PBL 388 13
Transmitter
channel output
24
4
Power amplifier
input
CTR
Tx
PBL 388 13
R
Rx
C
out
out
11
12
C
Control
P1
F6
F3
F2
3
5
+L
Receiver
channel input
10
16
GND
Rx
R
15
+
C
+
+
F5
R
R
C
+
C
C
C
14
13
R
1
2
in
Tx
Tx
F4
in
in
F1
Ref.
+
+
+
Rx
in
7
6
8
9
C
Tx
N
CMP
Rx
Det
C4
Det
C3
Det
R
C
Mic.
C
+
R5
+
+
C2
C1
Figure 16. Speech switching arrangement.
activation of background noise detection in
noiselessenvironment.Inthereceivermode
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.
decreaseswithfactorof10, thusincreasing connectedtotheCMPinput(summingpoint)
the detectors dynamic range. See fig.10. via a resistor R . The extent to which the
5
N
output will influence the potential at
Det
CMP input is set by the gain of the detector,
themaximumswingandR . Ifacontinuous
5
Background Noise Detector
inputsignalisreceivedfromthemicrophone
( > 10sec.) the voltage across C is pulled
4
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 C . The voltage across C is
positive (relative to the reference) with a
time constant set by C to e.g. 5 sec. A
4
continuous input signal is thus treated as
noise.Sincetheoutputofthenoisedetector
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
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
+
figure15. Tosetthecircuitintoamutestate
(resultsin,reducedgaininreceiverchannel
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
willemergewhere bothreceiverandtrans-
mitterchannelsareclosed.Seefig. 13and
15.
the voltage at Tx
decreases. If the
Detout
voltageacrossC3getslessthantheinverted
voltage across C4 divided by the detector
gainarapidchargeofC4towardsreference
will follow (all levels referred to the
reference). If the breaks are frequent as in
speech the background detector will not
influencetheswitchingcharacteristicofthe
system. See fig. 11. There is a threshold of
approx. 50mV at Tx
to prevent the
4
4
Detout
9
ERICSSON [ ERICSSON ]
