JPS59231959A - Side tone preventing circuit - Google Patents

Abstract

PURPOSE:To allow the circuit to apply to a loudspeaker telephone or the like by controlling automatically an impedance of a balancing circuit network to a line impedance of a telephone line to an optimum value so as to obtain a sufficient side tone attenuation. CONSTITUTION:Detectors 11, 12 are connected to each end of a transmitter 1 and a receiver 2 and the detectors 11, 12 detect each terminal voltage. An output of the detectors 11, 12 is applied to an attenuator 13, where a difference of the output of the detectors 11, 12 is taken, and an output of the attenuator 13 is converted into a digital value by an A/D converter 14. This digital value is inputted to a control circuit 15 comprising a computer so as to make the digital value constant. Further, the ratio of voltages between both terminals of the detectors 11, 12 is applied to a balanced circuit 7 as a fixed digital value so as to change the impedance of one side of the bridge circuit, the line impedance of the telephone line is controlled to an optimum value automatically, allowing the circuit to be used for the side tone prevention circuit for a loudspeaker telephone or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a side sound prevention 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. One with a Blitzno circuit configuration is known.

Figure 1 shows the basic configuration of a side sound protection circuit with a bridge circuit configuration. 1 is a transmitter, 2 is a receiver, 3 is a telephone line, 4 is a bridge circuit, and two adjacent Resistors 5 and 6 are connected to the sides, and the line end of the telephone line 30 and the balance network 7 are connected to the other two sides. Balanced network 7 is C, H
The line impedance zt of the telephone line 3 is
has an impedance zn similar to . That is, resistance 5
. 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 zt is an impedance due to the series resistance and line capacitance of the telephone line 3, and it changes depending on the frequency as well as the line length and type of line, so the constant of the balanced network 7 is fixed. In this case, the bridge circuit 4 cannot be sufficiently balanced, and the amount of sidetone attenuation decreases. A slight decrease in sidetone attenuation is usually not a problem, but in loudspeaker telephones that use a microphone and speaker as an interphone, the audio input from the microphone is transmitted via the speaker. There is a risk of howling occurring due to returning to the microphone.

[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 provides a balanced circuit network provided in a bridge circuit.
The resistance and capacitance corresponding to the line impedance of the telephone line are configured by a variable impedance circuit that can change the diimpedance by electronic control, and the impedance is controlled to the optimum value according to changes in the line impedance. It is characterized by the fact that

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 the present invention, a balanced state of the bridge circuit can always be obtained by controlling the impedance of the balanced circuit network to an optimum value in accordance with changes in the line impedance of the telephone line. 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 it is possible to prevent howling.

[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 the transmitter 1 and the receiver 2, respectively, and the terminal voltages of these detectors 11 and 12 are detected. In this example, the detectors 11 and 12 are ones that have logarithmic conversion characteristics by converting the input voltage into a current, passing the current through a diode, converting it back to voltage, and then rectifying and smoothing it. . The difference between the outputs of these detectors 11 and 12 is calculated by the subtracter 13, but this operation is performed by calculating the ratio of the terminal voltages of the transmitter 1 and the receiver 2, since the detection output is expressed in dB by logarithmic conversion. This corresponds to taking . After the output of this subtracter 13 is converted into a sidigital value by a converter 14, a control circuit 1 configured using, for example, a four-microphone grosser
given to 5. The control circuit 15 electronically controls the impedance of the variable impedance circuit in the balanced circuit network 7 so that the input dinotal value from the v-impedance filter 14 is constant, that is, the ratio of the voltages at both terminals is constant.

The balanced circuit network 7 is represented by, for example, an equal circuit shown in FIG.

In FIG. 3, C6l Ro corresponds to the terminal impedance of the exchanger and is a fixed value. Remaining resistance R1, R
, and capacity c1. c.

corresponds to the line impedance Zt of the telephone line 3, and in this embodiment, these R1+R2*c11C2 are constructed by an electronic 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 using this, a balanced circuit network 7 as shown in Fig. 3 can be constructed. An example of the configuration is shown in FIG. 21, 22
, 23.24 are R1* R, l cp + (
It is a variable impedance circuit that constitutes '@.

FIG. 5 shows R1r R2 using the variable impedance circuit described in Japanese Patent Application No. 53-138243.

In the example of Cl1C2, (,) shows an example applied to R1+C1 with both ends in a floating state, and (h) shows an example applied to R2 and C2 with one end in a grounded state.

In Fig. 5(,), the voltage applied between terminals 31 and 32 is applied to both input terminals of a differential input/differential output voltage/current converter (V/I converter) 350 via a voltage follower 33, 34. A current proportional to the voltage difference is caused to flow through the voltage-current conversion impedance element 36 through transistors Q3 and Q4.

Impedance element 36 is a fixed resistance or capacitance. Since the current of the current sources CS5*CS6 does not change depending on the emitter currents of the transistors Q3 and Q4, the change in the current flowing through the impedance element 36 is entirely the change in the difference between the emitter currents of Qj and Q4.
If the pace current is ignored, all of this becomes a change in the collector current and is taken out as a differential output current.

The differential output current is fed back to both ends of the V/I converter 35 via the current feedback circuit 37. In other words, the change in collector current of transistor QsrQa is a diode-connected transistor. 5.

Due to the change in the collector current of Q6, this transistor Q
5. Is the change in the pace-emitter voltage vIlffi of Q6 due to the transistor Q? , Qll changes in the collector current, which becomes the feedback current. This feedback current, DQy, the change in the collector current of Qj is the current source C
The current source C3lI,C varies depending on the current 211 of S.
If the current for 8 seconds is 2, then it is I 1 /I 2 times the change in the collector current of Qs and Q6. In other words, the amount of change in the current flowing through the impedance element 36 is It/I
Since the voltage is multiplied by s and returned to the terminals 31 and 32, the impedance between the terminals 31 and 32 is the value obtained by dividing the voltage difference by the feedback current, which means that the impedance of the impedance element 36 is equal to zo. Then Z=Zo'Iz
/h.

Therefore, by electronically changing the current feedback ratio of the Ig/It+stain current feedback circuit 37, the impedance 2
can be changed. Specifically, current ° sources C83 to C
The currents of S, may be controlled by bias control circuits 25 and 26 shown in FIG. 4 so that rl and r vary differentially.

The impedance z between the terminals 31.3;1 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 R1+C3.

On the other hand, the example shown in Fig. 5(b) used as %R2 or C2 is similar to Fig. 5(-) except for the point plate in which current feedback is applied only to the terminal 31 side because one terminal 32 is grounded. be.

By the way, if you look at terminal 3 in Figures 5(a) and 5(b) and terminal 32 in Figure 5(,), the current source C is 83°C.
84 is connected to transistors Qs + Q7 as a spot current source, and the impedance of the current source is extremely high, so if there is even a slight difference in current between C83 and Q a, + c s 4 and Q7, a large DC offset will occur. Voltage 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 3 in the variable impedance circuit 24
6 is a capacitor, while the impedance element 36 in the variable impedance circuit 21 connected to this circuit 24 via the terminal P7*P2 is a resistor. It is also considered that the DC offset voltage at the terminal P7 of the circuit 2 is canceled at the same time as the DC offset voltage at the terminal P2 of the circuit 2. However, when the equivalent capacitance C2 of the circuit 24 is large and the impedance is low, that is, the amount of current feedback to the terminal P7 is large, and the equivalent resistance R1 of the circuit 21 is large, that is, the amount of current feedback to the terminal P2 is small. It is impossible to correct the DC current error in the circuit 24 with the DC negative feedback of the circuit 21, and as a result, the DC offset voltage at the terminals P2+P7 is not canceled and fluctuates greatly, and in the worst case, it becomes inoperable. becomes. This is exactly the same for circuits 22 and 23.

To avoid such problems, circuit 21.2 is shown in FIG.
4 connection points p, L (terminals P21P7) and the circuit 22.4.
DC offset voltage stabilizing circuits 27 and 28 are connected to the connection point Pb (terminal P3.P,) of 23.

FIG. 6 shows a specific example of a DC offset voltage stabilizing circuit, in which the terminal 4 connected to point Pa or Pb in FIG.
A voltage follower (emitter follower) 42 consisting of a transistor Qll and a current source C811 is connected to the terminal, and the output of this voltage follower 42 is connected to the resistor R11 and the capacitor C1l.
Only the DC component is extracted by the low L filter tie 3. The output potential vd of this low-thousand filter 43 and the reference potential vr are compared by a differential amplifier 44 consisting of transistors Q1z + Qts + Q10 whose common emitters are connected to a current source C81, and a transistor Q1s as a load is compared. Current mirror 45 consisting of +Qss
A current corresponding to the difference between vd and vr flows through the voltage follower 4
It is fed back to the input terminal of 2.

Due to such negative feedback Luno, 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, the alternating current impedance seen from the terminal 4 to the voltage 7 lower 42 and the current mirror 45 is extremely large, so that it hardly affects the original characteristics of the variable impedance circuit.

As described above, the side sound prevention circuit of the present invention has a configuration in which the impedance of the balanced circuit network is automatically controlled to the optimum 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 the drawing]

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 a diagram showing a balanced circuit network. Equivalent circuit diagram, Figure 4 is a configuration diagram of a balanced circuit network constructed using a variable impedance circuit, Figure 5 (,) (b) is a circuit diagram of a variable impedance circuit used in the balanced circuit network of Figure 4, FIG. 6 is a circuit diagram of a DC offset voltage stabilizing circuit in the balanced circuit network of FIG. 4. 1... Transmitter, 2... Receiver, 3... Telephone line, 4
...Bridge circuit, 5, 6...Resistor, 7...Balanced circuit network, 11.12...Detector, 13...Subtractor,
14... Converter, 15... Control circuit, 21-24
...Variable impedance circuit, 25.26...Bias control circuit, 27,2.8...DC offset voltage stabilization circuit, 33.34...Voltage follower, 35...
・Differential input/differential output type voltage-current converter, 36... Voltage-current conversion impedance element, 37... Current feedback circuit, 42... Voltage follower, 43... Ronosu Fini, 44...・Differential amplifier, 45...Current mirror. Applicant's representative Patent attorney Takehiko Suzue Figure 1 Figure 2 Figure 31 Figure 4 Figure 6 Continued from page 1 0 Author: Takeshi Horiuchi 3-1 Asahigaoka, Hino City Tokyo Shibaura Electric Co., Ltd. Hinouchi@ Applicant Toshiba Corporation 72 Horikawa-cho, Saiwai-ku, Kawasaki City

Claims (4)

[Claims] (1) In a sidetone prevention circuit with a bridge circuit configuration having a balanced circuit network on one side having an impedance similar to the line impedance of a telephone line, the resistance and capacitance corresponding to the line impedance in the balanced circuit network are controlled electronically. It is characterized by comprising a variable impedance circuit capable of changing the line impedance, and comprising a control means for controlling the impedance of the variable impedance circuit to an optimum value according to the change in the line impedance. side sound protection circuit. (2) Claims characterized in that the control means detects the terminal voltages of the transmitter and receiver and controls the impedance of the variable impedance circuit so that the ratio of the voltages at both terminals is constant. The side sound protection circuit according to item 1. (3) A balanced circuit network consists of a variable impedance circuit that converts voltage to current. A differential input/differential output voltage/current converter that uses a resistor or capacitor as an impedance element, and a differential output current of this converter that converts the differential output current of this converter. and a current feedback circuit that feeds back to at least one input side of the converter, and by controlling the current feedback ratio of this current feedback circuit, the impedance (resistance or capacitance) between the differential input terminals of the converter is changed. The side sound prevention circuit according to claim 1, characterized in that the side sound prevention circuit is (4) The balanced network connects the connection point of the resistive variable impedance circuit and the capacitive variable impedance circuit to a voltage follower, and guides the output of this voltage follower to a low-nos filter. By comparing the output potential of the filter and the reference potential and feeding back a current corresponding to the difference between the two potentials to the input terminal of the voltage follower, the DC potential at the above connection point is kept equal to the reference potential. The side sound prevention circuit according to claim 1, characterized in that: