PBL 388 14
Hints how to design a handsfree system with PBL 388 14.
To design the speech control function,
seven different signal paths have to be
considered and understood. See fig. 28.
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.
G3 starts the same as G1 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,furthertothetransmitterofthespeech
circuit and out on the telephone line.
G4 is the corresponding signal to G3
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
an acoustic signal of the loudspeaker.
G5 starts the same way as G4 ends.
From the receiver rectifier through
loudspeaker amplifier, loudspeaker,
acoustic signal path (loudspeaker -
microphone) and is terminated, like G1, at
transmitter rectifier.
G6 is the corresponding signal to G5
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.
The signal paths:
G1 is the acoustic signal into the
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.
G2 is the corresponding signal to G1
on the receiver side. The signal from the
line that goes via the sidetone balancing
network and an amplifier which gain can be
G7 is the closed loop signal that can
be considered to start or end at any point in
the loop. The summ of G5 and G6.
Figure 20. Schematic
diagram of the various
signal paths that affect on
the design of a handsfree
G7
G3
G
5
telephone.
Transmitter channel
G6
G1
acoustical
COMPARATOR
coupling
G2
Receiver channel
VOLUME
G4
General:
The first thing that comes into ones sound harsh. This is normally not the most
mind when looking at a ”handsfree” solu- difficult requirement on the gain in the G7
of a ”hansfree” telephone with a speech
control starts with the acoustical design of
tion like the one with PBL 388 13 is, that it loop. The most difficult requirement is set the casing. PBL 388 14 makes a good
must be able to prevent oscillation in the by the telephone set impedance towards
closed loop G7. The circuit does this by theline. Thesignaloriginatesfromtheline,
acousticaldesigntosoundascloseaperfect
”handsfree” as it is possible. This means
having 50 dB less gain in the opposite rounds the loop G7 and enters the line that there are no audible swiching noises
direction against the open channel this again. This way the impedance of the
being either the receiving or transmitting telephonesettowardsthelineisinfluenced
and speech is conveyed in one direction at
the time. In opposite case having a bad
direction. Nor does it oscillate when having by the gain in the loop G7. The impedance acoustic design with a large coupling
proper gain values, sidetone balance, of the telephone towards the line has to
loudspeaker volume and small acoustic measured in the ”handsfree” mode under
between the loudspeaker and the
microphone, no electronics in the world,
coupling between the loudspeaker and correct acoustic circumstances and at using the speech switching principle, can
microphone. Actually, one needs a lot of maximum loudspeaker volume.
marginagainstoscillationsothatnopositive
A major problem in many cases is
makeittosoundgood.Why, willbestudied
later.
feedback is created in the loop G7. This the acoustical coupling between Acoustic design:
would destroy the frequency characteristic loudspeaker and microphone.The
Any amount of time can be spent on
through the increasing gain at the "would telephonedesignergetsoftenanordertofit the acoustic design. It depends largely if
oscillate frequency" in case of somewhat a ”handsfree” telephone system into a fully the task is to make a "just
working
higher gain in the loop. The speech would unsuitableready madecasing. Thedesign handsfree” telephone or to make the best
12
ERICSSON [ ERICSSON ]
