U4091BM Datasheet, PDF (11/29 Pages) TEMIC Semiconductors – Programmable Telephone Audio Processor

U4091BM
Acoustic Feedback Suppression
Acoustical feedback from the loudspeaker to the hands-
free microphone may cause instability of the system. The
U4091BM has a very efficient feedback-suppression
circuit which offers a 4-point- or alternatively a 2-point-
signal-sensing topology (see figure 7).
Two attenuators (TXA and SAI) reduce the critical loop
gain via the serial bus either in the transmit 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 block AFSCON
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.
MICRO
MICO
TXA
CTU
RTU
INLDT
CTLO CBNMT
TLDT BNMT
LOG
AGATX
STO
LOG
CALCT
BNM
Mode
control
Line
BNM
LOG
CALCR
AFSCON
AGARX
LOG
CT
BNMR TLDR
INLDR
RRU
CCT
CBNMR CRLO
CRU
RECO1 HV
SA
DTD
SAI
RECO2
14577
Figure 7. Basic system configurations.
Rev. A1, 02-Jun-98
Target Specification
11 (29)