TEA7540 datasheet(3/12 Pages) STMICROELECTRONICS

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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-

Name

Function

GND

Ground

Chip Select

OUTE

Transmit Attenuator Output

ATTE

Transmit Attenuator Input

HYST1

Transmit Channel Hysteresis

HYST2

Receive Channel Hysteresis

TIM

RC Timer

CCE

Time Constant of the Transmit Signal Compressor

CE3

Transmit Background Noise Memorization Output

CE2

Transmit Peak Detector Output

CE1

Transmit Rectifier Input

MICOUT

Transmit Signal Compressor Output

INE

Transmit Signal Compressor Input

Vref

V+/2 - Reference Voltage

INR

Receive Signal Compressor Input

RECOUT

Receive Signal Compressor Output

CR1

Receive Rectifier Input

CR2

Receive Peak Detector Output

CR3

Receive Background Noise Memorization Output

CCR

Time Constant of the Receive Signal Compressor

AMP IN

Handset Preamplifier Input

AMP SUP

Handset Preamplifier Power Supply

VOL

Volume Control

ATR

Attenuation Value

ATTR

Receive Attenuator Input

OUTR

Receive Attenuator Output

Iref

Reference Current Source

V+

PIN FUNCTION

TEA7540

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