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4872A–CORD–08/05

U4091BM-R

12. Acoustic Feedback Suppression

Acoustical feedback from the loudspeaker to the hands-free microphone may cause instability of

the system. The U4091BM-R has a very efficient feedback-suppression circuit which offers a

4-point or (alternatively) a 2-point signal-sensing topology (see Figure 12-1).

Two attenuators (TXA and SAI) reduce the critical loop gain via the serial bus either in the trans-

mit or in the receive path. The overall loop gain remains constant under all operating conditions.

The LOGs produce a logarithmically-compressed signal of the TX- and RX-envelope curve. The

AFSCON block determines whether the TX or the RX signal has to be attenuated.

The voice-switch topology can be selected by the serial bus. In 2-point-sensing mode, AFSCON

is controlled directly by the LOG outputs.

Figure 12-1. Basic System Configurations

CCT

CT

DTD

LOG

CALCR

AFSCON

SA

BNM

BNM

LOG

CALCT

CBNMR

BNMR

TLDR

CRLO

LOG

LOG

CBNMT

BNMT

CTLO

TLDT

MICO

INLDT

RTU

CTU

TXA

MICRO

RECO1

INLDR

CRU

RECO2

RRU

Mode

control

AGATX

STO

Line

AGARX

HV

SAI