Philips Semiconductors
Low voltage versatile telephone
transmission circuits with dialler interface
Product specification
TEA1112; TEA1112A
MUTE and MMUTE levels for different modes
Table 1 Required MUTE and MMUTE levels to enable the different possible modes
IC
Mode
Speech
DTMF dialling
Microphone mute
TEA1112
MUTE
MMUTE
L
L
H
X
L
H
TEA1112A
MUTE
MMUTE
H
H
L
X
H
L
SIDETONE SUPPRESSION
The TEA1112; TEA1112A anti-sidetone network
comprising RCC // Zline, Rast1, Rast2, Rast3, RSLPE and Zbal
(see Fig.10) suppresses the transmitted signal in the
earpiece. Maximum compensation is obtained when the
following conditions are fulfilled:
RSLPE × Rast1 = RCC × (Rast2 + Rast3)
k = --(--R-----a---s--t-(-2--R--×--a---s-(--tR-1----a×---s--tR--3---S-+--L---RP---E-S---)-L---P---E---)---)--
Zbal = k × Zline
The scale factor k is chosen to meet the compatibility with
a standard capacitor from the E6 or E12 range for Zbal.
In practice, Zline varies considerably with the line type and
the line length. Therefore, the value chosen for Zbal should
be for an average line length which gives satisfactory
sidetone suppression with short and long lines.
The suppression also depends on the accuracy of the
match between Zbal and the impedance of the average
line.
The anti-sidetone network for the TEA1112; TEA1112A
(as shown in Fig.14) attenuates the receiving signal from
the line by 32 dB before it enters the receiving amplifier.
The attenuation is almost constant over the whole audio
frequency range. A Wheatstone bridge configuration (see
Fig.11) may also be used.
More information on the balancing of an anti-sidetone
bridge can be obtained in our publication “Applications
Handbook for Wired Telecom Systems, IC03b”, order
number 9397 750 00811.
handbook, full pagewidth
Zline
VEE
LN
RCC
Rast1
RSLPE
Im
IR
Rast2
Rast3
SLPE
Zbal
Zir
MBE787
Fig.10 Equivalent circuit of TEA1112; TEA1112A family anti-sidetone bridge.
1997 Mar 26
9