TEA7540 データシートの表示（PDF） - STMicroelectronics

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TEA7540

MONITOR AMPLIFIER

STMicroelectronics 

PIN FUNCTION

N°

Name

1

GND

2

CS

3

OUTE

4

ATTE

5

HYST1

6

HYST2

7

TIM

8

CCE

9

CE3

10

CE2

11

CE1

12 MICOUT

13

INE

14

Vref

15

INR

16 RECOUT

17

CR1

18

CR2

19

CR3

20

CCR

21

AMP IN

22 AMP SUP

23

VOL

24

ATR

25

ATTR

26

OUTR

27

Iref

28

V+

Function

Ground

Chip Select

Transmit Attenuator Output

Transmit Attenuator Input

Transmit Channel Hysteresis

Receive Channel Hysteresis

RC Timer

Time Constant of the Transmit Signal Compressor

Transmit Background Noise Memorization Output

Transmit Peak Detector Output

Transmit Rectifier Input

Transmit Signal Compressor Output

Transmit Signal Compressor Input

V+/2 - Reference Voltage

Receive Signal Compressor Input

Receive Signal Compressor Output

Receive Rectifier Input

Receive Peak Detector Output

Receive Background Noise Memorization Output

Time Constant of the Receive Signal Compressor

Handset Preamplifier Input

Handset Preamplifier Power Supply

Volume Control

Attenuation Value

Receive Attenuator Input

Receive Attenuator Output

Reference Current Source

TEA7540

FUNCTIONAL DESCRIPTION

SWITCHED ATTENUATORS

Fig.A represents a block diagram of a handsfree

subset with attenuators in signal mode. To pre-

vent the system from howling, the total loop gain,

including acoustic feedback through the housing

and sidetone coupling, must be less than 0dB.

For this purpose, two switched attenuators are in-

serted in each mode (emission and reception).

The attenuation is shifted from one mode to the

other, resulting from the speech level comparison

between each way.

To prevent the circuit to switch continuously in

one way, the operation of the IC must be fully

symetrical in both ways. This involves signal com-

parison, attenuation value.

GAIN COMPRESSOR

In TEA7540, two signal compressors are inserted

in each mode before the signal comparison, so

the signal coming from each end has the same

level (100mV peak), the losses in each way (for

instance losses resulting from the line length in

receiving mode) are compensated and the signal

comparison is fully symetrical. The time constant

of each signal compressor decreases 80 times

more quickly than it increases to prevent from

noise increasing between words. The compress-

ing depth is 38dB.

BACKGROUND NOISE DISCRIMINATION

An additional feature provided in TEA7540 is

background noise level discrimination in each

way. The IC stores the background sound level

with a long time constant (3 to 5 seconds depend-

ing on an external RC) and compares it with the

incoming signal in order to distinguish a useful

signal (speech) from the background noise. This

background noise memorization is also used to

compensate the noise in each mode before signal

comparison: the noise level in each mode is sub-

3/12

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