PBL 388 14
Transmitter
channel output
CTR
24
PBL 388 14
12
Rx out
Power amplifier
input
C
Tx
out 4
C
Control
P1
F3
3
F6
13
+DC
5
11
17
Receiver
channel input
GND
R
+ C
C
+
R
C
16
+
F2
F5
15
C
F4
Rx
in
+
14
1
R
Tx
in
2
F1
+
8
6
9
10
Ref.
C
Mic.
C
+Tx
in
7
+ Rx
in
C
C
N Det
+
R5
Tx
Det
CMP
Rx Det
Ref.
C4
+
C3
C1
+
C2
Figure 14. Speech switching arrangement.
decreases with factor of 10, thus increasing
the detectors dynamic range. See fig.8.
Background Noise Detector
The general function of the backgro-
und noise detector in the transmitting
channel is to create a positive signal ( in
respect to the internal reference) so that,
when coupled to the summing point at the
CMP input, will counteract the continuous
type signal from the transmitter level
detector representing the actual sound
pressure level at the microphone. This
counteracts the noise from influencing the
switching characteristics. The input signal
to the back ground noise level detector is
taken from the output of the transmitter
detector, a voltage representing the
envelope of the amplified microphone sig-
nal. The detector inverts and amplifies this
signal 2 x (transmitting mode) and has on
it´s 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 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 received from the microphone
( > 10sec.) the voltage across C4 is pulled
positive (relative to the reference) with a
time constant set by C4 to e.g. 5 sec. A
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 transmitter 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 TxDetout 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. 9. There is a threshold of
approx. 50mV at TxDetout to prevent the
activation of background noise detection in
noiseless environment. In the receiver mode
some of the 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
transmitting mode periode.
CTR Input
For full speech control (50dB
attenuation between the channels) this in-
put can be left unconnected. To set the
function to 25dB attenuation the input has
to be higher than 600mV below V+. See
figure 15. To set the circuit into a mute state
(results in, reduced gain in receiver channel
for the DTMF confidence tone in the
loudspeaker and closed transmitter
channel) a voltage below Vref has to be
connected to the input. By lowering the
voltage at the input below 0.9V a condition
will emerge where both receiver and trans-
mitter channels are closed. See fig. 13.
9
ERICSSON [ ERICSSON ]
