Note: Descriptions are shown in the official language in which they were submitted.
8~
PHF 81.543 C 1 23~3,1982
"Arrangement for avoiding annoying sustained oscillations
in a closed~loop system and loud-speaking telephone set
employing this arrangement".
The invention relates to an arrangement for
avoiding annoying~ sustained oscillations in a closed-
loop system, the arrangement comprising a variable~gain
ampllfier incorporated in a first path which forms part
of the loop o.f the said system.
Among the closed-loop systems in which anno}~ing
sustained oscillations may be produced there are~ for
example~ systems for controlling electrical or n1echanical
or physical quantities. It is known that during the design
of such systems, the gain and phase conditions in the
control loop must have proper attention paid to them in
order to avoid sustained oscillations in the loop. But
there is never a certainty that due to unforeseen circum-
stances, for example parasitic couplings ~ sudden pertur-
bations, self-oscillations will not be produced which may .
attain high and even dangerous amplitudes in certain
systems. A different type of closed loop system is an
electro-acoustical system which comprises~ for example,
a microphone and a loudspeaker which are coupled to any
arbltrary manner by means of an electric circuit; if on
the other hand these two transducers are coupled by mea~s
of an acoustic circuit 7 an electro-acoustic loop is
obtained in which oscillations of a high amplitude may
be produced which cause unacceptable how~sO This phenome- I
non9 which is known as the Larsen effect, may be produced
in, for example, sound reproduction equipment or in
loudspeaking talephone sets.
To avoid oscillations in an electro-acoustical
loop the procedures which are generally employed consist
in the pro~ision in the loop of at least a ~-ariable
amplifier or attenuator circuit, which are controlled in
accordance with different criteria in order that the gain
., '~
8~
PH~ 81.~l~3C - 2 23 3.19~2
in the loop re~ains below unity. Thus, a procedure
emplo~-ed in loudspeaking telephone sets consists of
checking, by means of two envelope detectors, ~ihether
the speech signal is present in the loudspe~ker path
5 or in the microphone path and producing a gain increase
in the useful path and a gain reduction in the other path.
- These gain modifications which follow each other in both
paths during a conversation are very unpleasant for the
listeners and in addition this type of procedure is not
lO ~-ery effecti~e ~or the important acoustical couplings, t
as the gain control does not substantially depend on
the coefficient of acoustic coupling.
Another procedure described in ~rench Patent
Application No 2~461,412 also employes two signal en~elope
tS detectors in the two paths and a single amplifier in the
loudspeaker path, the gain of which is varied by a diffe-
rence signal between the output signal of the envelope
detector of the microphone path and the output signal
of the envelope detector of the loudspeaker path9 weighted
20 with a predetermined coefficient. For distances which are
lo~ger than a predetermined minimum distance between the
loudspeaker and the mic~ophone9 the gain of the amplifier
is in a constantDatio to the coefficient of acoustic
coupling and the oscillations can only be a~oid~d by means
25 of this procedure below said minimum distance. In
additiong the use of envelope detectors causes the recep-
tion to depend on the speech signals and particularly on
the speech signal produced by a speaker who speaks into
the microphone.
The pres2nt in~ention provides a different
means to avoid annoying loop oscillations 9 by mitigating
the disad~antages and the limitations of the prior art
procedures. The in~ention not only relates to electro-
acoustic loops, but also to all types of closed-loop
s~--remC such as control systems,
The in~-ention is based on the idea of causing
said loop oscillations 9 when the loop oscillation
-
PHF 81.543C 3 23.3~1982
conditions have been created, to pass through a second
path which has at least a distlnct porti.on of the first
path and controlling said osc.illati.ons b~ rneans o:f a
regulator so as to bring them to a constant level which
is low and not annoying in the remaining part of the
loop.
According to the invention, a second path which
has at least one part which is separate from the first
path is formed between the input and output terminals of
the first path, means being provided to ensure that the
g~in in the second path remains higher than the gain in
the first path, in the overall frequeney band where the
oscillations are liable to be produced, a signal formed
in the said part of the second path which is separate
from the first path being applied to the input of a linear
regulator controlling at least one variable-gain ampli-
fier which forms part of the second path to ensure that the
signal at the input of the said regulator remains constant
from a certain value of the said signal formed in the
seoond path.
In certain embodiments of the inven-tion the
second path is completely separate from the first path and
comprises a variable-gain amplifier whose output sig~al
is applied to the input of the said linear regulator, the
latter simultaneously controlling from the input terminal
which is used jointly by the two paths the said variable-
gain amplifier ~ the second path and a variable-gain
amplifier provided in the first path ar~d/or a variable-
gain amplifier provided in the first path from the output
terminal which is jointly used by the two pathsO
In further embodiments of the inven~on it is
possible to emit one variable-gain amplifier? because of the
fact that the first and second paths comprise one common
variable-gain amplifier which is provided at the input
terminal which is used jointly by the two paths and/or
a variable-gain amplifier provided at the output terminal
which is used jointly by the two paths, the one or the
other of these variable-gain amplifiers jointly used by
~ 4
PHF 81.543C - 4 ~3.3~1982
the t~-o paths being controlled by the said linear
re~ulator.
~d~-antageousl~r, the non-annoying oscillations t
occurring in the loop which is clos~d by the second path
5 are produced at a well-defined frequency. This result
is preferably obtained by means of a ~ilter having a
narrow passband which is connected into that part of the t
second path which is ~Eferent from the first path, before
the spot where the signal is taken off which is applied
to the input of the linear regulator, 30 as to selectively
increase the gain in the second path.
In addition~ in order to satisf~ the gain con~
ditions relative to the t~o paths it is possible to
include a further filter in that part of the first path
which is different from the second path~ this filter
increasing the gain in the same narrow passband as the
filter of the second path. It is alternatively possible
to provide such a filter before the input terminal which
is used jointly by the two paths, after the output termi-
~al which i5 used jointly by the two paths or in the parts
which are used jointly by the two paths.
The arrangement in accordance with the in~ention
can be advantageously employed in a loud$eaking telephone
set, the coupling circuit coupling the set to the telephone
line then being included in that part of the first path
which is dif~erent from the second pathl I~ tke loop which
will be denoted auxiliary loop hereinafter and which
comprises the acoustic patn between the loud~eaker and
the microphone of the telephone set and which is closed
by the second path, sustained amplitude oscillations are
obtained which are controlled by the regulator to ensure
that they are non-annoying and~ in particular, inaudible,
al the same time~ annoying Larsen oscillations are pre~en-
red from being produced in the electro~acoustic loop9
denoted main loop, ~hich is closed b~- the first path,
these annoying oscillations being produced o~-ing to imper-
fections in the coupling circuit coupling the local sub
~ 8 ~
PHF 81.543C 5 23~3.1982
scriber's set or owing to an acoustic ooupling in the
remote subscriber's set.
~ The arrangement in accordance with the in~ention
when us0d in a loudspeaking telephone set of the "hands
free" type renders it furthermore possible to effect in a
very simple manner an automatic change~over of the gains
in the transmis~ion and receiving path of the telephone
set. In this respect it should be noted that in many
prior art "hands free" telephone sets~ for example the
10 sets disclosed in French Patent Application No~ 2~376,576,
the gain change-over by speech is employed in order to
pre~ent annoying Larsen oscillations from being produced.
In teLephone sets of this type the speech si~lals in the
transmission and receiving paths are detected to determine i~
15 the path in which the speech signal is the higher; a
command to increase the gain is effected in this path
and a gain to decrease the gain is effected in the other
path, these two gain variations being complementary so as
to ensure that the total gain in the loop where the Larsen
20 oscillations may be produced remains below unity.
In a telephone set of the "hands-free" type
provided with the arrangement in accordance with the
invention it is not ~ecessary to effect a gain change-over
b~ the speech to prevent annoying Larsen oscillations
~5 from being produced, but it may be usefulg for other
reasons~ to effect a manual gain change-over9 or a
change-over controlled by the speech signal. For example~
when the "handsfree" s~ is operated in a very noisy
environment it may be important to increas& the gain
30 in the receiving link in order to increase the listening
comfort~ and to decrease the gain in the transmission link9
even if this may make it necessary to speak louder or
closer to the microphone. This gain change-over may be made
operati~e manually or controlled directly by the speech.
35 In this second hypothetical case, when the speaker does not
speak before the ''hands free" set, the gain in the recei-
~ing link must be increased9 which increases the listening
g8~
PHF 81.543C ~ 6 23 3.1982
comfort, and the gain in the transmission link must be
decreased, ~-hich also prevents the ambient noise of the
localit~- in which the "hands free" set is placed from
being heard comparative].y loud. In contrast therewith9 ', s ~-hen a speaker spec~s i~ltO the "hands free" set~ the gain
in the transmission and receiving li~cs may have the t
- nominal values again.
It has surprisingly been found that the arrange-
ment in accordance with the invention renders it possible
10 ro emplo~ an extremely simple means to effect these gain--
change-overs in the transmission and r0ceiving paths of a
relephone set.
In accordance ~-ith a first alternative~ in a
relephone set incorporating the arrangement in accordance
15 ~-ith the invention a~d comprising in the ~irst path a
~riable gain_amplifier contro~ed by the linear regulator
and arranged in the receiving path, a further variable-
gain amplifier having for its object to changeover the gain
in the two paths is pro~ided in the receiving path bet~een
20 the microphone and the inpu-t terminal which is jointly
used b~ the two distinct parts of the first and second
paths~ the said means provided to ensure that the gain in
the seco~d path remains higher than the gain in the
first path being adjusted in such a marn~r that the electro-
25 acoustic loop which is closed by the second path will
perma~entl~ be the source of unannoying oscillations of a
lo~ amplitude~ a variation caused by the gain of the said
further variable-gain amplifier determining a gain
~riation in the opposite sense to that in the receiving
30 path.
In accordance with another alternative~ if
~he variable-gain amplifier which is included in the first
pa.h and is controlled by the linear regulator is provi~
ded in the transmission path, the variable-gain amplifier
ch has for its object to change over the gain is
p-o~-ided in the receiving path between the loud-speaker
and ~he output terminal which is used jointly b~ the
PHF ~1~543C 7 23~3.~9~2
distinct parts of the first and second paths.
In the ~ariants of the arrangement in accor-
dance ~-ith the in~ention which emplo~- a ~-ariable-gain
amplifier which is controlled by the linear regulator
5 and is used ~ointly b~ the first and second paths 9
accurately complementary gain ~riations are obtained in
the transmission and recei~-ing paths by controlling a gain
variation only in the amplifier which has for its object
to change over the gain and which is situated either in
10 the transmission path or in the receiving path. ,;
In the two alternatives for the gain change-o~-er
system it is possible to control the gain ~-ariation of ~he
gain change-o~er amplifier to effect an automatic gain
change-o~-er in the two paths by employing a detector for
15 detecting the speech signal from the microphone.
The following description given by way of
e~ample with reference to the accompanying drawings will
make it better understood how the invention can be pui
into effect.
Figure 1 sho~s the arrangement in accordance
with the inYention used in an loudspeaking telephone set.
Figure 2 shows the characteristic of the
r~gulator employed in the arrangement in accordance
with the invention.
Figure 3 shows the diagrams 3a to 3f corTespon-
ding to the variations in the arrangements in accordan~e
~ith the in~ention.
Figure 4 shows the arrangement in accordance
with the invention used in a control system.
Fi~ure 5 shows an embodiment of the regulator
and the two ~ariable-gain amplifiers 9 controlled b~ the
regulator.
Figure 6 shows the arrangement of Figure 1,
additionall~- pro~-ided with means in accordance wi~h the
3~in~-enrion for changing o~-er the gains in the two paths o~
a telephone set.
1,
89~
PHF 81.5~3C 8 23.3 1982
Figure 1 sho~-s an electro-acousticalloop,
formed, for example, by a loudspeaking telephone set.
~aid telepho.ne set comprises a coupling circuit 1
~-hich couples the telephone line 2 to the transmission
path 3 of the set which i5 provided with the microphone
4 and an amplifier 5, and also the receiving path 6 of
- the set which is provided with the receiver amplifier
~ and the loudspeaker 8. Between the loudspeaker 8
and the microphone 4 there is a certain degree of
lO acoustic coupling, which in particular depends on the
distance between these transducer~ and their orientation
and which may vary in telephone sets of the type usually
referred to as "amplified reception", the microphone of
~-hich, ~hlch forms a fixed part of the combination, has an
15 indeterminate position with respect to the loudspeaker.
This c~upling may be characterized by a coupling
coefficient ~ ~ being the attenuation of the acoustic
po~-er transmitted by the loudspeaker and reaching the
microphone.
Moreover, due to inevitable imperfections in
the circuit 1~ the signal appearing in the transmission
path 3 of the set is not wholly retransmitted over the
telephone line 2 and a fraction of this signal is found
in the receiving path 6 of the setO A further inadvertent
~5 coupling be-tween the transmission and receiving paths of
the local set shown in the Figure may also be produ~sd
by the path incorporati~g the telephone line 2~ ~hich is
connected to a remote loud-speaker in which the~e also
exists an acoustic coupling between the loudspeaker and
30 the microphone r Whatever the origin of the inadvertent
coupling between the transmitting path 3 and the receiving
path 6 of the telephone s~t it is possible to establish a
device 9 arranged bet~-een the transmission and receiving
paths of the telephone set~ and having a defined gain G1
3v ac ~he standard of rhe rario bet~-een the signal appearing
on rhe transmission path 3 and the signal appearing on
the receiving path 6 of the setO No change is made to
~ 8
PHF gl.543C 9 23.3.1982
the generality of the definition of said device 9 if
a circuit 10t for example an amplifier, is arranged in
series with the coupling circuit 1, the gain &,l then
including the ga.in of said circuit 10.
The acoustic coupling between the lo~speaker
~ and the microphone 4 and the unwanted coupling pro~
duced in the device 9 between the transmission path 3 and
the receiving path 6 of the telephone set forms an
electro-acoustic loop in which oscillations may come into
being if the gain of the loop is equal to or higher
than unity. The fr0quency of these oscillations is not
accurately known and ls situated in the passband of the
components ~hich are included in the loop, that is to sa~
approximately 300-4000 H~, The ampli*ude of these oscil~
lations is only limited by the taturation of the
components of the loop.
In order to avoid these unwan~d loop oscilla
tions which render the use of the telephone set substan- ¦
tially impossible, it is known to include a variable-
gain amplifier 11 in the receiving path 6 of the set and
to control this gain g1 from the envelope signal of the
signals of the transmission and receiving pathsO This
procedure of controlling the gain is not adequate for a
hi,~h acoustic coupling and causes t,he.gain g1 to depend
on the useful speech signals.
The invention proceeds in a quite different
way9 rendering it possible to avoid these disadvantages
According to the invention~ connected to the terminals
A and B of a first path C1 of the loop incorporating the
device 9 and the variable~gain ampl~ier 11 there is
a second path C2 incorporating a further variable-gain
amplifier 12, The gain g2 of the amplifier 1~ is
controlled by a linear regulator 13 which receives as
its input the signal W produced in the second path at
the output o~ the amplifier 12 and which produces a gain-
control signal Vg2 in such a way that the level of the
signal W is kept constant beyond a certain value of the
PH~ 81.543C 10 23.3.1982
input signal x of the amplifier 12.
Figure 2 sho~s the characteristic of the output
signal 1; as a f~ction of the input signal x of the ampli-
fier 12 ~hich is so controlled. Up to a value x of
the input signal x~ for which the output signal has a
~-alue W0, the gain g2 of the amplifier is constant
- and equal to a maximum value g2M = ~ which ls
o
deiermined by the slope of the line OC. Beyond xO the level
10 Of the output signal W remains constant and equal to W .
For any arbitrary value x of the input signal the
operating point of the amplifier est~blishe~ itself at M
and the gain of the amplifier g2 = decreases 1~hen x
increases, The important fact is stressed here that with
15 the linear regulator l3 associated with the amplifier 12,
the last-mentioned amplifier does not introduce any non-
linearity in the second path.
The amplifier 11 in the first path C1 is
controlled by a gain-control signal vg1 supplied by
20 the regulator 13 in such a way that its gain g1 responds
to the gain g2 of the amplifier 12 in the second path C2.
~t each instant the gain g2 Qf the amplifier 11 may be
equal or proportional to the gain g2 of the amplifier 120
If the two amplifiers 11 and 12 are of the same construc~
25 tion~ the t~o control signals vg1 and vg2 may be equal and
ma~ be obtained from the same output of the regulator as
sho~ in the Figure. Fin~lly~ means are provided in the
one and/or in the other one of the two paths 50 as to
ensure that the gain in the second path C2 remains
30 higher than the gain in the first path C1 in the overall
fre~uency band where the oscillations are likel~ to occurO
Tn Figure 1, these means are represented by the amplifier
1~ ha~ing a gain G2~ which is arranged in the second path
C~ before the variable-gain amplifier 12 and is preferably
~, _ 1
~ accociated ~-ith an attenuator 15 having a gain -A ~ which
is connected bet~-een the output of the amplifier 12 and
rhe outtput rerminal B which is used jointly by the two
PH~ 810543 ~ 11 23~3.1982 t
paths. The abo~-e-described notations being used, this
gain condition in the t~;o paths ma~ be ~-ritten aâ: ¦
~ t:
G2 . A2 ~ g2 > Gl gl ( 1 )
It should be noted here that this condition
(1) must be realized for every possible frequency of the
loop oscillations t that is to say~ in the e~ample chosen
here, for ever~r ~requency in the band 300-4000 Hz~ r
In the event that the two amplifiers 11 and 12
are identical and controlled by the same signals,
condition (1) beco~nes:
G2 ~ 2 > G1
lS To explain how the system of the invention
operates and to demonstrate that it is possible to avoid
annoying loop oscillations therewith it is advantageous
to define the gain G of an electro-acoustic circuit ~-hich
is enclosed by the frame 6 and comprises the loudspeaker
8 and its amplifier 7, the microphone 4 and its a~plifier
5, a~d finally the acoustic path between the loudspeaker
and the microphone~ defined by the coupling coefficient ~ .
This gain G may be defined as the coefficien~ ¦
of the ratio between the signal v applied to the ampli- ¦
fier 7 and the signal u produced by the amplifier 5.
Several quantities may be defined to e~press said gai~ Go
Th~ acoustic power up to the output of the
loudspeaker is denoted ~ and the acoustic po~er just
be~ind the microphone is denoted q. Obviously it is then
apparent that:
P ~ 2
p
To introduce the transduction factor of the loudspeaker
it is possible to define a nominal acoustic po~;er p
cupplied by the loudspeaker 8 and ~;hich ~ould be producec
b~- a signal ~- at the input. of the amplifier ,. The
transduction factor of the assembly formed b~- the amplifier
98~
PHF 81.543 C ~ 12 2303.1982
7 and the loudspeaker 8 is ~ v ~ In the range iIl
~-hich said amplifier-loudspeaker assembly is linear~ it
mav be written that:
[ 0] (3)
- To introduce the transduction factor of the microphone
it is possible to define an acoustic power qO measured ,.
just before the microphone and produced by a speaker
lO ~-hich produces an acoustic power Kp (K being a constant)
and situated at a nominal distance characterized by a
coupling coefficient ~1 , A signal u at the output of
he amplifier 5 corresponds to this acoustic power qO -
~O The transduction factor of the assembly formed by
the microphone 4 and the amplifier 5 is ~ ~ = [K
In this range in which said microphone~amplifier assemblyis linear, it can be written that:
[ Kpo ~ o ] (4)
Taking account of the relations (2)~ (3) and
(4) the gain G of the electro-acoustic circuit 16 can
be brought to the form~ ¦
p u
u o o
G = ~ ~ ~ vO ~
Sin~e, in a~cordance with the oondition (1)
to be satis~ied7 *he gain in the second path C2 between
3~ A and B is higher than the gain in the first path C1
oscillations can only be present in the auxiliary loop
formed by the electro~acoustic circuit 16 and the second
parh C2 and cannot establish themselves in the main loop
formed by the circuit 16 and the first path C1 . Let it
first be ass~med that in said auxiliary loop ~here is no
speech signal produced b~r the microphone 4 or coming from
the telephone line 2. There are no oscillations if the
~9~9~ -
P~ 81.543 c 13 2303. 1982
i.
gain in this auxiliary loop is belo~ it~-~ that is to
sa~ if: 1
G . G2 . ~2 ~ g2 <-1 (6)
In this inequality, the gain g2 of the amplifier 12 must
be given its maximum value g2M~ deflned in the ~oregoing.
As in accordance ~ith formula (5) the gain G is propor- !
tional to the coupling coe~ficient ~ between the
loudspeaker and the microphone ? the inequality (6) simpl~r
confirms that there are no oscillations if said coupling
coefficient is lower than a predetermined ~alue.
If the coupling coef~icient between the loud- f
speaker and the microphone increases~ the gain G increase~
and abo~e a predetermined value of the coupling coefficient
the inequality (6) is no longer satisfied. This results
in oscillations starting in the the auxiliary loop formed
b~ the circuit 16 and the second path C2. The linear
regulator 13 causes a decrease of the gain g2 of the
ampli~ier 12 in such a way that the amplitude of these
20 oscillations at the output of the said ampl fier remains
limited to the value WO. The output signal o of the
attenuator circuit l5 then has a value which is s~ffi-
ciently low to ensure that all the components of the
loop, particularly the ampli~iers 5 and 7 of the circuit -¦
16, operate in the line~r mode. For this oscillating mode
the total gain of the loop establishes itself at the
~alue t, that is to say it can be written that:
G . G2 . A2 g2 ~ (7
Since the components o~ the circuit 16 operate
ln ~he linear mode, the formula (5) which furnishes
the gain G may be employed and by combining the formulae
(5) and (7) it is easily obtained that:
g2 ~ ~G~ ' uO ~ ~8)
This formula (8) sho~rs that ~or this oscillating
mode in the auxiliary loop formed by the circuit 16 and
~ 9 ~ ~
PHF 81.543 C ~ 14 23 3 1g~2
the second path C2 the gain G2 of the amplifier 12
establishes itself at a value which is proportional to
the acoustic attenuatio~ :~actor ~ bet~v-een the loud- j
spea~er and the nlicrophone~ which itself is substalltially proportional to the distance between the two transducers.
The gain of the amplifier ~11 in the first path,
~-hich responds to the gain g2 establishes itself there
fore automatically also at a value which is proportional
to the acoustic attenuation factor ~ ~ that is to say
l substantially proportiona]. to the distance bettveen the
loudspeaker and the microphoneO This gain g1 varies
in the same sense as the gain g2~ whic}l renders it possLble
to satisf~ the conditioll (1) in all circumstances. This
condition (1) being satisfied~ the total gain in the main
lS loop formed by the circuit 16 closed by the first path
Cl always remains below lln;ty~ so ~at this first path Cl
does not contribute to the production ofbigh-amplitude and
anno~ing Larsen oscillations at the terminals of the
electro-acoustic circuit 16.
Thus, with the device in accordance with the
in~ention, when the acoustic attenuation factor becomes
sufficiently low to create oscillating conditions in the
loop these oscillations are forced to pass through the
second path C2, particularly because the gain g1 of the
25 amplifier 11 of the first path C1 responds to the gain
g2 of the amplifier 12 of the second path. In the seço~d
path the loop oscillations are controlled in such a way
that they are not annoying in the electro-acoustic circuit
16, that is to say they are inaudible and do not saturate
30 the amplifiers 7 and 5; thus) for a given maximum amplitu-
de W of the oscillations at the output of the amplifier
12 it is always possible to increase the attenuation
factor A2 of the circuit 15 so as to ensure that the
amplitude of the oscillations at the input of the circuit
~ ~ is extremel~- small and not annoying~ the gain G2 of
.he circuit 14 being increased in a corresponding ~a~- to
sa~sf~ the condition (1) The signal produced at the output
PHf 81.543 C 15 23~3~1982
terminal B b~- the oscillations ci~culating in the
second path C~ has the fi~ed value ~rL = WO
Let no~ the case be considered ln which the
first path C1 and the electro-acoustic circuit 16 are
the source of the usef~l speech signals. Let it first t
be ass~ed that the speech signal produced by the micro- s
phone 4 is totally directed, starring from point A~ along
the first path A1. Loop oscillations may then come into
e~istence in the-second path C2, e~actl~ the same as des-
cribed in the foregoing9 without being affected by the
speech signal coming from the microphone. At point B~ ,~
these loop oscillations have a very small amplitude and
superpose themselves on the speech signal coming from the
telephone line 2 v~a the output of the first path C1.
A listener in front of the loudspeaker 8 will not
observe these loop oscillations of a very small amplitude.
In the h~pothetical case posed above t the gain g2 f
the amplifier 12 and consequently gl of the amplifier 11
20 of the first path are not influenced by the speech signal
coming ~rom the microphone 4 and only depend on the
acousti~ coupling coefficient ~ .
Actually~ if no p~ecautions are ta~en9 th~re
is a risk that a fraction of the signal coming from the t
25 microphone 4 is applied to the second path C2 and
disturbs the regulation of the amplitude of the loop
osci~ations by means of the regulator 13. In that event~
the gain g2 of the amplifier 12 and consequently the
gain g1 of the amplifier 11 will not only depend on the ',
30 acoustic coupling coefficient, but also on the amplitude
of the speech signal coming from microphone 4O A means
to avoid this disadvantage is to provide a filter 17
which is indicated by means of dotted lines in the Figure
in the second path C2 before the amplifier 12. The gain
35 G2 in the second path then includes the gain of the ampli-
fier 14 and the filter 17.
If, in the first path C1, the gain for the
frequencies above, for example, 3OOO Hz is higher than
~ ~ ~9~ ~ 8 ~
PHP 81.5~3C 16 23.3,1982
the gain for the frequencies~ower than 3OOO Hz, it is
possible to use a high-pass filter as the filter 17,
~;hich high-pass filter lncreases the gain in the second
path for the frequencies higher than 3OOO Mz. The
condition (1) is thus easily satisfied and the speech
signals affect the regulation procedure of the loop
- oscillations to a lesser extent. These loop oscillations
occur at a frequency above 3OOO Hz but still poorly
defined, ~hich is a disadvantage.
It would be more advantageous if the filter 17
~ere a bandpass filter having a very narro~ passband~
~is filter selectively increases the gain G2 in its pass-
band and establishes an adequate phase shi~t so as to
ensure that the gain in the auxiliary loop formed by the
circuit 16 and the second path C2 may take a positive
value in the said passband and that the oscillations in this
au~iliary loop establish themselves substantially at the
c~ntral~requency of the said passband~ independent of
the acoustic coupling between the loudspeaker and the
20 microphone; at the same time the speech signals in the
second path have a ver~r low level compared with the lo~p
oscillations and do ~ot substantially disturb the regula
tion of the amplitude of~hese oscillations.
It should here be noted that ~ith a filter 17
25 having a ~arro~ passband it becomes much easier to
satisfy the ~ndition (1) which ensures that the loop
oscillation~ pass only ~ia the second path C2~ To satisfy
this condition it is sufficient that the gain of the
seoond path fGr the central frequency of the narrow band
30 of the filtsr 17 is superior to the gain provided by the
first path for all the frequencies of the band of said
first path (for example 3OO-4OOO Hz). It becomes still
easier to satisfy this condition (1) by providing a filter
18, ~-hich is represented by the dotted line in the first
~ pa~h, increasing the gain in the same narro~- passband as
~he filter 17. The gain G1 in the first path then includes
PHF 81 o543C 17 23~30 1982
the gain of the device 9 and the filter 18. This
filter 18 selectively increases the gain G1 in its
passband, so that the gain of the first path C1 provides
a maximum value in said narrow passband of the filter t
5 18, To satisfy the condition (1) and consequently to
ensure that the loop oscillations pass only along the
second path C2 it is then sufficient that the gain of
the second path C2 be higher than the gain of the *irst
path C1 for the common central ~requencies of the pass-
lO bands of the filters 17 and 18 ~ It will be seen here
that the filter 18 causes substantially no perturbations
in the speech signals~ because of its narrow band. It
will be ob~rious that the amplifier 14 and the filter 17 on
the one hand and the arnplifier 10 and the filter 18 on the
15 other hand may be in the form o~ two selective ampli~iers,
~inally~ instead of employing a ~ilter 18 in the first
path C1 it would alternatively be possible to employ a
filter 19, which is represented by dotted lines and is
provided in the path between the output of the amplifier
20 5 and the input terminal A which is used jointly by the
two paths. ~s said filter 19 increases the gain in the
same narrow passband as the filter 17~ the condition
(1) is of necessity satisfied automatically arld the
loop oscillations at the central frequencies of the two
25 filters 17 and 18 pass via the second path~ ~inally~ it
will be obvious that in the above-described variant in
which the filter 17 o~ the second path is ~a high-pass
filter which increases the gain G2 for the frequencies
higher than for example 3OOO Hz, it is alternatively
30 possible to employ a filter 18 or a high-pass filter 19,
but with a small attenuation difference between the
transmitted high-frequency band and the attenuated lo~-~
~requency band, in order to prevent the speech signals
from being distorted to a consi~rable e~tent.
Several different variations of the arrangemen~
in accordance with the invention are possible which will
now be described with reference to Figure 1 which shows
PHF 81~543C 18 23.301982
circuit diagrams corresponding to some of the possible
~-ariations. In these circuit diagrams the components
sho~ .in ~he circult diagram of ~igure l ~hich are
essential and sufficient for an ~mde.rstanding of the
description are given the same reference numer~s, namel~
the electro-acoustic circuit 16 having a gain G, the
de~-ice 9 having a gain G1 proYided in the first path C1,
the device 14 having a gain G2 and the attenuator circ~t
ha~-ing a gain ~ provided in the second path C2 and
~
finall~- the linear regulator 13. Also the ampli~iers 11 and
12 ha~ing the respective gain g1 and g2 ma~r be used,
as ~ill be described herei~lafter. For the sake of` simplici-
r~- ~he f`ilters 17~ ~8~ 19 which are shown bv mealls of
dotted lines in Figure 1 have been omitted.
In the arrangement shown in ~igure 3a, the
t~-o paths C1 and C2 connected between the terminals A and
B are absolutely separatey as in the arrangement shown in
the Figure 1. The difference between the arrangement sho~n
in the Figure 3a and the arrangement of Figure 1 is that
the variable-gain amplifier of the first path is provid0d
bet~ee~ the input termina7 A which is ueed jcintly by the
t~o paths and the de~ice 9 9 instead of being provided
bet~e~n the device 9 and the common output terminal B. In
its ne~ positiony this variable~gain amplifier of the
first path is glven the re~rence numeral 11' and has a
gain g910 It is controlled in exactly the same manner as
the amplifier 11 of the ~igure 1. As regards the control
of the Larsen oscillations, everything described for the
arrangement of ~igure 1 is also wholly valid for the
arrangement show~ in ~igure 3a, In particular, the
Larsen oscillations are limited to a small and unan~o~ing
amplitude ha~ing at the terminal B the fixed value
~-L = A 9 ~ being, as explained ~-ith reference to
~~ i--~re 2, rhe maximum fixed ~alue of the signal ~
applied b~- Ihe ~-ariable-gain amplifier 12 to the input of
Ihe linear regulator 130
PHF 8~ 543C 19 23.3.1982
In a further variation, not shown, the arrange~
ments shown in Figures 1 and 3a can be combined by emplo~-ing
t~o variable-gain amplifiers 11 and 1ll provided in the
first path on both sides of the device 9 and both con-
trolled by the signal processed by the regulator 13~
In the arrangement shown in Figure 3b the
- two paths C1 and C2 connected between the terminals A and
B are not absolute separate. The circuit diagram of Figure
3b differs from the circuit arrangement shown in the Figure
10 3a in that the input of the device 14 o~ the second path is
not connected to terminal A but to the output of the
uariable-gain amplifier 11 ' . When the terminal ~ is always
considered to be the input terminal which is used Jointly
by the two paths C1 and C2 the ampllfier 1l~ is used
15 ~ointly by these two paths~ while after the output of
the amplifier 11 I to the terminal B the two paihs have
separate parts. If measures are taken to ensure that in
the separate parts the second path has a gain which is
higher than the gain of the ~irst path it is obtained in
20 this arrangement 3b that the Larsen oscillations are only
produced in the part of the second path which is separate
from the first path and that the amplitude ~f these
oscillations is small and unannoying, being limited at the
terminal B to the fixed value vL = o . In a variation, not
25 shown~ of the arrangement of Figure 2 3b,the linear
regulator 13 may control, in addition to the variable-gain
amplifiers 12 and 119~ a variable-gain amplifier7 not
shown~ which is provided between the output of the deu~e
9 and the terminal Bo
The arrangement of ~igure 3c may be considered
to be a further improvement of the arrangement of
Figure 3b as it is possible to do without one variable gain
amplifier. It is different in that the amplifier 12 of the
second path is no longer controlled b~ the regulator 137
35 and consequentl~- has a fixed gain g2 In the arrangement
of Figure 3c 9 taking account of the fact that the amplifier
1~' having the gain g'1 ~orms part of the first as well as
of the second paths between the terminals A.and B~ it can
g84~
PH~ 81.543C - 20 23.3.1982
be said that the regulator 13 acts on the gain g'l of
rhe amplifier 11 ' for keeping the signal W ~ormed in the
second path at the output of the amplifier li~c 11l, 14, 12
co~stant and equal to WO~ which signal is to be applied
5 to the input of the regulator 13. The amplitude of the
oscillations on the terminal B is still limited to a lo~-
- and fi~ed ~-alue o . In a variation9 not showny of the
~ 2
arrangement of ~igure 3c the regulator 13 can inter alia
10 con~rol a ~-ariable-gain amplifier provided between the
output of the de~-ice g and the terminal B,
In the arrangement sho~l in ~igure 3d the t~o
paths C1 and C2 connected between the two terminals ~ and
B are aiso not absolute separate, 1~hen the arrangen~ent oI`
15 Figure 3d is compared with the arrange~nent of Figure 1 i-t
~ill be seen that they differ in that the attenuator circuit
ha~ing a gain ~ is connected between the output of the
device 14 and t~e input of the variable gain amplifier 11.
When the terminal B is still considered to be the output
20 terminal which is used jointly by the two paths C1 and C2,
these two paths ha~e the amplifier 11 in commonO
The separate parts of the two p~hs comp~ising the arrange-
ment 9 for the first path C1 and the cascade arrangement of
the de~ice 14 and the attenuator circuit 15 for the second
25path C2 are connected between the terminal A and the i~put
of the amplifier 11. It should be noted that in this
arr~ugement of Pigure 3d the amplifier 12 is ~ot incorpora-
t~d in the second path, A signal is ~ormed in that part of
the second path which is separate from the first path~ in
30a place situated between the device ~4 and the attenua-tor
circuit 15 and this signal is applied to the input of the
regulator 13 by means of the variable-gain amplifier 12~
This regulator 13 controls the gain g2 of the amplifier 12
and the gain g1 of the amplifier 1 in such a manner that the
3--ig-al ~~ coming from amplifier 12 and applied to the input
o-^ .he regula~or 13 is kept at a constant ~-alue 1; . It
can be easil~- demonstrated that on the terminal B the
PHF ~.543C 21 23.3.1982
Larsen oscillations have an amplitude which is limitecl
to the value vL = o gl which is a fixed value because
' ~ g2
the gains gl and g2 of the ~mplifiers 1l and 12 are
controlled by the same signal. Particul~rl~ when the
amplifiers are identical, g1 ~ g2 and the limit value o~ !
-the amplitude of the Larsen oscillations is vL = o
In a variation, not shown~ of the arrangement of Figure 3d
the regulator 13-may control inter alia a variable-gain
ampli~ier which must then be provided between the terminal
A and the input of the device 9.
The arrangement shown in Figure 3e i9 a further
improvement of the arrangement shown in Figure 3d, an
improvement which is comparable to the improvement effec-
ted in the arrangement of Figure 3c with respect to the
arrangement shown in Figure 3b. The arrangement 3e
differs from the arrangement 3d in that the gain g2 of
the amplifier 12 is no longer controlled by the regulator
20 13 and consequently remains fixed. The r~gulator 13 acts
on the gain g1 of the amplifier 11 which is assumed to be
part of the second path, so that the signal ~ormed in the
second path bet~een the device 14 and the attenuator ci~cuit
15 and amplified thereafter in the arnplifier 12 having a
25 fixed gain g2 9 is applied to the input of the regulator
with a fixed amplitude WO. In the arrangement of Figure 3e
the amplitudes of the Larsen oscillations on the terminal
B ha~e the value vL = o g1 ~ But 9 in contrast with the
~ariations described in2 g2 the foregoing, this value vL
30 is vuria~le and-the gain g2 is fixed. It is nevertheless
always possible to choose an attenuation coefficient A2
and a gain g2 in such a manner that the variable amplitude
of the Larsen oscillations remains low and non-annoying~
As the amplifier 11 having the variable gain g1 i.s located
35 in the path of the useful speech signals which come frorn
the telephone line and are to be applied to the loudspeaker
incorporated in the electro~acoustic device 16,it can be
said that the variation shown in Figure 3e makes it
possible to obtain on the terminal B a constant ratio
PHF ~1.543C 22 23.3.1982
ber~een the useful speech signal and the noise generated
b~- the unannc)~ring L,arsen oscil:Lations produced b~- means
of the second path, In a ~ariation, not S}10~, of the
arr~ngement of Figure 3e the regulator 13 might inter
alia control a variable~gain a~plifier provided between
the terminal A and the device 9~
The arrangement of Figure 3f dlffers from the
arrangcment of Figure 3e in that an amplifier 11 t having
a variable gain g~1, controlled b~ the regulator l3t is
provided bet~een the terminal A and the arrangement 9 and
furthermore in that the input of` the device ~4 is co~nec- j
ted between the amplifier 1l' and the device 9. Thus~ in
this arrangement of Figure 3f the variable-gain
ampli~ers 11' and 'I 1~ which are connected to the
terminal A and the terminal B7 respectively, are used
jointly by the two paths C1 and C2 and arc controlled
by the same signal supplied by the regulator 13O The
separate parts of the two paths comprise the same elements
as the ~rrangement shown in Figure 3e and the regulator 13
has its input connected in the same manner to the secorLd
path3 by means of the amplifier 12 having a fixed gain g20
As in the arrangement shown in Figure 3f~ the amplitude
of the Larsen oscillations on the terminal B has the value
~ g
~ A g1 which varies in the same way as g1
It can be seen that in the arrangements of
Figures 3c~ 3e and 3f where an amplifier 12 having a
fi~ed gain g2 is described9 it is possible to omit this
amp1ifier completely, if a gain g2 equal to unity is
sufficient for a correct opera-tion of the arrangements,
In the arrangements shown in Figures 3a to 3f
it is advantageous to provide filters having the same
characteristics and the same functions as the filters
1,, 18 and 19 in the arrangement sho~n in Figure 1 D A
fil~er ha~-ing the function of the filter 17 would then be
p-o~-ided in that part of the second path ~-hich is separate
from the first pathl A filter having the function of the
filter 18 ~ould then be provided in that part of the
.g~
P~ 81.543C 23 23,30 982
first path whlch is separate from the second path, a
filter ha~ing the function of the filter 19 would then
be di~posed in the par-ts which are common to the main
and -the auxiliarr loops~ ~
In the foregoing the use of the invention for t
an electro~acoustic system which is inadvertently looped ~!
by acoustic and/or electric coupling is described. How-
ever~ the invention is suitable for use in a more general
way in all systems in which a loop may be formed in which
lO uncontrolled oscillations of a high amplitude may be
produced~ So the invention may be used in all types of
control systems~ as shown in Figure 4.
Figure 4 shows the well known circuit diagram
of a control system intended to have any output quantity
5 S respond to an electric input signal E. To make this
more readily understood the same re~erences and the same ~-
notations are used as in Figure 1 to indicate the corres-
ponding components and quantities. The input signal E is
applied ~ia the point A to the forward path of the contrDl
20 system9 comprising the cascade arrangement of the device 9
ha~ing a gain Gl~ the variable-gain amplifier ll and
finally the device 20 which supplies the output quantity
S. The feedback path of the control system is connected
between the output of the de~ice 20 and the point Ao T~is
25 feedback path is represented by the device 16 which has a
transfer function modulus G. In response to the output
quantity S the device 16 produces an electric signal u
which is added with a suitable phase to the input signal
Eo During adjustment of this control system or in abnormal
30 circumstances during its operation it may happen that
un~anted oscillations arise in the loop formed by the
forward path and the feedback path and that the output
quantity S attains uncontrolled and dangerously high
amplitudes.
The invention renders it possible to a~oid loop
oscillations of this typeO By employing, for example, the
embodiment shoun in ~igure '~ ~he invention lS utilized bv
84
PHF. 81.543C 24
connecting a second path C2 which comprises in accordance
with one of the variations of Figure 1 a filter 17 having
a narrow passband and an amplifier 14 which together have
a gain G2, an amplifier 12 having a variable gain G2 and
an attenua-tor 15 having a gain 12 to the terminals A and
B of a first path Cl of the control. loop which comprises
the components 9 and 11. The linear regulator 13 re~u-
lates the output signal of the amplifier 12 and renders it
possible to have the gain gl of the amp.li~ier 11 respond -to
the gain g2 of the amplifier 12. Everything which has been
described for the electro-acoustic system shown in Figure 1
remains valid. Particularly, the loop oscillations which
may come into being are passed along the second path C2 and
in said second path their amplitude is controlled by the
regulator 13. Thanks to the attenuator circuit 15 the
amplitude of these oscillations can be brought to a very
low, unannoying yalue at the output S of the control
system.
An embodiment of the assembly formed by the
variable-gain amplifier 12:and its regulator 13, and by
the variable-gain~amplifier 11 is shown in Figure 5.
In accordance with Figure 5, the ampliier 12
which is provided in the second path C2 comprises a npn
transistor 22 the emitter of which is connected to a
negative supply terminal which serves as a reference, the
collector of which receives the input signal x of the
amplifier via the series arrangement of the resistor 23
and the capacitor 24 and the base of which is connected to
the output of an integrating circuit 25. This integrating
circuit 25 receives the pulse-shaped signal Pc formed in
the regulatox 13, as w.ill be described hereinafter. The
transistor 22 is the adjustable component of the amplifier
12. Actually, on the terminal 26 between the resistor 23
. - '
,~,
g~
PHF 81o543c 25 23~3~1982 t
and the capacitor 24 a voltage is obtained which is a
variable ~raction of the input signal x of the amplifier,
as its level depends on the more or less conductive state ,'
of the emitter-collector path of the transistor 22 and
5 consequently on the output voltage of the integrating
circuit 25~ It can be easily seen that on the terminal 26
an increase of the output voltage of the integrating
circuit 25 corresponds to a decrease of the level of
the signal which is available on terminal 26 and conversely,
10 This variable-level signal available on terminal 25 is,
for example~ a current im which is applied to a fixed
gain amplifier circuit 27~ At its output 28 the circuit
27 produces a var~le current Im ~hich i5 identical to
the output signal W of the variable~gain ampli~ier 12. At
15 its output 29 the circuit 27 produces a current Io ~ Im9
wherein Io is a direct voltage having a constant amplitude~
In the regulator 13 the current Io ~ Im is applied to a
pulse-width modulator 30, The latter further receives
clock pulses from the clock generator 31 and produces a
2n signal Pm ~ormed by pulses the width of which is modulated
by the current Io + Im~ The techniques of forming pulse-
width modulated pulses are well known, It is useful to
state here that the clock pulses are9 in particular~ used
to sample the input current of the modulator~
5 When9 at the sampling instants 9 the variable portion Im f
this current is zero 9 the pulses of the signal P have a
width PO~ Depending on whether the variable curre~t Im is
positive or negative at the sampling instants, the pulses
o~ the signal Pm have a width which is larger or smaller
than PO. Pulses having a width P ~ P 1 and PO - Pm~ 9
respectively correspond to a predetermined level of the
variable current I for ~hich this current may take
the value I 1 and -I ~. The modulated signal Pm is applied
to a pulse-length detector 32, which produces a pulse-
shaped signal P formed b~- pulses which are commonly
referred to as compression pulses 9 which have a fixed
PH~ 8 ~ . ~43C 26 23 . 3 . ~ 982 r
duration and are produced each time the pulses of
the signal P reach the values P + P 1, The pulse shaped r
cignal P is applied to the integrating circuit 25 which t
produces a vo~tage whi.ch is representative of the pulse
5 repetition rate of the compression pulses. ~asically, said
integrating circuit comprises a capacitor which is charged
- b~- a constant current during the duration of the compres-
sion pulses and discharged by a current which is lower
thar the charging current~ In this ma~ler~ when compression
10 pulses appear ~hich indicate that the level o.f the current
Im is e.xceeded, the voltage produced by the integrating
circuit 25 increa.ses, which makes the transistor 22 more
conductive and produces an attenuation o.f the current
le~-el Im, Finall~ the voltage produced by the integrating
li circuit 25 stabilizes around an average value which
determines a constant level of the current Im. The amplifier
12, ~-hich is associated with the r0gulator 13~ in accor-
dance with the embodiment described in the foregoing, has
precisely the required characte~istic shown in Figure 2~
20 while the constant le~el ~O indicated in said Figure becomes
identical to the constant level at which the current Im
is established~
The variable-gain amplifier 11 incorporated
in the first path C1 is of the same construction as ampli
25 fier 12, it comprises the same compone~ts designated by
the same references9 but provided with ~n accent notation.
Said amplifier 11 is controlled by the pulse-shaped signal
Pc which is formed in the regulat~r 13 as described in the
foregoingO The transistor 22l9 which is the variable compo
30 nent of the amplifier 11, is controlled by the sa~le signal
as the transistor 22 which is the variable component of the
amplifier 12. If the components of the two amplifiers, par-
r_cularly the transistors 22 and 22', are arranged pair-
~ise in a suitable manner, the ~ain of the amplifier 1' is
~~ a~.omatically adjusted at each instant to the same value
as .he gain of the amplifier 12.
Since the amplifier 11 has for its object to
9~38~
PX~ 81.543C 27 23.3 1982
process the useful signal (for e~ample the speech signal),
and thc ampli-fier 12 to process the oscillation signal
of the lltp~ it mayb useful to give the integrating
circuits 25 and 25' different time constants~ adapted to $
5 the signals processed by said amplifier On the other hand,
by having the pair-wise arranged transistors 22 and ~2'
controlled by the same signal, it may be advantageous $o
give the fi~ed amplifiers 27 and 27' different gains, the
gains of the amplifiers 11 and l2 remaining in a constani
lO rario. ~inall~, it will be clear that one sing~e integra-
ting circuit may perform the fnnction of the two integra-
ting circuits 25 and 25~ having the same time constant.
The circuit diagram sho~n in ~igure 5 is a
suitable CiI`CUit diagram to realize the assembly of the
15 amplifiers ~1 and 12 ~nd the regulator 13 in the arrange-
ment of Figure 1. But it will be obvious that with the
same embodiment, the regulator 13 can control two variable-
gain amplifiers which are connected in a different ~anner,
or to co~trol only one of these two amplifiers to reali~e
20 the several variations shown in Figure 3.
The present invention9 when applied in a
telephone set of the ~'hands free't type to avoid unanno~ing
Larsen oscillations makes it also possible to realize 9
in a very simple way~ a further very useful function~ ¦
25 n~mely to realize complementary gai~ variations in the
transmission and receivir~ paths of the set~ ¦
In the known tel~phone sets 9 in order to obiain
complementary gain variations in its two paths 7 it is
necessary to effect ~ith the aid of an amplifier of the
30 transmission path a galn variation in one direction and to
effect at the same time with the.aid of a~ amp~fier of
the receiving path a gain variation of the same amplitude~ I
but in the other direction. In order to effect a change-
over of these gains b~- speech, two speech detectors whose
ou~put signals are used to change-over ihe gain are of.en
used.
The present invention makes it possible to
PHF 810543C ~8 23.3.1982
to obtain e~tre~el~ complementary gain variations in
the t~o pa~hs o~ the set by co.ntrolling only the gain
of one single amplifier. To describe this ~eature
offered by the lnvention~ let~ for e~ample~ the case be
S considered of a telephone set which is associated with
the arrangements according to the invention as shown in
Figure 1 and it will be described how the change-over
gains can be effected w:ith re~erence to Figure 6 which
shows, referenced in the same wa~9 the majority of the
1n components sho~ in Figure 1. In this embodiment, the
single amplifier havin~ for its function to control the
gain in the two paths of the telephone set is the ampllf`ier
~; which must no~ be assumed to have a variable gaill and
is included in the transmission path between the microphone
15 4 and the input A wh~ch is used jointly by the first path
C1 and the second path ~2. If, for the minimum gain value
of the amplifier 5, that is to say the minimum value of
the gain G the gain o.f the components of the second path
C2 is controlled, as described in the foregoing~ in such
20 a ~ay that the sustained~ unannoying oscillations are
only produced in the auxiliary loop formed by the circuit
16 and the second path, it is certain that these anannoy-
ing oscillations are also produced for all the other
values of the gai~ ~ of the amplifier 5~ For all the
25 values of the gain g, the total gain in the auxiliar~ loop
remains equal to unity which is expressed by the above
fo~mula (7)0
As the oscillations passing through the circuit
16 have a very small amplitude9 the components of this
30 circuit operate in the linear mode and the gain G of the
circuit 16 is substantially proportional to the gain
Assuming that G = hg, h being a fixed coefficient, the
formula (3) may no~- be written:
3 h.g~g2~ 2 A2 = 1
~or ~he event in ~-hich the two amplifiers 11
and '2, having variable gains g1 and g2 are identical and
are controlled by the same signal, it is obtained that
PH~ 81~543C ~9 23.3019~2
g1 = g2 and the formula (9) may-be ~Titten~
g-gl'~T2 ~2 = 1 (10)
From this it can be deduced that to ever-
induced variation of the gain ~ of the amplifier 5 ~hich
amplifies the signal from microphone 4 there corresponds
-automaticaI~ a variation of thesame amplitude and the
opposite sense of the gain g1 of the amplifier 11 which
amplifies the signal received in the telephone set~
In the event that the gains g1 and g2 are
controlled such that g1 = a g2~ it can be easily seen
that to ever~ ~ariation of the gain ~ of the amplifier 5
there corresponds in the recei~ing path a gain ~ariation
in the opposite direction and having an amplitude divicled
15 by a.
It should be noted that so as to obtain opposi
tely directed gain variations in the transmission and
receiving paths an amplifier~ for example amplifier 5,
included in the transmission path between the microphone
20 4 and the input terminal A which is used jointly by the
two paths Ct and C2 must be acted upon~ An induced gai~
variation i~ any place in the receiving path, for example
a gain va~iation of the ampli~ier 7~ will not cause auy
gain variation in the transmission pa*hO
The gain variation of the amplifier 5 may be
effected progressively or suddenly between two gain ~aluesO
I-t can be controlled for example by a speech signal
detector 40~ so as to automatically realize "the change
o~er of the speech " in the set. When a subscriber speaks
30 into the microphone 4g his speech detected by the
detector 40 serves to con-trol a normal gain of the
amplifier 5. This automatically ~esults in a gain g1 ~
a low value for the amplifier 11 and consequently a ~-ea~
energizing signal for the loudspeaker 80 When the
3s subscriber stops speaking into the microphone 4~ the
detector 40 controls a lo~ ~ain of the amplifier 5~ This
automatically results in the amplifier 5 having a gain g1
PHF 81.543C ~ 3O 23,3.1982
of a higher ~-alue~ which produces a normal energizing
cignal for ths loudspeaker 8. It is of co~se possible
~o pro~-ide manual controls for the subscriber, to ensu~e
the,best possible listening comfort.
It will be readily understood that the cha~ge-
oYer of the ,gains in the two paths may also be effected
- in Ihe other variations of the arrangement in accordance
~-ith the invention, by means of one single ampli~ier. If a
variable-gain amplifier of the first path C1~controlled
lO b~- the regulator 13~ is provided in one of the two paths ``
of the.telephone set it is sufficient for the variable~ ~
gain amplifier used to change-over the gain to be pro~ided ,`
in the other path~ between the acoustic transducer and the
terminal which is used jointly by the two separate parts
15 of the two paths C1 and C2. For example, in the arrangement
sho~ in Figure 3a~ i~ which the variable~gain ~mplifier
11' controlled by the regulator 13, is provided in the
transmission path of the telephone set, the variable-
gain amplifier used for changing over the gai~ might be
20 provided in the receiving path between theoutput terminal
B7 which is used jointly by the two paths and the loud-
speaker incorporated in the electro-acoustic circuit 16.
~or the automatic change--over of the gain by the speech
signal it would be possible to emplo~ the speech signal
25 detector 4O of the microphone~ Finally9 it should be noted
that i~ the arr~gements such as those shown in the
Figures 3c, 3e and 3f in wh~ch the regulator 13 controls
only one or two variable~gain amplifiers used jointly b~
the two parts C1 and C2p gain variations of the opposite
30 direction and having automatically the same amplitude are
obtained by means of the amplifier changing over the gain
pro~-ided in the suitable path.
