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NJW1124

Voice Switched Speakerphone circuit

Japan Radio Corporation  

NJW1124

7.2 Background Noise Monitor

Background Noise Monitor judges whether the input signal is noise or sound or voice by TLO2 and RLO2 voltage,

and change the mode.

The NJW1124 includes the Background noise monitor on transmit side and receive side.

Fig.10 shows Block diagram of Background noise monitor.

The voltage difference between TLO2 or RLO1 and Ref is amplified 8.6dB on AMP1.

The signal from AMP1 inputs 2nd stage AMP2 and comparator (COMP).

The COMP non-inverting input voltage becoming 36mV higher than inverting, COMP output 1,which shows the

NJW1124 is transmit or receive mode.

At the same time, external capacitor charged from 0.8µA internal current source, until the CCP voltage becomes

46mV higher than AMP2 input voltage.

The equivalent below shows CCP voltage charging.

∆VCCP= 0.8µA/CCP

For example, CCP=1µF, δVCP=0.8V/sec.

Without the input signal, C CP discharged and finally reset the Background noise monitor.

Response example is ex.3.

The signal like continued sign wave inputting, COMP output ‘0’ which is noise-monitoring mode (idle-mode).

The signal like conversation sound inputting, CCP continues to charge and discharge. COMP output continued

to ‘1’, which is transmit or receive mode.

Small Ccp shortens the time shifting to ‘0’ condition. Too small CCP attenuates even the conversation signal.

Large CCP keeps ‘1’ condition long, which lengthen attenuating time.

Capacitor should be adjusted appropriately on actual application.

(Use high input impedance like FET probe measuring voltage of CPT, CPR pin.)

CCP

(C2,C22)

Tx : TLO2

Rx : RLO1

Tx : CPT

Rx : CPR

19kΩ 32kΩ

46mV

0.8µ A

Level

Detector

AMP1

Ref

AMP2

COMP

Tx : S3

Rx : S4

36mV

Fig.8 Background Noise Monitor Block Diagram

Function

Noise Detection

1300

1250

1200

TLO2

CPT

recommendation value

100nF～1µF

Detail

-

1300

1250

1200

Memo

The time for noise detection depends on this.

TLO2

CPT

1150

1150

1100

1100

1050

1050

1000

1000

950

950

900

900

850

850

800

0

100

200

300

400

500

time [m sec]

Response example.3

TLO2 vs. CPT signal (input start)

MCO = 200mVrms/1kHz

C5 = 0.1µF, C4=1µF

800

0

10

20

30

40

50

time [m sec]

Response example.4

TLO2 vs. CPT signal (input finish)

MCO = 200mVrms/1kHz

C5 = 0.1µF, C4=1µF

– 21 –

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