ucb1200 NXP Semiconductors, ucb1200 Datasheet - Page 34

• Manufacturer Part Number: ucb1200
• Description: Advanced Modem/audio Analog Front-end
• Manufacturer: NXP Semiconductors

Philips Semiconductors
APPLICATION INFORMATION
In this chapter some application information is contained. More information will be available when an Application Note
on UCB1200 is published.
Sidetone suppression circuit
An important built-in feature of the telecom codec is the sidetone suppression circuit. The sidetone suppression circuit is
activated when TEL_SIDE_ENA in the telecom control register B is set. The telecom input signal contains a large part
of the telecom output signal, when the sidetone suppression circuit is disabled. The available dynamic range of the
telecom input is occupied largely by the telecom output voltage.
The sidetone suppression circuit subtracts a part of the telecom output signal from the telecom input signal when
activated. The available dynamic range is in that case used more effectively than without sidetone suppression.
The built in side tone suppression circuit, shown in Fig.30, has a fixed subtraction ratio, set be the resistors R
which equals 600/456. This ratio is calculated from the following relations.
The impedance seen by the telephone line equals:
in which Rt represents winding resistance of the transformer, divided by 2. Assuming Ri >> Ro then
1998 Jul 22
Z
R
line
Advanced modem/audio analog front-end
line
=
=
2
R
t
A
B
+
R
R
t
1:1 transformer
t
Fig.30 Typical telecom codec sidetone suppression circuit (without protection circuits).
+
+
Rt
Rt
R
R
t
o
+
=
------------------ -
R
R
600 2
o
0
+
R
R
Rt
Rt
i
i
=
,
300
TOUTP
TOUTN
TINP
TINN
11
15
14
12
UCB1200
34
Ri
Rs
Rs
Ri
-
+
+
-
Rg
Rg
Product specification
UCB1200
s
and R
i
,