TEA7540 ST Microelectronics, Inc., TEA7540 Datasheet - Page 3

• Manufacturer Part Number: TEA7540
• Description: Handsfree Controller
• Manufacturer: ST Microelectronics, Inc.

PIN FUNCTION
FUNCTIONAL DESCRIPTION
SWITCHED ATTENUATORS
Fig.A represents a block diagram of a handsfree
subset with attenuators in signal mode. To pre-
vent the system from howling, the total loop gain,
including acoustic feedback through the housing
and sidetone coupling, must be less than 0dB.
For this purpose, two switched attenuators are in-
serted in each mode (emission and reception).
The attenuation is shifted from one mode to the
other, resulting from the speech level comparison
between each way.
To prevent the circuit to switch continuously in
one way, the operation of the IC must be fully
symetrical in both ways. This involves signal com-
parison, attenuation value.
GAIN COMPRESSOR
In TEA7540, two signal compressors are inserted
in each mode before the signal comparison, so
15
N
10
11
12
13
14
16
17
18
19
20
21
22
23
24
25
26
27
28
1
2
3
4
5
6
7
8
9
AMP SUP
RECOUT
MICOUT
AMP IN
HYST1
HYST2
OUTE
OUTR
Name
ATTR
ATTE
GND
CCE
CCR
CE3
CE2
CE1
CR1
CR2
CR3
VOL
ATR
TIM
INE
INR
V
CS
V+
I
ref
ref
Ground
Chip Select
Transmit Attenuator Output
Transmit Attenuator Input
Transmit Channel Hysteresis
Receive Channel Hysteresis
RC Timer
Time Constant of the Transmit Signal Compressor
Transmit Background Noise Memorization Output
Transmit Peak Detector Output
Transmit Rectifier Input
Transmit Signal Compressor Output
Transmit Signal Compressor Input
V+/2 - Reference Voltage
Receive Signal Compressor Input
Receive Signal Compressor Output
Receive Rectifier Input
Receive Peak Detector Output
Receive Background Noise Memorization Output
Time Constant of the Receive Signal Compressor
Handset Preamplifier Input
Handset Preamplifier Power Supply
Volume Control
Attenuation Value
Receive Attenuator Input
Receive Attenuator Output
Reference Current Source
the signal coming from each end has the same
level (100mV peak), the losses in each way (for
instance losses resulting from the line length in
receiving mode) are compensated and the signal
comparison is fully symetrical. The time constant
of each signal compressor decreases 80 times
more quickly than it increases to prevent from
noise increasing between words. The compress-
ing depth is 38dB.
BACKGROUND NOISE DISCRIMINATION
An additional feature provided in TEA7540 is
background noise level discrimination in each
way. The IC stores the background sound level
with a long time constant (3 to 5 seconds depend-
ing on an external RC) and compares it with the
incoming signal in order to distinguish a useful
signal (speech) from the background noise. This
background noise memorization is also used to
compensate the noise in each mode before signal
comparison: the noise level in each mode is sub-
Function
TEA7540
3/13