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4872A–CORD–08/05

U4091BM-R

15. Sidetone System

Figure 15-1. Principle Circuit of Sidetone Balancing

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.

-10 dB

LF

f

P

SL

g

STC

STRC

0 - 7 dB

STOAMP

STO

8.2 k

Ω

STO

CTO

33 nF

8 dB

MOD

RECIN

LINE

-10 dB

AMP1

9 dB

STO_DIFF

DIFF1

AMP2

+

-

AGARX

LTX

LRX

CK

ZL

Sidetone balancing

LF

P

SL