signal inputted from PBX 

→ Gain adjustment by DSP through a built-in echo canceller Of IC1→ D/A conversion

and outputted from IC1-pin116 

→ Amplified by IC21 and outputted to Speaker.

 Power Amp IC21 is controlled as follows;
 IC21-pin1 L: ON
 IC21-pin1 H: OFF