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DESCRIPTION JP2002314465

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DESCRIPTION JP2002314465
[0001]
FIELD OF THE INVENTION The present invention relates to a half-duplex hands-free phone.
[0002]
2. Description of the Related Art In conventional hands-free phones, an AGC circuit (auto gain
control) is used for a signal input from a microphone, and the output signal level is kept constant
regardless of the level of the microphone input signal. That is, when the distance between the
microphone and the mouth is large and the microphone input signal is small, the gain of the
microphone amplifier is increased. On the other hand, when the distance between the
microphone and the mouth is short and the microphone input signal is large, the gain of the
microphone amplifier is reduced.
[0003]
Alternatively, as shown in FIG. 7, the output signal is made proportional to the input signal. That
is, the gain G = 20 × log (51 kΩ / 5.1 kΩ) = 20 dB of the microphone amplifier 55 was
constant. Also, as shown in FIG. 8, conventional hands-free phones generally use full-duplex
communication that simultaneously performs transmission and reception.
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[0004]
In full-duplex communication, acoustic coupling causes the microphone 51 to collect sound
uttered from the speaker 50 having the amplifiers 55 and 56, and a signal transmitted to the
circuit side is reflected by the hybrid transformer 52 and the line is received again There is a
bond. Both of these cause howling. Echo cancelers 53 and 54 are used to prevent howling due to
this acoustic coupling and circuit coupling. Both echo cancellers have the function to compare
and cancel transmit and receive signals.
[0005]
SUMMARY OF THE INVENTION According to such a handsfree phone, when a microphone
amplifier with a fixed gain is used, increasing the gain causes a howling, and decreasing the gain
separates the mouth from the microphone. In this case, there is a problem that the volume heard
from the handset of the other party's telephone is reduced.
[0006]
In addition, the echo canceller has a limit on the signal level that can be canceled, and increasing
the input signal level by increasing the output signal level of the speaker or increasing the gain of
the microphone amplifier causes the limit of the echo canceler to cause howling to occur. There's
a problem.
Furthermore, the echo canceller has a problem that the transmission signal becomes small
because the signal collected from the microphone is also attenuated.
[0007]
According to the present invention, there is provided a hands-free phone having a hybrid
transformer for performing two-wire to four-wire interconversion in a communication path
between a speaker connected to an amplifier and a microphone. Attenuator for controlling the
transmission signal and an attenuator for controlling the reception signal are provided, and a
level detector for comparing the transmission signal with the reception signal is provided before
and after both attenuators, and half duplex by the signal from the both level detectors Attenuator
control is provided to perform control. Furthermore, a slice circuit using a diode is provided in
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the microphone amplifier.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be
described below with reference to the drawings. First Embodiment FIG. 1 is a circuit diagram of a
half duplex system, and FIG. 2 is a circuit diagram of a microphone amplifier. Two diodes 4 and 5
are connected. 3 shows the signal waveforms of the microphone amplifier of FIG. 2, (1) is an
input signal waveform to the microphone amplifier, (2) is an output signal 6 waveform when the
signal input to the microphone amplifier is small, and (3) is a microphone It is 7 output signal
waveforms when the signal input into an amplifier is large.
[0009]
FIG. 4 is an explanatory view of the connection state of the hands-free phone. In the figure, the
hands-free phone 13 installed in the private house 11 is connected to the public network 15, and
the telephone 14 installed in the private house 12 is the public network. Connected to 15. FIG. 5
is an explanatory view of a front panel of the hands-free phone. In the figure, 21 is a push button,
22 is a speaker for outputting voice and voice synthesized voice of the other party from the
public network 15, 23 is a speakerphone button used when using the handsfree phone function,
24 is a handsfree phone It is a microphone that collects the voice of the user.
[0010]
In FIG. 1, 22 is a speaker for ringing the reception signal from the public network 15 as
described above, 24 is a microphone for collecting the voice of the hands-free phone user as
described above, and 32 is a microphone for amplifying the microphone input signal. The
amplifier 33 is a transmission amplifier for amplifying a signal to be transmitted to the public
network 15, the 34 is a hybrid transformer for performing two-wire to four-wire interconversion
of the communication path, 35 is a reception amplifier for amplifying a signal received from the
public network 15, 36 Is a speaker amplifier that performs power amplification for driving the
speaker 22.
[0011]
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Reference numeral 38 denotes a transmission signal TLIA, 39 denotes a reception signal RLIA,
40 denotes a transmission signal TLIB, and 41 denotes a reception signal RLIB.
42 is a level detector A, which compares the TLIA 38 with the RLIA 39. 43 is a level detector B,
which compares the TLIB 40 with the RLIB 41. Reference numeral 44 denotes attenuator control,
which performs half-duplex control based on the signals from the level detector A42 and the
level detector B43.
[0012]
45 is a TX attenuator that controls a transmit signal, and 46 is an RX attenuator that controls a
receive signal. The half-duplex control operation of the above circuit will be described with
reference to the flowchart of FIG. The case of making a call from the handsfree phone 13 of the
private house 11 to the telephone 14 of the private house 12 using the public network 15 will be
described.
[0013]
First, when the speakerphone button 23 is pressed, a tone signal is emitted from the speaker 22.
The telephone number of the private house 12 is pressed with the push button 21. Ringback tone
(hereinafter referred to as RBT) from the speaker 22 of the private house 12 ON for 1 second
and OFF for 2 seconds. ) Sounds. At this time, the telephone 14 of the private house 12 is also
turned on for one second and off for two seconds.
[0014]
When the handset of the telephone set 14 of the private house 12 is off-hooked, the ringing tone
is stopped, and a call can be made between the hands-free phone 13 of the private house 11 and
the telephone set 14 of the private house 12. The voice of the conversation is collected by the
microphone 24 and amplified by the microphone amplifier 32. The transmission signal TLIA 38
amplified by the microphone amplifier 32 is input to the level detector A 42 and the TX
attenuator 45.
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[0015]
The level detector A42 also receives the reception signal RLIA39. The half duplex scheme
operates as follows. At the time of transmission, the level detector A 42 compares the levels of
the transmission signal TLIA 38 and the reception signal RLIA 39, and when TLIA> RLIA, the
level detector A 42 detects transmission, and transmits a switching message to the attenuator
control 44. The attenuator control 44 receiving this switching message turns off the TX
attenuator 45 and turns on the RX attenuator 46.
[0016]
At the time of reception, the level detector B43 compares the levels of the transmission signal
TLIB40 and the reception signal RLIB41, and when TLIB <RLIB, the level detector B43 detects
reception and transmits a switching message to the attenuator control 44. The attenuator control
44 that has received this switching message turns on the TX attenuator 45 and turns off the RX
attenuator 46.
[0017]
The level detector A42 compares the levels of the transmission signal TLIA38 and the reception
signal RLIA39, and when the state of TLIA> RLIA continues for about 1 second, the level detector
A42 detects it as a reception signal and switches the attenuator control 44 to a switching
message Send. The attenuator control 44 receiving this switching message turns off the TX
attenuator 45 and turns on the RX attenuator 46.
[0018]
At the time of transmission, the voltage is amplified by the transmission amplifier 33 and is
output to the telephone line. The signal is converted from 4 lines to 2 lines by the hybrid
transformer 34. At this time, all the signals are not propagated to the telephone line, and go
around to the receiving amplifier 35 side. This looped-in signal is amplified by the reception
amplifier 35, but since the RX attenuator 46 is ON, howling due to line coupling does not occur.
Further, since the transmitting side and the receiving side operate alternately, even if a sound
uttered by the speaker 22 at the time of reception is collected by the microphone 24, since the
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TX attenuator 45 is turned on, howling due to acoustic coupling does not occur.
[0019]
When the call ends, the speakerphone button 23 of the handsfree phone 13 is pressed to close
the line. The handset of the telephone set 14 is hooked on. According to the embodiment
described above, it is possible to prevent the occurrence of howling due to acoustic coupling and
line coupling, and it is possible to raise the speaker level of the hands-free phone.
[0020]
Second Embodiment Example In order to configure a high quality hands-free phone that reports
an emergency, it is necessary to be able to have a sufficient conversation even at a location far
away from the hands-free phone. At the time of transmission, the level of the transmission signal
TLIA 38 and the reception signal RLIA 39 are compared by the level detector A 42. However,
when the amplification factor of the microphone amplifier 32 is adjusted to a position where the
microphone and the mouth are close, In the case of the conventional microphone amplifier as
shown in the above, there is a problem that the signal output to the telephone line becomes very
small, and the volume heard from the handset of the telephone 14 of the private house 12
becomes small and hard to hear.
[0021]
Also, although the level of the transmission signal TLIA38 and the level of the reception signal
RLIA39 are compared by the level detector A42, when the amplification factor of the microphone
amplifier 32 is adjusted to a position where the microphone and the mouth are close, the
conventional microphone amplifier shown in FIG. In this case, the level detector B43 compares
the levels of the transmission signal TLIB40 and the reception signal RLIB41 at reception, but
since the amplification factor of the microphone amplifier 32 is very high, it is possible to switch
to the reception state without TLIB <RLIB. There is a problem that it disappears.
[0022]
Therefore, in the present embodiment, the problem is solved by using a microphone amplifier as
shown in FIG.
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The microphone amplifier shown in FIG. 2 has a gain G of 20 × log (180 kΩ / 5.1 kΩ) = about
31 dB, considering the case where the diode 4 and the diode 5 are not provided.
[0023]
Also, consider the case where the diode 4 and the diode 5 are present. Let the forward voltage of
the diode 4 and the diode 5 be VF. VF is usually 1V. If the output signal of the microphone
amplifier is a positive voltage and is equal to or higher than the VF of the diode 4, the operational
amplifier negative input 3 is logically shorted to 0 V, so it is clipped at +1 V. The clipped state is
shown in FIG. 3 (3) A.
[0024]
In addition, when the output signal of the microphone amplifier is a negative voltage and lower
than the VF of the diode 5, the operational amplifier negative input 3 is logically shorted to 0 V,
and therefore, is clipped at -1 V. The clipped state is shown in FIG. 3 (3) B. Since the bare gain of
the microphone amplifier shown in FIG. 1 is about 31 dB, the input signal 1 which the amplifier
does not clip at +1 V or -1 V is about ± 28 mV. Therefore, when the amplitude C of FIG. 3 (1)
input signal is 56 mVp-p or less, a sine wave is output from the output signal 2 as shown in FIG.
3 (2) output signal 6. On the other hand, when the amplitude C of the input signal in FIG. 3 (1) is
56 mVp-p or more, a signal clipped at ± 1 V as shown in FIG. 3 (3) output signal 7 is output
from the output signal 2.
[0025]
A description will be given below by comparing the time when the microphone and the talker's
mouth are close and far when the user talks. When the microphone and the mouth are close, if
the microphone and the mouth are close, the amplitude of the input signal of the microphone
amplifier 32 becomes about 56 m Vp-p or more.
[0026]
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Therefore, the output voltage of the microphone amplifier 32 is clipped at about 2 Vp-p, and
becomes like the output signal 7 of FIG. 3 (3). By this, the level detector B43 compares the levels
of the transmission signal TLIB40 and the reception signal RLIB41 at the time of reception, but
when the output voltage becomes 2Vp-p or more, the output voltage of the microphone amplifier
32 is 2Vp-p or less with respect to the reception signal RLIB41. Therefore, the problem of not
being able to switch to the reception state can be solved.
[0027]
When the microphone and the mouth are far apart, the voice which is spoken is collected by the
microphone 31 and amplified by the microphone amplifier 32. When the microphone and the
mouth are far, the amplitude of the input signal of the microphone amplifier 32 becomes about
56 m Vp-p or less. Therefore, the output voltage of the microphone amplifier 32 becomes
approximately 2 Vp-p or less, and the output signal 6 is as shown in FIG. 3 (2). As a result, the
signal output to the telephone line can be increased, and the problem that it is difficult for the
telephone 14 of the B private house 12 to hear can be solved.
[0028]
As shown in FIG. 3 (3), the output signal 7 has a clipped waveform, but since the frequency band
includes the voice band, the voice maintains the quality without causing any problem in
conversation. When the call ends, the speakerphone button 23 of the handsfree phone 13 is
pressed to close the line. The handset of the telephone set 14 is hooked on. According to the
embodiment described above, by providing the slicing circuit using the diode in the microphone
amplifier, a large volume level can be output to the telephone line even when the microphone is
far from the mouth. Also, with the half duplex system, it is possible to suppress the occurrence of
howling due to acoustic coupling even when the microphone and the mouth are close.
Furthermore, the switching failure of the half duplex system will be improved.
[0029]
Third Embodiment In the call state, priority is given to transmission, that is, transmission from
the hands-free phone 13 of the A private house 11 to the telephone set 14 of the B private home
12. That is, although the level of the transmission signal TLIA38 and the level of the reception
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signal RLIA39 are compared by the level detector A42, in the initial state, the signal level of the
TLIA38 is set slightly higher than the RLIA39 so that TLIA> RLIA. At this time, a switch message
is sent to the attenuator control 44. The attenuator control 44 having received the switching
message turns off the TX attenuator 45 and turns on the RX attenuator 46.
[0030]
At the time of reception, the level detector B43 compares the levels of the transmission signal
TLIB40 and the reception signal RLIB41, and when TLIB <RLIB, the level detector B43 detects
reception and transmits a switching message to the attenuator control 44. The attenuator control
44 receiving the switching message turns on the TX attenuator 45 and turns off the RX
attenuator 46.
[0031]
When the call ends, the speakerphone button 23 of the handsfree phone 13 is pressed to close
the line. The handset of the telephone set 14 is hooked on. According to the embodiment
described above, when priority is given to the telephone from the hands-free phone side, when
the hands-free phone is used in an emergency or the like, the informing person does not do
anything other than the operation to report, and a message Can be sent to the recipient.
[0032]
According to the present invention described in detail above, the occurrence of howling due to
acoustic coupling and line coupling can be prevented, and this has the effect of being able to
raise the speaker level of the hands-free phone. Further, by providing a slice circuit using a diode
in the microphone amplifier, it is possible to output a large volume level to the telephone line
even when the microphone is far from the mouth. In addition, the half-duplex system has an
effect of suppressing howling due to acoustic coupling even when the microphone is close to the
mouth. This has the effect of improving the switching failure of the half duplex system.
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