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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hands-free telephone set providing a
microphone and a loudspeaker for making telephone communication in
hands-free. In particular, the present invention relates to a hands-free
telephone set including an acoustic echo suppression system and a
side-tone echo suppression system.
2. Description of the Related Art
Lately, a hands-free telephone set, which will be simply called a
"hands-free telephone" hereinafter, comes into use because of convenience.
By virtue of the hands-free telephone, telephone communication can be
performed in hands-free, using a microphone, a loudspeaker and single chip
digital signal processing integrated circuits associated with them.
In a telephone set, generally, an echo phenomenon occurs, and in an
ordinary hand-set telephone set, an echo phenomenon occurs due to a
side-tone echo produced by impedance mismatching at a well known
two-wire/four-wire converter in the telephone set. However, in the
hands-free telephone, not only the side-tone echo but also an acoustic
echo due to sound coupling between the loudspeaker and the microphone is
produced. Therefore, in the hands-free telephone, there are two echo
suppression systems, an acoustic echo suppression system including the
loudspeaker and the microphone and a side-tone echo suppression system
including the two-wire/four-wire converter.
Since the microphone and the loudspeaker are apart from the mouth and ears
of telephone talker, which will be simply called "talker" hereinafter,
respectively, the hands-free telephone is susceptible to influences from
surrounding noise and reverberation, in comparison with the ordinary
hand-set telephone.
Further, since the microphone and the loudspeaker are arranged closely on
the hands-free telephone, the acoustic echo caused by acoustic coupling
between the microphone and the loudspeaker occurs easily. Still further,
since the microphone is apart from the mouth of talker, voice sound coming
from talker into the microphone becomes small, which causes the microphone
to produce a small output signal. As a result, gains must be increased for
amplifying the small signal, which causes to produce easily well known
howling in the hands-free telephone.
Great efforts have been exerted for solving the above problems. FIG. 1
shows a block diagram of a most modern hands-free telephone 1' of the
prior art, developed in consideration of the above problems.
In FIG. 1, a microphone (MIC) 10 receives voice of talker and produces an
analog signal which will be called a "transmitting analog signal"
hereinafter. The transmitting analog signal is sent to an analog digital
converter (A/D) 21 by which the transmitting analog signal is converted to
a digital signal which will be called a "transmitting digital signal"
hereinafter. The transmitting digital signal is sent to an acoustic echo
canceler (AEC) 60, which is a key device of the acoustic echo suppression
system, by which an acoustic echo caused by acoustic coupling between MIC
10 and a loudspeaker 20 is suppressed. The transmitting digital signal
having passed through AEC 60 is sent to a voice-switched variable
attenuator (V-SW ATT) 90 whose function will be explained later. The
transmitting digital signal having passed through V-SW ATT 90 is sent to a
digital analog converter (D/A) 31 and partly to an sidetone echo canceler
(SEC) 70 which is a key device of the side-tone echo suppression system
and will be also explained later. In D/A 31, the transmitting digital
signal is converted back to an analog signal called a transmitting analog
output signal hereinafter. Then, the transmitting analog output signal is
sent to a hybrid circuit (H) 50 which is a well known two-wire/four-wire
converter used in a conventional telephone set. That is, the four-wire
transmitting analog output signal is converted to a two-wire transmitting
analog signal at H 50. The two-wire transmitting analog signal is sent to
a local switch, not depicted in FIG. 1, through a two-wire telephone line
51.
When the hands-free telephone 1' receives a two-wire received analog signal
from the local switch through the two-wire telephone line 51, the two-wire
received analog signal is converted to a four-wire received analog signal
at H 50. The four-wire received analog signal is converted to a received
digital signal at an analog digital converter (A/D) 22. The received
digital signal output from A/D 22 is sent to SEC 70 by which a side-tone
echo caused by the leakage signal produced due to the impedance
mismatching at H 50 is suppressed in cooperation with the transmitting
digital signal sent to SEC 70 partly through V-SW ATT 90. The received
digital signal having passed through SEC 70 is sent to V-SW ATT 90. The
received digital signal having passed through V-SW ATT 90 is sent to a
received signal amplifier (REC-SIG AMP) 81 at which the received digital
signal is amplified to a manually controlled level, producing an amplified
received digital signal. The amplified received digital signal is sent to
a digital analog converter (D/A) 32 where the amplified received digital
signal is converted to a received analog signal. The received analog
signal output from D/A 32 is sent to a loudspeaker (SPK) 40 so that
received telephone voice is loudly spoken from SPK 40. Hereupon, a part of
the received analog signal output from REC-SIG AMP 81 is fed to AEC 60 for
suppressing the acoustic echo.
In the above, the echo suppression performed by AEC 60 (SEC 70) is
accomplished by synthesizing an echo replica and subtracting the echo
replica from the output signal of MIC 10 (H 50). Wherein, the echo replica
is a signal simulating the echo, produced in accordance with an estimation
process performed through MIC 10 and SPK 20 in case of AEC 60 and through
H 50 in case of SEC 70. By virtue of applying AEC 60 (SEC 70) to the
hands-free telephone 1', the echo suppression can be performed well.
However, it takes a processing time to perform the synthesization and the
subtraction of the echo replica, and during the processing time, the
hands-free telephone 1' becomes unstable in operation, so that the
hands-free telephone 1' happens to fall into oscillation such as howling.
The V-SW ATT 90 is an automatic attenuator mainly for preventing such
oscillation from occurring in the hands-free telephone 1'. The V-SW ATT 90
compares amplitude of the transmitting digital signal and the received
digital signal applied to V-SW ATT 90 for determining which digital signal
is smaller and reduces amplitude of the smaller digital signal so as to
maintain the hands-free telephone 1' in a stable state during the
processing time.
A paper on the hands-free telephone 1', especially about AEC 60, SEC 70 and
V-SW ATT 90, has been read in Abstracts of Meeting on Acoustic
Engineering, held by Acoustic Society of Japan in March, 1990, titled "A
study on loudspeaker telephone using small echo cancellets", by Hiroyuki
Masuda, Kensaku Fujii and Juro Ohga who are inventors of the present
invention. The same subject as the above is presented at 119th Meeting
held by Acoustic Society of America, titled "Hands-free telephone using
compact echo cancelers and voice-switched attenuators", by Juro Ohga,
Hiroyuki Masuda, Kensaku Fujii and Yoshiro Sasaki, and the abstract of the
presentation is published in J. Acoustic Soc. Am. Suppl. 1, Vol. 87,
Spring 1990. Further, a paper theoretically explaining the acoustic echo
canceler has been read in "Special Section on Acoustic System Modeling and
Signal Processing", of IEICE TRANS. FUNDAMENTALS. VOL. E75-A, NO. 11
NOVEMBER 1992, with a title "A Fast Adaptive Algorithm Suitable for
Acoustic Echo Cancellet", by Kensaku Fujii and Juro Ohga.
The AEC 60 is the key device of the acoustic echo suppression system.
However, MIC 10 is another important device of the acoustic echo
suppression system. Because, loosening the acoustic coupling between MIC
10 and SPK 40 is very important for suppressing the acoustic echo.
However, since SPK 40 and MIC 10 are arranged closely, SPK 40 and MIC 10
easily form the acoustic echo coupling. Therefore, in order to prevent the
acoustic echo coupling from occurring between SPK 40 and MIC 10, a
bidirectional microphone system shown in FIG. 2(a) is applied to MIC 10.
In FIG. 2(a), the bidirectional microphone system of MIC 10 consists of a
pair of omnidirectional microphones (OMN MICs) 10A and 10B, a pair of
microphone amplifiers (MIC AMPs) 10a and 10b connected with
omnidirectional microphones 10A and 10b respectively and an operational
amplifier (OPE AMP) 10C connected with MIC AMPs 10a and 10b at a minus
terminal and a plus terminal of OPE AMP 10C respectively. The
bidirectional microphone system is for making MIC 10 receive sound
directly from SPK 40 substantially as little as possible. Since the
outputs from OMN MICs 10A and 10B are sent to OPE AMP 10C, when the
outputs from OMN MICs 10A and 10B are equal to each other, no output
signal (transmitting analog signal) is produced from OPE AMP 10C
theoretically. Therefore, when OMN MICs 10A and 10B are arranged on the
hands-free telephone 1' so that distances directly from SPK 40 to OMN MICs
10A and 10B are equal to each other, the output from OPE AMP 10C can be
reduced. In this case, if talker speaks toward MIC 10 in a direction
perpendicular to a line connecting OMN MICs 10A and 10B, the transmitting
analog signal due to voice of talker can be produced from OPE AMP 10C in
maximum. Because, the difference between talker's voice arrived at OMN
MICs 10A and 10B becomes maximum, resulting in producing a maximum output
from OPE AMP 10C. A bidirectional characteristic of MIC 10 (the
bidirectional microphone system) is shown in FIG. 2(b). In FIG. 2(b), the
bidirectional characteristic is represented by angular co-ordinates. The
OMN MICs 10A and 10B are positioned on a line including 0.degree. and
180.degree. axes and origin "0" of the angular coordinates is placed at a
middle point between OMN MICs 10A and 10B. The bidirectional
characteristic shows in FIG. 2(b) that the output of MIC 10 becomes
maximum at angle 0.degree. and 180.degree. and minimum at angle 90.degree.
and 270.degree.. It can be realized from the bidirectional characteristic
that SPK 40 should be placed in a plane including 90.degree.-270.degree.
axis and perpendicular to 0.degree.-180.degree. axis for minimizing the
acoustic echo and talker should speak toward point "0" along
0.degree.-180.degree. axis for obtaining the maximum output from MIC 10.
In FIG. 2(b), a zone including 90.degree. and 270.degree. axes will be
called a dead zone hereinafter.
Problems in the Prior Art
However, the hands-free telephone 1' of the prior art has a problem that
levels of the transmitting analog signal sent to A/D 21 and the four-wire
received analog signal arrived at A/D 22 fluctuate respectively as much as
20 dB in difference. Since talker speaks to MIC 10 without handling the
hands-free telephone 1', talker speaks extremely near to or far from MIC
10 occasionally, so that the transmitting analog signal sent to A/D 21
fluctuates as much as 20 dB in level, and since the telephone loss changes
every time when line is changed at the local switch, the four-wire
received analog signal arrived at A/D 22 also fluctuates as much as 20 dB
in level. As a result, when the fluctuation occurs in such large amount,
the fluctuation exceeds dynamic ranges of elements such as A/D 21, A/D 22,
D/A 31, D/A 32 and REC-SIG AMP 81, causing saturation of the elements,
which results in production of non-linear elements in the hands-free
telephone 1'.
Since AEC 60 (SEC 70) is required to synthesize the echo replica in the
estimation process, if there is a non-linear element in a circuit linked
to AEC 60 (SEC 70), it becomes hard to carry out the estimation process
for producing the echo replica. Wherein, the circuit linked to AEC 60 is a
circuit including MIC 10, A/D 21, AEC 60, D/A 32 and SPK 20, making
linkage for the acoustic echo suppression, and the circuit linked to SEC
70 is a circuit including H 50, A/D 22, SEC 70 and D/A 31, making linkage
for the side-tone echo suppression.
As a result, the hands-free telephone 1' becomes unstable and falls into
oscillation, making the hands-free telephone 1' impossible to perform the
telephone communication.
Meanwhile, there is another problem in regard to the acoustic echo
suppression system. The bidirectional microphone system of MIC 10 has a
great merit to the acoustic echo suppression system, decreasing the sound
directly coming into MIC 10 from SPK 40 and the reverberation in room.
However, when talker speaks in the dead zone of MIC 10, the level of the
transmitting analog signal-output from MIC 10 decreases extremely.
Therefore, when persons gather for holding a voice meeting, encircling the
hands-free telephone 1', the voice from the persons positioned in the dead
zone becomes small, so that it becomes hard to hold the voice meeting.
SUMMARY OF THE INVENTION
Therefore, a first object of the present invention is to improve the
acoustic echo suppression system in the hands-free telephone so that the
system operates in stable though a level difference of the transmitting
signal in a linked circuit to the system is large as much as causing
saturation of elements in the linked circuit.
Another second object of the present invention is to improve the side-tone
echo suppression system in the hands-free telephone so that the system
operates in stable though a level difference of the received signal in a
linked circuit to the system is large as much as causing saturation of
elements in the linked circuit.
Still another third object of the present invention is to increase
operation reliability of the echo suppression in the hands-free telephone.
Yet another fourth object of the present invention is to improve the
bidirectional microphone in the acoustic echo suppression system of the
hands-free telephone so that the bidirectional characteristic of the
microphone is changed in response to the circumstances of the hands-free
telephone.
Further another fifth object of the present invention is to expand
usefulness of the hands-free telephone.
The above first and third objects are achieved by providing transmitting
signal level control means consisting of an automatic gain controller and
a limiter to a circuit not linked to the acoustic echo suppression system
so that no non-linear element appears in the linked circuit though the
transmitting signal in the linked circuit has a large difference in level.
In the above, the limiter is provided for limiting the level of the
transmitting signal, until the automatic gain controller becomes a steady
state of operation.
The above second and third objects are achieved by providing received
signal level control means consisting of an automatic gain controller and
a limiter to a circuit not linked to the side-tone echo suppression system
so that no non-linear element appears in the linked circuit though the
received signal in the linked circuit has a large difference in level. In
the above, the limiter is provided for limiting the level of the received
signal, until the automatic controller becomes a steady state of
operation.
By virtue of providing the automatic gain controller and the limiter to the
acoustic echo suppression system and the side-tone echo suppression system
respectively, the acoustic echo and the side-tone echo can be suppressed
in stable though the level of the signal treated in the hands-free
telephone increases so high as a level having been impossible to perform
the echo suppression in the prior art.
The above fourth and fifth objects are achieved by providing microphone
direction control means consisting of a switching circuit to the
bidirectional microphone system in the hands-free telephone, for
controling the bidirectional microphone system in the hands-free
telephone. By virtue of controlling the bidirectional microphone system,
the microphone of the hands-free telephone is used as a bidirectional
microphone when a level of the received signal is higher than a designated
level and as a omnidirectional microphone when the received signal is
lower than the designated level,and further the bidirectional
characteristic of the microphone can be changed so as to be matched to
circumstances of the hands-free telephone. Since the bidirectional
microphone unit consists of two omnidirectional microphones and an
operational amplifier connected to the two omnidirectional microphones
through a microphone amplifier respectively, when one of two inputs to the
operational amplifier is disconnected by the switching circuit, the
microphone of the hands-free telephone becomes an omnidirectional
microphone, and when amplification of one of the microphone amplifiers is
controlled by the switching circuit, the bidirectional characteristic of
the microphone can be changed.
Further, the fourth object of the present invention is achieved by
providing a low-pass filter in the bidirectional microphone system for
raising a frequency response in a low frequency range. Generally, the
output of the bidirectional microphone system has a frequency
characteristic that a frequency response in a low frequency range
decreases. Providing the low-pass filter thus, the transmitting signal
from the hands-free telephone is improved so that the telephone sound from
the hands-free telephone becomes clearer. Consequently, it becomes less
necessary to raise amplitude of the transmitting and received signals in
the hands-free telephone, which is effective in suppressing the acoustic
echo.
Providing the microphone direction control means to the bidirectional
microphone system, the hands-free telephone can be used flexibly and
usefully. For instance, when the circumstances of the hands-free telephone
is calm, the microphone can be used as the omnidirectional microphone for
making the service area of the hands-free telephone broad, and when the
circumstances become noisy, the microphone is changed to be used as the
bidirectional microphone for preventing the occurrence of the acoustic
echo. Further, when the hands-free telephone is used to, for example, the
voice meeting, the microphone can be used as the omnidirectional
microphone or the bidirectional microphone in corresponding to the
circumstances around the voice meeting.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a hands-free telephone of the prior art;
FIG. 2(a) is a block diagram of a biodirectional microphone system of the
microphone in the hands-free telephone of the prior art;
FIG. 2(b) is a bidirectional characteristic of the bidirectional microphone
system shown in FIG. 2(a);
FIG. 3 is a block diagram of a hands-free telephone, for illustrating a
principle of the present invention;
FIG. 4 is a block diagram of a microphone direction control means, for
illustrating a principle of the microphone direction control means;
FIG. 5 is a block diagram of the hands-free telephone, for illustrating the
first embodiment of the present invention;
FIG. 6 is a block diagram of the hands-free telephone of the present
invention, for illustrating arranging positions of automatic gain controls
and/or limiters of transmitting signal level control means and received
signal level control means in the hands-free telephone;
FIG. 7 is a block diagram of the hands-free telephone of the present
invention, for illustrating the second embodiment of the present
invention;
FIG. 8 is a block diagram of the hands-free telephone of the present
invention, for illustrating the third embodiment of the present invention;
FIG. 9 is a block diagram of the hands-free telephone of the present
invention, for illustrating the fourth embodiment of the present
invention;
FIG. 10 is a block diagram of the bidirectional microphone system of the
hands-free telephone of the present invention and of the microphone
direction controller provided to the bidirectional microphone system, for
illustrating the fifth embodiment of the present invention;
FIG. 11 is a block diagram of the hands-free telephone of the present
invention, for illustrating the sixth embodiment of the present invention;
FIG. 12(a) is a graph representing acoustic sensitivity of the
bidirectional microphone controlled by a switch in the microphone
direction controller shown in FIG. 10, to levels of a received digital
signal in the hands-free telephone;
FIG. 12(b) is a graph representing acoustic sensitivity of the
bidirectional microphone controlled by an amplitude controller in the
microphone direction controller shown in FIG. 10, to levels of a received
digital signal in the hands-free telephone;
FIG. 12(c) is another graph representing acoustic sensitivity of the
bidirectional microphone controlled by the amplitude controller, to the
levels of the received digital signal;
FIG. 13(a) is a block diagram of a low-pass filter provided to the
microphone in the hands-free telephone 1 of the seventh embodiment of the
present invention; and
FIG. 13(b) is a graph representing a frequency characteristic of a
transmitting analog signal passed through the low-pass filter shown in
FIG. 13(a).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 is a block diagram for showing the principle of a hands-free
telephone 1 of the present invention. In FIG. 3, the same reference
numeral as in FIG. 1 designates the same part as in FIG. 1. In FIG. 3,
transmitting signal level control means (Tx-SIG CONT) 100 and received
signal level control means (Rx-SIG CONT) 200 are provided to circuits not
linked to the AEC 60 and SEC 70 respectively. The transmitting signal
level control means 100 is for controlling the level of the transmitting
digital signal produced from A/D 21, and the received signal level control
means 200 is for controlling the level of the received digital signal
produced from A/D 22, so as not to produce saturation of elements in the
linked circuits to AEC 60 and SEC 70 respectively. As explained before,
the linked circuit to AEC 60 is a circuit including MIC 10, A/D 21, AEC
60, D/A 32 and SPK 40 in FIG. 3, and the linked circuit to SEC 70 is a
circuit including H 50, A/D 22, SEC 70 and D/A 31 in FIG. 3. These
transmitting signal level control means 100 and received signal level
control means 200 control the levels of the transmitting signal and the
received signal so that no non-linear element appears in the linked
circuits of AEC 60 and SEC 70 respectively.
FIG. 4 is a block diagram for showing the principle of microphone direction
control means (MIC DIRECT CONT) 300 provided to the bidirectional
microphone system (MIC 10) in the hands-free telephone 1. In FIG. 4 the
same reference numeral as in FIG. 2 designates the same part as in FIG. 2.
In FIG. 4, microphone direction control means 300 has three operation
modes, bidirection mode, omnidirection mode and variable bidirection mode.
In the bidirection mode, means 300 performs no control to the
bidirectional microphone system, so that MIC 10 operates as the
bidirectional microphone as explained in the prior art in reference to
FIG. 2. In the omnidirection mode, means 300 controls the bidirectional
microphone system so that MIC 10 operates as a omniderectional microphone.
In the variable bidirection mode, means 300 controls the bidirectional
microphone system so that MIC 10 has a variable bidirectional
characteristic.
In the omnidirection mode, the control is performed by disconnecting one of
two inputs of OPE AMP 10C, and in the variable bidirection mode, the
control is performed by changing amplitude of one of MIC AMPs 10a and 10b.
The selection of the modes is performed by either way, automatically or
manually. However, usually, the selection of the bidirection mode and the
omnidirection mode is performed automatically. For instance, when the
level of the received signal is less than a level designated at REC-SIG
AMP 81, the ominidirection mode is selected and when the level of the
received signal exceeds the designated level the bidirection mode is
selected.
FIG. 5 shows a block diagram for the hands-free telephone 1 of the first
embodiment of the present invention. In FIG. 5, the same reference numeral
as in FIG. 1 designates the same part as in FIG. 1. In the hands-free
telephone 1 shown in FIG. 5, Tx-SIG CONT 100 shown in FIG. 3 is composed
of a transmitting signal automatic gain controller (TAGC) 110 and a
transmitting signal limiter (TLIM) 120, and Rx-SIG CONT 200 is composed of
a received signal automatic gain controller (RAGC) 210 and a received
signal limiter (RLIM) 220. In the above composition of Tx-SIG CONT 100 and
Rx-SIG CONT 200, TAGC 110 and RAGC 220 automatically control amplitude of
the transmitting digital signal and the received digital signal in
corresponding to their levels respectively, so that no non-linear element
appears in circuits linked to AEC 60 and SEC 70 respectively. However,
since TAGC 110 and RAGC 220 require a little time (several millisecond) to
build up to their normal states, TLIM 120 and RLIM 220 are provided for
suppressing a large level of the transmitting digital signal and the
received digital signal during the rising up time.
Hereupon, in the hands-free telephone 1, arranging positions of AGCs and
LIMs are important. Because, if AGCs and LIMs were arranged in the
circuits linked to AEC 60 and SEC 70, AGCs and LIMs would be changed to
non-linear elements because of their saturation, spoiling the estimation
process of AEC 60 and SEC 70.
FIG. 6 shows the same block diagram as in FIG. 1, however, in which nodes
(a).about.(m) are illustrated to explain positions for arranging AGC or
LIM. Since the linked circuit of AEC 60 includes nodes (a), (b), (l) and
(m) and the linked circuit of SEC 70 includes nodes (e), (f), (g) and (h),
AGC or LIM cannot be positioned at these nodes. Therefore, AGC or LIM can
be positioned at nodes (c), (d), (i), (j) and (k) in which TAGC or TLIM
can be positioned at nodes (c) and (d) and RAGC or RLIM can be poisoned at
nodes (i), (j) and (k).
FIG. 7 shows a block diagram for the hands-free telephone 1 of the second
embodiment of the present invention. In FIG. 7, the same reference as in
FIG. 5 designates the same part as in FIG. 5. In the second embodiment in
FIG. 7, TAGC 110 and TLIM 120 are arranged at node (c) and RAGC 210 and
RLIM 220 are arranged at node (i) in FIG. 6. As shown in FIG. 7, TAGC 110
and TLIM 120, and RAGC 210 and RLIM 220 can be gathered respectively at
the same node.
FIG. 8 shows a block diagram for the hands-free telephone 1 of the third
embodiment of the present invention. In FIG. 8, the same reference as in
FIG. 5 designates the same part as in FIG. 5. In the third embodiment in
FIG. 8, only TLIM 120 is used as Tx-SIG CONT 100 and arranged at node (d)
in FIG. 6 and only RLIM 220 is used as Rx-SIG CONT 200 and arranged at
node (k) in FIG. 6. In FIG. 8, TAGC 110 and RAGC 210 can be arranged in
the handsfree telephone 1 instead of TLIM 120 and RLIM 220 respectively.
FIG. 9 shows a block diagram for the hands-free telephone 1 of the fourth
embodiment of the present invention. In FIG. 9, the same reference as in
FIG. 5 designates the same part as in FIG. 5. The fourth embodiment in
FIG. 9 shows a case where the hands-free telephone 1 is connected to
four-wire telephone line as seen in ISDN. In this case, since H 50 shown
in FIG. 5 is not necessary to be provided in the hands-free telephone 1,
there is no leakage signal due to the impedance mismatching at H 50.
Therefore, SEC 70 shown in FIG. 5 is also not necessary to be provided in
the hands-free telephone 1. As a result, in the fourth embodiment in FIG.
9, TAGC 110, TLIM 120, RAGC 210 and RLIM 220 are arranged at nodes (c), (d
or e), (h or i) and (k) in FIG. 6, respectively.
FIG. 10 shows a block diagram for a microphone direction controller (MIC
DIR CONTROLLER) 310 included in the hands-free telephone 1, of the fifth
embodiment of the present invention. In FIG. 10, the same reference
numeral as in FIGS. 2(a) and 5 designates the same part as in FIGS. 2(a)
and 5.
In FIG. 10, RES-SIG AMP 81 has a setter for setting a designated level and
a sensor for sensing whether a level of the received digital signal sent
from V-SW ATT 90 exceeds the designated level and producing a switching
signal to MIC DIR CONTROLLER 310 when the level exceeds the designated
level. (The setter and sensor are not depicted in FIG. 10.) A switch (SW)
311 in MIC DIR CONTROLLER 310 is for switching one of input circuits of
OPE AMP 10C to earth or to one of MIC AMPs 10a and 10b. In FIG. 10, the
input circuit of OPE AMP 10C is a circuit connected to "+" terminal of OPE
AMP 10C and the input circuit connects MIC AMP 10b through SW 310.
When the level of the received digital signal sent from V-SW ATT 90 is
lower than the designated level, no switching signal is produced from
REC-SIG AMP 81. Then, SW 311 does not operate, so that the input circuit
of OPE AMP 10C is grounded. As a result, MIC 10 operates as the
omnidirectional microphone.
When the level of the received digital signal exceeds the designated level,
the switching signal is produced from REC-SIG AMP 81 and sent to SW 311.
Then, SW 311 operates so that OPE AMP 10C is connected with MIC AMP 10b as
shown in FIG. 10. As a result, MIC 10 operates as the bidirectinal
microphone.
The results of the operation of SW 311 is graphed in FIG. 12(a). In FIG.
12(a), the vertical axis represents acoustic sensitivity of MIC 10 (MIC
SENSITIVITY) and the horizontal axis represents the level of the received
digital signal (REC-SIG LEVEL). In FIG. 12(a), when REC-SIG LEVEL
increases and exceeds the designated level, the directional characteristic
of MIC 10 is changed from omnidirectinal characteristic to bidirectional
characteristic.
In FIG. 10, an amplitude controller (AMP CONT) 312 is provided in MIC DIR
CONTROLLER 310. The amplitude of MIC AMP 10b is changed by MIC DIR
CONTROLLER 310 when the switching signal is sent from REC-SIG AMP 81.
Changing the amplitude of MIC AMP 10b, two input signals of OPE AMP 10C
are unbalanced, so that the bidirectional characteristic of MIC 10 is
changed in response to the change of the amplitude of MIC AMP 10b.
The results of the operation of SW 311 is graphed in FIGS. 12(b) and 12(c).
The vertical axis and the horizontal axis are same as those in FIG. 12(a)
respectively. When the amplitude is changed in steps, MIC SENSITIVITY is
changed in steps as shown in FIG. 12(b). As the amplitude of MIC AMP 10b
is increased thus, the decrement of the bidirectional characteristic at
the dead zone is changed in steps like 0 dB (omnidirection), 6 dB, 10 dB
and infinity. When the amplitude is changed gradually, MIC SENSITIVITY is
changed also gradually as shown in FIG. 12(c). As the amplitude of MIC AMP
lob is increased thus, the decrement of the bidirectional characteristic
at the dead zone is changed gradually.
Not depicted in FIG. 10, SW 311 and AMP CONT 312 can be used together or
independently, switching of SW 311 can be made manually, and switching of
AMP CONT 312 can be made manually or automatically.
FIG. 11 shows a block diagram for the hands-free telephone 1 of the sixth
embodiment of the present invention. In FIG. 11, the same reference
numeral as in FIG. 5 designates the same part as in FIG. 5. FIG. 11 shows
a case where MIC DIR CONTROLLER 310 is included in the prior art
hands-free telephone 1' shown in FIG. 1. However, as a matter of course,
MIC DIR CONTROLLER 310 can be included in the hands-free telephone 1 shown
in FIG. 5, 7, 8 or 9. In FIG. 11, MIC DIR CONTROLLER 310 is depicted
simply so that MIC AMPs 10a and 10b are omitted to be depicted.
FIG. 13(a) shows a partial block diagram concerned to a low-pass filter
provided in the hands-free telephone 1 of the seventh embodiment of the
present invention. In FIG. 13(a), the same reference numeral as in FIG. 10
designates the same part as din FIG. 10. Generally, the output of the
bidirectinal microphone has a frequency characteristic that a frequency
response decreases in a low frequency range. Therefore, in FIG. 13(a), a
low-pass filter (LOW-PASS FILTER) 320 is provided at the output of OPE AMP
10C. Providing LOW-PASS FILTER 320 thus, the frequency response increases
in the low frequency range as shown in FIG. 13(b). A dotted curve depicted
in FIG. 13(a) shows that the frequency response is raised up in the low
frequency range.
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