U4091BM-N Datasheet, PDF (27/38 Pages) ATMEL Corporation – PROGRAMMABLE TELEPHONE AUDIO PROCESSOR

U4091BM-N
Side Tone System
Figure 20. Principle Circuit of Side Tone Balancing
LTX
8dB
LRX
0-7dB
AGARX
+ DIFF1
-
STO_DIFF
-10dB
AMP1
MOD
-10dB
9dB AMP2
STOAMP
Sidetone balancing
LF
g
P
SL
LINE
RECIN
CK
ZL
STO
STO
8.2 k Ω
STRC
STC
CTO
33 nF
f
LF
P
SL
The Side Tone Balancing (STB) has the task of reducing the cross-talk from LTX (micro-
phone) to LRX (earpiece) in the frequency range of 0.3 to 3.4 kHz. The LTX signal is
converted into a current in the MOD block. This current is transformed into a voltage sig-
nal (LINE) by the line impedance ZL. The LINE signal is fed into the summing amplifier
DIFF1 via capacitor CK and attenuator AMP1.
On the other hand the LTX buffered by STOAMP drives an external low-pass filter (RST,
CST). The external low-pass filter and the internal STB have the transfer function drawn
in the STB box. The amplified STB-output signal drives the negative input of the sum-
ming block. If both signals at the DIFF1 block are equal in level and phase, we have
good suppression of the LTX signal. In this condition, the frequency and phase
response of the STB block will represent the frequency curve on line.
In real life the line impedance ZL varies strongly for different users. To obtain good sup-
pression with one application for all different line impedances, the STB function is
programmable.
The 3 programmable parameters are:
1. LF (gain at low frequency)
LF has 15 programming steps of 0.5 dB
LF(0) provides -2 dB gain, LF(15) provides 5.5 dB gain
STO_DIFF(LF) = (-10 dB - 2 dB + 0.5 dB × LF + 9 dB) × LTX
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