KX-NT321RU (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 8

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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 between NT300 series, The telephone call uses G722 codec.

1. Handset Call

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

→ input to IC1-pin112 → A/D conversion and

Gain adjustment by a built-in DSP of IC1 

→ then sent to PBX.

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

→ Gain adjustment by a built-in DSP.

Receiving signal input from PBX 

→ Gain adjustment by a built-in DSP of IC1 → D/A conversion and output from IC1-pin115.

→ Amplified by IC20, and output to handset.

2. Headset Call

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

→ input to IC1-pin112 → A/D conversion and

Gain adjustment by a built-in DSP of IC1 

→ then sent to PBX.

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

Receiving signal input from PBX 

→ Gain adjustment by a built-in DSP of IC1 → D/A conversion and output from IC1-pin115.→

Amplified by IC20, and output to headset.

3. Speakerphone Call

Speakerphone call realize a full duplex call by a built-in DSP (echo-canceller) of IC1. For a call In the most appropriate condi-
tion, DSP checks voice level of the calling party, acoustics in a room, voice level of the other party and state of the line of con-
tact, and cancels echo in transmitting and receiving at the start of calling. It operates in a half duplex state in a few seconds for
the check.

Transmitting signal is input from microphone and amplified by IC23 

→ input to IC1-pin113 → A/D conversion and Gain adjust-

ment by a built-in DSP of IC1, then sent to PBX through echo canceller.

Receiving signal input from PBX 

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

output from IC1-pin116 

→ Amplified by IC21 and output to Speaker.

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