u4091bm-r ATMEL Corporation, u4091bm-r Datasheet - Page 27

• Manufacturer Part Number: u4091bm-r
• Description: Programmable Telephone Audio Processor
• Manufacturer: ATMEL Corporation

15. Sidetone System
Figure 15-1. Principle Circuit of Sidetone Balancing
4872A–CORD–08/05
LRX
AGARX
0 - 7 dB
The Sidetone Balancing (STB) has the task of reducing the cross-talk from LTX (microphone) to
LRX (earpiece) in the frequency range of 0.3 kHz 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 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 summing block. If both sig-
nals 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 impedances, the STB function is programmable.
9 dB
STO_DIFF
-
+
AMP2
DIFF1
g
LF
LF
LTX
Sidetone balancing
P
P
-10 dB
AMP1
STOAMP
MOD
SL
SL
-10 dB
8 dB
f
RECIN
LINE
STRC
STO
STC
U4091BM-R
CK
CTO
33 nF
8.2 k
STO
ZL
27