TEA7540
HANDSFREE CONTROLLER
NOISE/SPEECH DISCRIMINATION IN EMIS-
SION AND RECEPTION
INTEGRATED SIGNAL GAIN COMPRESSOR
IN BOTH MODES
PROGRAMMABLE ATTENUATORS IN BOTH
MODES
ADAPTED TO ACOUSTIC PARAMETERS OF
ALL CABINETS
LOW OPERATING VOLTAGE 2.5V
LOW OPERATING CURRENT 2.1mA
CHIP SELECT BETWEEN HANDSFREE AND
MONITORING MODES
DESCRIPTION
This 28 pins IC is an innovative approach to quality
handsfree telephone sets. It results from an extensive
research on speech signal.
July 1998
T
is advanced information on a new product now in development or undergoing evaluation. Details are subject to change without
HYST2
TIM
CCE
CE3
CE2
MICOUT
CE1
INE
VREF
1
3
2
4
5
6
7
8
9
INR
RECOUT
CR1
CR3
CR2
CCR
AMP IN
AMP SUP
VOL
23
22
21
20
19
17
18
16
15
D93TL005
10
11
12
13
14
28
27
26
25
24
GND
CS
OUTE
ATTE
HYST1
ATR
ATTR
OUTR
IREF
V+
PIN CONNECTION (top view)
SO28
ORDERING NUMBERS:
TEA7540FP
1/12
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
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-
N
Name
Function
1
GND
Ground
2
CS
Chip Select
3
OUTE
Transmit Attenuator Output
4
ATTE
Transmit Attenuator Input
5
HYST1
Transmit Channel Hysteresis
6
HYST2
Receive Channel Hysteresis
7
TIM
RC Timer
8
CCE
Time Constant of the Transmit Signal Compressor
9
CE3
Transmit Background Noise Memorization Output
10
CE2
Transmit Peak Detector Output
11
CE1
Transmit Rectifier Input
12
MICOUT
Transmit Signal Compressor Output
13
INE
Transmit Signal Compressor Input
14
V
ref
V+/2 - Reference Voltage
15
INR
Receive Signal Compressor Input
16
RECOUT
Receive Signal Compressor Output
17
CR1
Receive Rectifier Input
18
CR2
Receive Peak Detector Output
19
CR3
Receive Background Noise Memorization Output
20
CCR
Time Constant of the Receive Signal Compressor
21
AMP IN
Handset Preamplifier Input
22
AMP SUP
Handset Preamplifier Power Supply
23
VOL
Volume Control
24
ATR
Attenuation Value
25
ATTR
Receive Attenuator Input
26
OUTR
Receive Attenuator Output
27
I
ref
Reference Current Source
28
V+
PIN FUNCTION
TEA7540
3/12
stracted from the incoming signal before the com-
parison. So very high noise level in one mode
cannot trouble the comparison between the useful
signals.
The result of the comparison manages the attenu-
ators in the following way:
- The maximum attenuation is switched on
the mode where the speech signal is the
lowest. The maximum attenuation is fixed
by two external resistor (maximum 52dB).
The time constant of the switch is fixed by
the timer via an external capacitor.
- When neither party is talking both attenu-
ators are set to a medium attenuation.
Thus each mode is in idle mode. The time
constant of the switch from active mode to
idle mode must be long enough to prevent
from switching to idle mode between two
words (see fig.B). This time constant is
fixed by an external RC.
Figure A
Em Rec
1
0
SPEECH/NOISE
A max dB
0dB
0
1
IDLE (A max/2)
D93TL009A
Figure B
TEA7540
4/12
TEA7540 OPERATION
TEA7540 is powered through an external shunt
regulator (for instance the shunt regulator of the
monitor amplifier TEA7532) or an external zener
diode.
It can work at a very low voltage (2.5V) over the
circuit and it has a low current consumption
(2.1mA).
It's also possible via the chip select pin (CS) to
put the handsfree function in standby to use the
circuit in monitoring mode with the handset micro-
phone.
TEA7540 is designed to work with all kind of mi-
crophone, including Electret.
TEA7540 also handles the handset microphone
signal (AMP IN) when the system is set to normal
conversation mode.
Figure C: Application Diagram (Example of high range telephone set using TEA7540).
Symbol
Parameter
Value
Unit
V
+
Supply Voltage
12
V
Top
Operating Temperature
-20 to 70
C
T
stg
Storage Temperature Range
-65 to 125
C
ABSOLUTE MAXIMUM RATINGS
TEA7540
5/12
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