pbl388141sot ETC-unknow, pbl388141sot Datasheet - Page 12

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

Hints how to design a handsfree system with PBL 388 14.
PBL 388 14
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 20. Schematic
diagram of the various
signal paths that affect on
the design of a handsfree
telephone.
General:
mind when looking at a ”handsfree” solu-
tion 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 transmitting
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 speech would
12
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
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 and at
maximum loudspeaker volume.
the
loudspeaker
telephone designer gets often an order to fit
a ”handsfree” telephone system into a fully
unsuitable ready made casing. The design
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
coupling
Transmitter channel
microphone.The
Receiver channel
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.
of a ”hansfree” telephone with a speech
control starts with the acoustical design of
the casing. PBL 388 14 makes a good
acoustical design to sound as close a perfect
”handsfree” 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
handsfree” telephone or to make the best
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
G
G
2
6
G
7
through
working