PBL 388 12
Transmitter
channel output
CTR
R
Tx
out
5
C
Control
P1
F3
4
+L
6
R
+ C
C
+
R
C
2
R
Tx
in
3
C
Mic.
C
C4
+
R5
C3
+
C1
+
C2
+Tx
in
+
8
N Det
Tx
Det
7
CMP
9
10
Rx Det
C
R
F1
Ref.
F4
14
Rx
in
C
15
+
F2
F5
R
11
16 GND
F6
13
Receiver
input
1
PBL 388 12
12
Rx out
C
Power amplifier
input
Figure 14. Speech switching arrangement.
a logarithmic rectifier characteristic
whereby gain and sensitivity is high at
small signals. There is a break point in the
curve at a level of
±
200mV from the
internal reference voltage app. 2V, where
the sensitivity for increasing input signals
decreases with factor of 10, increasing the
detectors dynamic range. See fig.10.
Background Noise Detector
The general function of the background
noise detector in the transmittng channel
is to create a positive signal ( in respect to
the referrence) so that, when coupled to
the summing point at the CMP input, will
counteract the 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 backround noise
level detector is taken from the output of
the transmitter detector , a voltage
representing the envelope of the amplified
microphone signal. 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
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 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 continuous input sig-
nal is thus treated as noise. Since the out-
put 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. 11. 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 input can be
left unconnected. To set the function to
25dB attenuation the input has to be higher
than 600mV below V+. See figure 13. To
set the circuit into a mute state (results in,
redeced 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 in-
put. By lowering the voltage at the input
below 0.9V a condition will emerge where
both receiver and transmitter channels are
closed. See fig. 11 and 15.
9
ERICSSON [ ERICSSON ]
