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參數(shù)資料
| 型號(hào)： | U3900BM |
| 英文描述： | Programmable Telephone Audio Processor |
| 中文描述： | 程控電話音頻處理器 |
| 文件頁數(shù)： | 13/34頁 |
| 文件大?。?/td> | 762K |
| 代理商： | U3900BM |
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U3900BM
Rev. A2, 25-Aug-98
Target Specification
20 (34)
10 Acoustic Feedback Suppression
Acoustical feedback from the loudspeaker to the
microphone may cause instability of the system. The
U3900BM has a very efficient feedback-suppression
circuit which uses a modified voice switch topology.
Figure 20 shows the basic system configuration.
Two attenuators (TXA and SAI) reduce the critical loop
gain via the serial bus either in the transmit or in the
receive path. The sliding control in block AFS control
(figure 19) determines whether the TX or the RX signal
has to be attenuated. The overall loop gain remains
constant under all operating conditions.
Selection of the active channel is made by a comparison
of the logarithmically compressed TX- and RX- envelope
curve. Figure 19 shows the AFS control.
The system configuration for group listening, which is
implemented in the U3900BM, is illustrated in figure 21.
TXA and SAI represent the two attenuators, whereas the
logarithmic envelope detectors are shown in a simplified
way (operational amplifiers with two diodes).
Receive and transmit signals are first processed by
logarithmic rectifiers in order to produce the envelopes of
the speech at TLDT and RLDT. After amplification a
decision is made by the differential pair as to which
direction should be transmitted.
The attenuation of the controlled amplifiers TXA and SAI
is determined by the emitter current IAFS which is bus
programmable.
–
+
I
AFS
BUS
TLDT
TLDR
IDTXA
IDSAI
14613
Figure 20. AFS control
14591
AFS
control
SA
MICRO
Line
SAI
LOG
TXA
Figure 21. Basic system configurations
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