pbl388131sot ETC-unknow, pbl388131sot Datasheet - Page 14

• Manufacturer Part Number: pbl388131sot
• Description: Voice-switched Speakerphone Circuit With Built Loudspeaker Amplifier
• Manufacturer: ETC-unknow

PBL 388 13
seven different signal paths have to be
considered and understood. See fig. 28.
The signal paths:
microphone, further transformed to an
electrical signal in an amplifier which gain
can be controlled 12,5 dB up or down from
an idle point, further to a point where it is
rectified to a negative signal and compared
with its counterpart from the receiver
channel.
on the receiver side. The signal from the
line that goes via the sidetone balancing
network and an amplifier which gain can be
Figure 28. Schematic
diagram of the various
signal paths that affect on
the design of a handsfree
telephone.
General:
mind when looking at a ”handsfree”
telephone solution like the one with PBL
388 13 is, that it must be able to prevent
oscillation in the closed loop G7. The circuit
does this by having 50 dB less gain in the
opposite direction against the open channel
this being either the receiving or transmit-
ting direction. Nor does it oscillate when
having proper gain values, sidetone
balance, loudspeaker volume and small
acoustic coupling between the loudspeaker
and microphone. Actually, one needs a lot
of margin against oscillation so that no
positive feedback is created in the loop G7.
This would destroy the frequency
characteristic through the increasing gain
at the "would oscillate frequency" in case
of somewhat higher gain in the loop. The
14
Hints how to design a handsfree telephone with PBL 388 13.
To design the speech control function,
G1 is the acoustic signal into the
G2 is the corresponding signal to G1
The first thing that comes into ones
G
5
speech would sound harsh. This is normally
not the most difficult requirement on the
gain in the G7 loop. The most difficult
requirement is set by the telephone set
impedance towards the line. The signal
originates from the line, rounds the loop G7
and enters the line again. This way the
impedance of the telephone set towards
the line is influenced by the gain in the loop
G7. The impedance of the telephone
towards the line has to measured in the
”handsfree” mode under correct acoustic
circumstances
loudspeaker volume.
the
loudspeaker
telephone designer gets often an order to fit
a ”handsfree” telephone system into a fully
controlled 12,5 dB up or down from an idle
point, further to a point where its rectified to
a positive signal and compared with its
counterpart from the transmitter channel.
go to the rectifier, instead passes through
further an amplifier which gain can be
controlled 12,5 dB up or down from an idle
point, further to the transmitter of the speech
circuit and out on the telephone line.
on the receiver side. Starts the same as G2
but does not go to the rectifier, instead
passes through further an amplifier which
gain can be controlled 12,5 dB up or down
from an idle point, via loudspeaker volume
control, loudspeaker amplifier and out as
acoustical
coupling
G
A major problem in many cases is
G3 starts the same as G1 but does not
G4 is the corresponding signal to G3
1
G
acoustical
G
3
4
VOLUME
and
and
coupling
Transmitter channel
microphone.The
Receiver channel
at
maximum
between
COMPARATOR
an acoustic signal of the loudspeaker.
From the receiver rectifier
loudspeaker amplifier, loudspeaker,
acoustic signal path (loudspeaker -
microphone) and is terminated, like G1, at
transmitter rectifier.
but goes through the sidetone network.
Starts the same way as G3 ends. From the
transmitter rectifier, amplifier via speech
circuit transmitter, sidetone balancing
network and the line, to be terminated at
receiver rectifier like G2.
be considered to start or end at any point in
the loop. The summ of G5 and G6.
unsuitable ready made casing. The design
of a ”hansfree” telephone with a speech
control starts with the acoustical design of
the casing. PBL 388 13 makes a good
acoustical design to sound as close a perfect
”handsfree” telephone as it is possible.
This means that there are no audible
swiching noises and speech is conveyed in
one direction at the time. In opposite case
having a bad acoustic design with a large
coupling between the loudspeaker and the
microphone, no electronics in the world,
using the speech switching principle, can
make it to sound good. Why, will be studied
later.
Acoustic design:
the acoustic design. It depends largely if
the task is to make a "just
G
G5 starts the same way as G4 ends.
G6 is the corresponding signal to G5
G7 is the closed loop signal that can
Any amount of time can be spent on
2
SPEECH
CIRCUIT
SIDETONE
BALANCE
G
6
G
Line
7
through
working