JPH0247907B2 - BOSOKUONKAIRO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a side sound protection circuit having a bridge circuit configuration used in a telephone set.

[Technical background of the invention and its problems]

Telephones are equipped with a side sound prevention circuit to suppress sidetone from the transmitter to the receiver during the 4-wire to 2-wire conversion that connects the transmitter and receiver to the telephone line. Bridge circuit configurations are known.

Figure 1 shows the basic configuration of a side sound protection circuit with a bridge circuit configuration, where 1 is a transmitter, 2 is a receiver, and 3 is a receiver.
4 is a telephone line, and 4 is a bridge circuit. Resistors 5 and 6 are connected to two adjacent sides of the bridge circuit 4, and the line end of the telephone line 3 and a balance circuit network 7 are connected to the other two sides. The balanced network 7 is composed of C and R, and has an impedance Z _o similar to the line impedance Z _l of the telephone line 3. That is, resistance 5, 6
Letting the values of R _a and R _b be R _a /R _b =Z _l /Z _o (1), that is, Z _o is selected so that the bridge circuit 4 is balanced. At this time, the output of the transmitter 1 is not applied to the receiver 2, and in principle, an extremely large amount of sidetone attenuation can be obtained.

However, the line impedance Z _l is the impedance due to the series resistance and line capacitance of the telephone line 3, and it changes depending on the frequency and also varies greatly depending on the line length and type of line. If the constant of 7 is fixed, bridge circuit 4
A sufficient balance cannot be obtained, and the amount of sidetone attenuation decreases. A slight decrease in sidetone attenuation is not normally a problem, but in loudspeaker telephones that use a microphone and a speaker as an interphone, the audio input from the microphone is transmitted via the speaker. There is a danger that howling may occur if the signal returns to the microphone again.

[Purpose of the invention]

An object of the present invention is to provide a side sound prevention circuit that can always obtain a large amount of side sound attenuation regardless of changes in the line impedance of a telephone line.

[Summary of the Invention] This invention comprises a variable impedance circuit capable of changing the impedance by electronic control, in which the resistance and capacitance of the balanced circuit network provided in the bridge circuit correspond to the line impedance of the telephone line. , the impedance is controlled to an optimal value according to changes in line impedance.

Changes in the line impedance of the telephone line can be detected, for example, from the ratio of the voltages at each terminal of the transmitter and receiver, and if the impedance of the variable impedance circuit in the balanced network is controlled so that this ratio is constant, This impedance is set to an optimal value corresponding to changes in line impedance.

〔Effect of the invention〕

According to this invention, the impedance of the balanced circuit network is always controlled to an optimum value in accordance with changes in line impedance of the telephone line, so that a balanced state of the bridge circuit can always be obtained. Therefore, a large amount of side sound attenuation can be achieved, sufficient side sound prevention characteristics can be obtained even in a loudspeaker telephone, and howling can be prevented.

[Embodiments of the invention]

FIG. 2 shows the configuration of a side sound prevention circuit according to an embodiment of the present invention. In the figure, detectors 11 and 12 are connected to both ends of a transmitter 1 and a receiver 2, respectively, and these detectors 11 and 12 detect the respective terminal voltages. In this example, the detectors 11 and 12 are configured to have logarithmic conversion characteristics by converting the input voltage into a current, passing the current through a diode, converting it back into voltage, and then performing rectification and smoothing. It will be done. The difference between the outputs of these detectors 11 and 12 is calculated by a subtracter 13, but this operation is performed using the difference between the transmitter 1 and the receiver 2 because the detected output is logarithmically converted and displayed in dB. This corresponds to taking the ratio of terminal voltages. The output of this subtracter 13 is converted into a digital value by an A/D converter 14, and then applied to a control circuit 15 constructed using, for example, a microprocessor. The control circuit 15 electronically controls the impedance of the variable impedance circuit in the balanced circuit network 7 so that the input digital value from the A/D converter 14 is constant, that is, the ratio of the voltages at both terminals is constant. .

The balanced circuit network 7 is represented by the equivalent circuit shown in FIG. 3, for example. In FIG. 3, C ₀ and R ₀ correspond to the terminal impedance of the exchanger, and are fixed values. The remaining resistances R ₁ , R ₂ and capacitances C ₁ , C ₂ correspond to the line impedance Z _l of the telephone line 3, and in this embodiment, these R ₁ , R ₂ , C ₁ , C ₂ are Consists of a variable impedance circuit.

Although various types of variable impedance circuits are known, for example, the one proposed in Japanese Patent Application No. 138243/1984 can be used, and this can be used to construct the balanced circuit network 7 as shown in Fig. 3. An example of this is shown in Figure 4. 21, 22, 23, 24 are respectively R ₁ ,
This is a variable impedance circuit that includes R ₂ and C ₁ C ₂ .

FIG. 5 shows R ₁ , R ₂ , C ₁ , C ₂ using the variable impedance circuit described in Japanese Patent Application No. 53-138243.
In this example, a shows an example applied to R ₁ and C ₁ with both ends floating, and b shows an example applied to R ₂ and C ₂ with one end grounded.

In FIG. 5a, the voltage applied between terminals 31 and 32 is applied to the differential input via voltage followers 33 and 34.
Differential output type voltage-current converter (V/I converter) 3
5, and the transistors Q ₃ ,
Voltage current conversion impedance element 3 through Q ₄
A current proportional to the voltage difference is applied to 6. Impedance element 36 is a fixed resistance or capacitance.
Since the currents of current sources CS ₅ and CS ₆ do not change due to the emitter currents of transistors Q ₃ and Q ₄ , all changes in the current flowing through impedance element 36 are
This is a change in the difference between the emitter currents of Q ₃ and Q ₄ , and if the base current is ignored, this is all a change in the collector current and is taken out as a differential output current.

This differential output current is fed back to both input ends of the V/I converter 35 via the current feedback circuit 37. In other words, changes in the collector currents of transistors Q ₃ and Q ₄ result in changes in the collector currents of diode-connected transistors Q ₅ and Q ₆ , and these transistors Q ₅ and
A change in the base-emitter voltage V _BE of Q ₆ becomes the collector current of transistors Q ₇ and Q ₈ , which becomes a feedback current. This return current, that is, the change in the collector current of Q ₇ and Q ₈ varies depending on the current 2I ₁ of the current source CS ₇ , and if the current of the current sources CS ₅ and CS ₆ is I ₂ , then Q ₅ , I ₁ /I ₂ times the change in collector current of Q ₆ . In other words, the change in the current flowing through the impedance element 36 is multiplied by I ₁ /I ₂ and returns to the terminals 31 and 32, so the impedance between the terminals 31 and 32 is calculated by dividing the voltage difference by the feedback current. If Z ₀ is the impedance of the impedance element 36, then Z=Z ₀ ·I ₂ /I ₁ .

Therefore, by electronically changing I ₂ /I ₁ , that is, the current feedback ratio of the current feedback circuit 37, the impedance Z can be changed. Specifically, _the bias control _{circuit 25} shown in _FIG .
26. In addition, the terminal 31,
The impedance Z between 32 becomes resistive if a resistor is used as the impedance element 36, or capacitive if a capacitor is used as the impedance element 36, and can be used as R ₁ and C ₁ respectively.

On the other hand, the example shown in FIG. 5b used as R ₂ or C ₂ is the same as that shown in FIG. 5a except that current feedback is applied only to the terminal 31 side because one terminal 32 is grounded.

By the way, the terminals 31 in Figures 5a and 5b and the terminals 31 in Figure 5a
Looking at terminal 32, both current sources CS ₃ and CS ₄
After all, transistors Q ₈ and Q ₇ are connected as current sources, and the impedance of the current sources is extremely high, so if there is even a slight difference in current between CS ₃ and Q ₈ or between CS ₄ and Q ₇ , a large DC offset voltage will occur. appears. If the impedance element 36 is a resistor, this DC offset voltage is canceled by applying negative DC feedback through it, but if the impedance element 36 is a capacitor, no such feedback is applied. In FIG. 4, for example, the impedance element 36 in the variable impedance circuit 24 is a capacitor,
On the other hand, if the impedance element 36 in the variable impedance circuit 21 connected to this circuit 24 via the terminals P ₇ and P ₂ is a resistor, the above-mentioned DC negative feedback in the circuit 21 causes the terminal P of the circuit 24 to It is also considered that the _DC offset voltage at terminal _P2 of circuit 21 is canceled at the same time as the DC offset voltage at terminal P2 of circuit 21. However, the equivalent capacitance C ₂ of the circuit 24 is large and the impedance is low, that is, the amount of current feedback to the terminal P ₇ is large, and the equivalent resistance R ₁ of the circuit 21 is large, that is, the amount of current feedback to the terminal P ₂ is large. If the voltage is small, it becomes impossible to correct the DC current error in the circuit 24 by the DC negative feedback of the circuit 21, and as a result, the DC offset voltages at the terminals P ₂ and P ₇ are not canceled and fluctuate greatly, resulting in the worst case. In this case, it becomes inoperable. This is exactly the same for the circuits 22 and 23.

In order to avoid such problems, in Fig. 4, a DC offset is applied to the connection point P _a (terminals P ₂ , P ₇ ) between circuits 21 and 24 and the connection point P _b (terminals P ₃ , P ₆ ) between circuits 22 and 23. Voltage stabilizing circuits 27 and 28 are connected.

Figure 6 shows a specific example of a DC offset voltage stabilizing circuit, in which a transistor _Q11 and a current source _CS11 are connected to the terminal 41 connected to point P _a or P _b in Figure 4.
A voltage follower (emitter follower) 42 consisting of
is connected, and the output of this voltage follower 42 is a resistor
Only the DC component is extracted by a low-pass filter 43 consisting of _R11 and capacitor _C11 . The output potential V _d of this low-pass filter 43 and the reference potential V _r are connected to the transistor whose common emitter is connected to the current source CS ₁₂ .
A comparison is made by a differential amplifier 44 consisting of Q ₁₂ , Q ₁₃ , and Q ₁₄ , and transistors Q ₁₅ and Q ₁₆ as loads.
A current corresponding to the difference between V _d and V _r is fed back to the input end of the voltage follower 42 via a current mirror 45 consisting of a current mirror 45 .

Due to such a negative feedback loop, the DC potential of the terminal 41 is kept equal to the reference potential _Vr . That is, the aforementioned DC offset voltage is kept constant. In this case, since the AC impedance seen from the terminal 41 to the voltage follower 42 and current mirror 45 is extremely large, it hardly affects the original characteristics of the variable impedance circuit.

As described above, the side sound prevention circuit of the present invention is configured such that the impedance of the balanced circuit network is automatically controlled to the optimal value for the line impedance of the telephone line, so that sufficient side sound attenuation can be achieved. It can also be used for loudspeaker telephones, etc.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the basic configuration of a side sound prevention circuit with a bridge circuit configuration, FIG. 2 is a diagram showing the configuration of a side sound prevention circuit according to an embodiment of the present invention, and FIG. 3 is an equivalent diagram of a balanced circuit network. 4 is a configuration diagram of a balanced circuit network constructed using a variable impedance circuit, FIG. 5 a and b are circuit diagrams of a variable impedance circuit used in the balanced circuit network of FIG. 4, and FIG. FIG. 4 is a circuit diagram of a DC offset voltage stabilizing circuit in the balanced circuit network of FIG. 4; 1...Telephone, 2...Telephone receiver, 3...Telephone line,
4... Bridge circuit, 5, 6... Resistor, 7... Balanced circuit network, 11, 12... Detector, 13... Subtractor, 14... A/D converter, 15... Control circuit,
21-24...variable impedance circuit, 25,
26... Bias control circuit, 27, 28... DC offset voltage stabilization circuit, 33, 34... Voltage follower, 35... Differential input/differential output type voltage-current converter, 36... Voltage-current conversion impedance Elements, 37... Current feedback circuit, 42... Voltage follower, 43... Low pass filter, 44... Differential amplifier, 45... Current mirror.

Claims (1)

[Scope of Claims] 1. In a side sound prevention circuit having a bridge circuit configuration having on one side a balanced circuit network having an impedance similar to the line impedance of a telephone line, a resistance and a capacitance corresponding to the line impedance in the balanced circuit network are provided. , comprising a variable impedance circuit capable of changing impedance by electronic control, and comprising control means for controlling the impedance of the variable impedance circuit to an optimum value in accordance with changes in the line impedance, the control means comprising: 1. A side sound protection circuit that detects terminal voltages of a transmitter and a receiver, and controls the impedance of the variable impedance circuit so that the ratio of the voltages of both terminals is constant. 2. A balanced circuit network consists of a variable impedance circuit, a voltage-to-current converter, a differential input/differential output type voltage-to-current converter using a resistor or capacitor as an impedance element, and a differential output current of this converter to convert the differential output current of this converter into a voltage-current converter. A current feedback circuit that feeds back to at least one input side, and the impedance (resistance or capacitance) between the differential input terminals of the converter is changed by controlling the current feedback ratio of this current feedback circuit. The side sound prevention circuit according to claim 1, characterized in that: 3. The balanced network connects the connection point between the resistive variable impedance circuit and the capacitive variable impedance circuit to a voltage follower, leads the output of this voltage follower to a low-pass filter, and combines the output potential of this low-pass filter with a reference potential. A patent claim characterized in that the direct current potential at the connection point is kept equal to the reference potential by comparing the two potentials and feeding back a current corresponding to the difference between the two potentials to the input terminal of the voltage follower. The side sound protection circuit according to item 1.