PBL 3881
output a RC network consisting of an
internal resistor of 100k and an external
capacitor C4. The voltage across C4 is
connected to the CMP input (summing
point) via a resistor R5. The resistor R6 is
important in order to keep the charging
current of C4 within safe limits in regard
of high charge peaks that could be
audible in the system. The extent to
which the NDet output will influence the
potential at CMP input is set by the gain
of the detector, the maximum swing and
R5. If a continuous input signal is
LS 17
Input
0.22 µ
50 ohm
22 B
in
100 µF
16V
+
+
PBL 3881
+ Line
1000 µF
16V
+L 19
–C 21
C
VLine
R
20
R
GND
16
DC
E
18
received from the microphone ( > 10sec.)
the voltage across C4 is pulled negative
(relative to the reference) with a time
constant set by C4 to e.g. 5 sec. A
– Line
R
E
V
RDC
continuous input signal is thus treated as
noise. Since the output of the noise
detector is going negative it thereby
counteracts the signal from the transmit-
ter detector and thus helping the receiver
detector signal to maintain a set relation
to the transmitter detector signal. If the
transmitter input signal contains breaks
like breath pauses the voltage at TxDet
output decreases. If the voltage across
C3 gets less than the inverted voltage
across C4 divided by the detector gain a
rapid charge of C4 towards reference will
follow (all levels referred to the
reference). If the breaks are frequent as
in speech the background detector will
not influence the switching characteristic
of the system. See fig.13. There is a
threshold of approx. 50 mV at TxDet to
prevent the activation of background
noise detection in noiseless environment.
In the receive mode some of the
Figure 21. Power supply in parallel with speech circuit.
LS 17
Input
50 ohm
22 B
in
L
0.22µ
100µF
1
16V
+
PBL 3881
+ Line
+L 19
–C 21
(2 Alt.)
2
+L
+
1000µF
16V
Regulated voltage
from
R
20
R
DC
18
GND
16
E
mains supply
– Line
Figure 22. External power supply option. Line supply with inductor or mains supply.
loudspeaker output signal will be sensed
by the microphone. In order not to treat
this input signal as noise, the noise
detector goes into a hold state and
”remembers” the level from the previous
V
out
(Vp)
V
2.4
2.0
VLine
1.6
1.2
0.8
VR
0.4
ILine
ILine
(mA)
100
10
20
30 40
50
60
70
80
90
Figure 23. Typical loudspeaker output swing.
10
Figure 24. Speech circuit DC characteristics.
ERICSSON [ ERICSSON ]
