U4091BM TEMIC Semiconductors, U4091BM Datasheet - Page 18

• Manufacturer Part Number: U4091BM
• Description: Programmable Telephone Audio Processor
• Manufacturer: TEMIC Semiconductors

Sidetone System
The SideTone Balancing (STB) has the task of reducing
the crosstalk from LTX (microphone) 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 signal (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 lowpass 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 summing 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 suppression with one
application for all different line impendances, the STB
function is programmable.
U4091BM
18 (29)
LRX
AGARX
0–7dB
9dB
STO_DIFF
–
+
AMP2
DIFF1
g
Figure 19. Principle circuit of the sidetone balancing
LF
LF
Target Specification
LTX
–10dB
Sidetone balancing
P
P
AMP1
The 3 programmable parameters are:
1. LF (gain at low frequency)
STO_DIFF(LF) = (–10 dB – 2 dB + 0.5 dB
2. P (the pole position of the lowpass)
3. SL
LF has 15 programming steps of 0.5 dB. LF(0) gives
–2 dB gain, LF(15) gives 5.5 dB gain.
The P adjustment has 31 steps. P(0) means the
lowpass determined by the external application
(RST, CST). The internally processed lowpass
frequency is fixed by this equation
(sidetone slope; the pole frequency of the highpass)
The SL has 3 steps. SL(0) is a lower frequency of the
highpass. SL(3) is a higher frequency of the highpass.
With SL, can be influenced the suppression at high
frequencies.
f(P)
STOAMP
MOD
+
SL
SL
–10dB
8dB
2
f
p
CST
1
RECIN
LINE
STRC
RST
STO
STC
Rev. A1, 02-Jun-98
1.122
LF + 9 dB)
CK
CTO
33 nF
STO
8.2 k
P
14579
ZL
LTX