pbl388141sot ETC-unknow, pbl388141sot Datasheet - Page 9

• Manufacturer Part Number: pbl388141sot
• Description: Voice Switched 2-channel Circuit With Loudspeaker Amplifier
• Manufacturer: ETC-unknow

Figure 14. Speech switching arrangement.
decreases with factor of 10, thus increasing
the detectors dynamic range. See fig.8.
Background Noise Detector
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
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 4 . The voltage across C 4 is
The general function of the backgro-
Mic.
C
+
+DC
R
R
Transmitter
channel output
R
C
C
C
C
+ C
+
CTR
Tx
Tx
Tx
out
in
in
16
24
4
3
5
1
2
+
level
F1
+
N Det
F2
C4
F3
connected to the CMP input (summing point)
via a resistor R 5 . The extent to which the
N Det output will influence the potential at
CMP input is set by the gain of the detector,
the maximum swing and R 5 . If a continuous
input signal is received from the microphone
( > 10sec.) the voltage across C 4 is pulled
positive (relative to the reference) with a
time constant set by C 4 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 Tx Detout 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
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 Tx Detout to prevent the
+
8
R5
Tx
(all levels referred to the
C3
Det
6
+
CMP
Control
9
C1
PBL 388 14
10
+
Rx Det
C2
Ref.
Ref.
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
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.
F6
F5
7
For full speech control (50dB
F4
+
13
17
11
15
14
12
Rx out
+ Rx
GND
Rx
in
in
P1
C
C
C
PBL 388 14
Power amplifier
input
Receiver
channel input
C
9