TEA7540 datasheet(4/12 Pages) STMICROELECTRONICS

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stracted from the incoming signal before the com-

parison. So very high noise level in one mode

cannot trouble the comparison between the useful

signals.

The result of the comparison manages the attenu-

ators in the following way:

- The maximum attenuation is switched on

the mode where the speech signal is the

lowest. The maximum attenuation is fixed

by two external resistor (maximum 52dB).

The time constant of the switch is fixed by

the timer via an external capacitor.

- When neither party is talking both attenu-

ators are set to a medium attenuation.

Thus each mode is in idle mode. The time

constant of the switch from active mode to

idle mode must be long enough to prevent

from switching to idle mode between two

words (see fig.B). This time constant is

fixed by an external RC.

Figure A

Em Rec

SPEECH/NOISE

A max dB

0dB

IDLE (A max/2)

D93TL009A

Figure B

TEA7540

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