JPH02168761A - Side tone prevention circuit for telephone set - Google Patents

Abstract

PURPOSE:To prevent a side tone by revising an impedance of a balance adjustment circuit in response to the measured impedance of a telephone line. CONSTITUTION:The line impedance ZL by an impedance measurement circuit 32 is measured by switching a switch 38 to the position of an oscillator 40 with a command of a control circuit 34, and outputting signal outputted from the oscillator 40 is outputted to a telephone line. Thus, the line impedance ZL of the telephone line measured by an impedance measurement circuit 32 is inputted to the control circuit 34. The control circuit 34 calculates the impedance from the line impedance of the telephone line to decide the resistance and static capacitance in the balance adjustment circuit 14. Then the resistance and capacitance of the balance adjustment circuit 14 are adjusted by the operation of the switch in the balance adjustment circuit 14.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a sidetone prevention circuit for a telephone, that is, a circuit that prevents the output of its own transmitter from going around to the receiver.

[Prior Art] The majority of current telephones are connected to a two-wire telephone line, which is separated into a receiver side circuit and a transmitter side circuit by a 2-wire to 4-wire conversion circuit to transmit signals. . If the output of the user's own transmitter is routed to the receiver, the user's own voice can be heard from the receiver, making it impossible to have a good conversation. For this reason, conventional telephones often have a sidetone prevention circuit using an impedance bridge in order to prevent this sidetone.

Therefore, a sidetone prevention circuit in a conventional telephone will be explained based on FIG. 9. In the figure, a telephone office 100 has an exchange that connects a plurality of telephone lines, and this exchange has one trunk circuit 10 for one connected telephone line. A telephone line from this telephone office 100 is connected via a telephone line 200 to a telephone 300 provided in each home.

Here, as schematically shown in the figure, the telephone line 200 consists of the resistance of the telephone line itself, which changes depending on the wire diameter, distance, etc., and the stray capacitance riding on this resistance. For this reason,
Telephone line 200 can be represented by a resistor and a capacitor as shown.

The line impedance ZL seen from the telephone 300 is the telephone a! The connection end of 300 is determined by the impedance of the trunk circuit 10 and the telephone line 200 at the central office 100 as seen from L2.

The telephone 300 has an internal circuit 12 that connects the connection end and the L2, and the internal circuit 12, the telephone line 200, and the trunk circuit 10 are connected in parallel. The connecting end L2 is grounded, and balanced impedance elements 'f-R, , R, , for configuring a bridge circuit are connected in series to this connecting end L2. In addition, a balance adjustment circuit 1 is connected to the connection end L1.
One end of the balance adjusting circuit 14 is connected to the other end of the balancing resistor R8.

As a result, the resistance for harasis R,,R,. , balance adjustment circuit] 4 and the internal circuit 12 and the external line are connected in parallel to form an impedance bridge.

Then, a handset] 6 is connected between the connecting end L2 and the connecting point of the resistor R4o and the balance adjustment circuit 14, and the transmitter 1
8 is connected between the connection point and the connection end of the resistor R9 and the resistor R1o.

Therefore, if these impedances satisfy the relationship (ZL //ZSN) ・R+o=ZuN-R9, this impedance bridge will satisfy the equilibrium condition, and the output of the transmitter 18 will enter the receiver 16. There isn't.

[Problem to be solved by the invention] In such a conventional impedance bridge,
If the above-mentioned equilibrium conditions are met, sufficiently good sidetone prevention can be achieved.

However, the line impedance Z seen from the telephone 300 side
1. varies significantly depending on the wire diameter and distance of the telephone line 200. Zunawaji, the wire diameter of the telephone line 200 is 0.32.0.
4, 0.5.0.65.0. Depending on the distance of the telephone line 200, the signal level loss may be 0.
There is a change of about 7 dB. Therefore, the line impedance ZL of the telephone line changes significantly depending on the installation state of the telephone 300.

However, the noise adjustment circuit 1 in conventional telephones
4 is constituted by a fixed CR circuit network, so it cannot cope with the impedance Zl of the telephone line, which changes greatly.

In other words, the impedance bridge satisfies the equilibrium condition only when the impedance Z1 of the telephone line changes significantly.
．． There is no equilibrium under other conditions. Therefore, if the line impedance Zl changes,
There is a problem in that it is not possible to effectively prevent sidetone.

This invention was made with the aim of solving the above-mentioned problems, and is a telephone side system that can balance impedance bridges and obtain good sidetone prevention characteristics under a wide range of telephone line conditions. It is an object of 181 to provide a tone prevention circuit.

[Means for Solving the Problem] A sidetone prevention circuit for a telephone according to the present invention includes an impedance measurement circuit that is connected to a connection end of a telephone line and measures impedance on the telephone line side; A circuit that connects the handset and the handset, and includes an internal circuit of the phone having a predetermined impedance, a balance adjustment circuit having a balanced impedance element, an ijJ variable resistance element and a 1J variable capacitance diode, and an impedance adjusting circuit at the handset connection end. Depending on the balance bridge circuit and the measurement results in the impedance measurement circuit,
a control circuit that controls impedance by changing the resistance value of the variable resistance element and the capacitance value of the variable capacitance diode of the balance adjustment circuit; By changing the impedance of the balance adjustment circuit, the balance condition of the bridge circuit is increased by 7IN, and sidetone is prevented.

[Function] Telephone site dawn prevention circuit according to the present invention 1. O. Since it is configured as described above, the impedance measurement circuit can measure the 1'-width of the telephone line6, and 2. The impedance is adjusted by changing the resistance value of the variable resistance element of the adjustment circuit and the capacitance value of the variable capacitance diode.

Therefore, even if the line impedance in the telephone line fluctuates greatly, the bridge circuit can always be balanced, and sidetone can be effectively prevented. In particular, since the resistance value of the variable resistance element and the capacitance value of the variable capacitance diode can be directly changed by an electric signal, the adjustment can be performed very easily.

[Example] Hereinafter, an example of a telephone according to the present invention will be described based on the drawings.

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of a telephone according to the present invention. In the figure, the connection end of the telephone line, L2, is connected to a two-wire external telephone line, and is a rectifier circuit 30 consisting of a bridge connection of diodes.
It is connected to the.

The negative connection end of this rectifier circuit 30 is grounded, and the positive connection end is connected to an impedance measurement circuit 32.

The impedance measuring circuit 32 measures the impedance on the telephone line side as appropriate based on instructions from the balance control circuit 34. The output side of the impedance measurement circuit 32 is connected to a bridge circuit 36 for sidetone prevention. In addition, under normal conditions, impedance mill + foot circuit 3
2 is not necessary, so it is better to bypass it and allow the signal to be transmitted.

This sidetone prevention bridge circuit 36 includes the telephone internal circuit 12, balance adjustment circuit 14, balance impedance elements R, ,R,
Consists of o. Then, in this bridge circuit 36,
A receiver 16 and a transmitter 18 are connected.

Here, the receiver 16 includes an operational amplifier 16a and a speaker SP, and the receiver 18 includes a current regulator 18a1.
It consists of an operational amplifier 18b3 microphone MIC. In this invention, this microphone MI
A switch 38 is inserted between C and the operational amplifier 18b, and this switch 38 can connect the oscillator 40 to the operational amplifier 18b.

In the telephone configured as described above, when making a normal call, when the caller inputs voice into the microphone MIC of the transmitter 18, the current regulator 18a responds accordingly.
A predetermined current flows through the connection end, L2,
It is output to the telephone line.

Further, the signal input from the telephone line is output as voice from the speaker SP of the handset 16.

Then, the impedance of the bridge circuit 36 is in a balanced condition (Z+, // Z SN) ・ R 0 - Z
If BN-Ro is satisfied, the signal input from the transmitter 18 will not be output from the receiver 16.

Here, a feature of the present invention is that the impedance measuring circuit 32 can measure the impedance of the telephone line to which the telephone is connected, that is, the line impedance ZL. The measurement of the line impedance Z by the impedance measurement circuit 32 may be performed when the telephone is installed or at an appropriate timing.

This measurement is then controlled by a control circuit 34.

That is, the switch 38 is switched to the oscillator 40 side by a command from the control circuit 34, and the oscillator 40 emits a signal at a predetermined frequency. At this time, the balance adjustment circuit 14 is disconnected from the circuit by a signal from the control circuit 34. Therefore, a predetermined signal outputted from the oscillator 40 is outputted to the telephone line, and the impedance measurement circuit 3
2, line impedance ZL can be measured.

The line impedance Zl of the telephone line thus measured by the impedance measuring circuit 32 is input to the control circuit 34. The control circuit 34 performs a predetermined calculation based on the line impedance of the telephone line to determine the resistance value and capacitance value in the balance adjustment circuit 14. Then, the resistance value and capacitance value in the balance adjustment circuit 14 are adjusted to predetermined values by operating a switch in the balance adjustment circuit 14 or the like.

Since the line impedance ZL of the telephone line is measured as described above and the resistance value and capacitance value of the balance adjustment circuit 14 are adjusted accordingly, the bridge circuit 36 in the present invention ensures that the impedance 2+- is always balanced. Become. Therefore, the line impedance ZL of the external telephone line
Good sidetone prevention can always be achieved even when there are large fluctuations in sidetone.

Measurement of Line Impedance Zl Here, the measurement of line impedance Zl will be explained. Examples of this measurement method include a resistance substitution method and a method of measuring voltage and current.

Therefore, first, the line impedance ZL using the resistance replacement method is
The measurement will be explained based on FIG. 2(A). In the figure, the line impedance of the telephone line at the connection terminal L2 of the telephone 300 is schematically shown as ZL. Then, the telephone circuit 52 for operating the telephone as a regular telephone is disconnected by the changeover switch 50, and the oscillator 40 is connected. In this example, impedance measurement circuit 32 includes an oscillator 40 therein.

Furthermore, as shown in FIG. 2(B), a switch 50 may be provided that bypasses the impedance measurement circuit 32 during normal times.

The impedance measuring circuit 32 has a resistor R and two voltage measuring devices 54 and 56, and includes a voltage V1 on the oscillator 40 side of the resistor R and a voltage V2 after passing through the resistor R.
can be measured. For this reason, the oscillator 40
By transmitting a signal at a predetermined frequency f, the line impedance ZL at that frequency f can be determined from the following equation.

ZL I I=R/ (Vl /V2 1) Also,
When measuring the circuit impedance ZL using the voltage-current measurement method, a voltage measuring device 60 is used as shown in FIG. 2(C).
and a current measuring device 62 are provided on the path from the oscillator 40 to the external telephone line.

Then, the circuit impedance zI can be obtained from the voltage V and current I, which are the detected values of the voltage measuring device 60 and the current measuring device 62, as shown in the following equation.

Z,, t=V/I Calculation of line impedance ZL In this way, line impedance ZL is calculated, and this line impedance Z1. consists of a combination of resistance components R, , R2 and stray capacitance NC shown in FIG. Therefore, calculation of these values will be explained below.

Here, the line impedance zL is equivalent to the circuit shown in FIG. 3, which is a combination of a resistor and a capacitor. That is,
A circuit in which a parallel connection circuit of a resistor R0, a resistor R2, and a capacitor C are connected in series is equivalent to the line impedance ZL. In such a circuit, the line impedance 21 changes depending on the frequency f of the signal applied thereto.
will be different.

The frequency characteristics of this line impedance ZL are shown in FIG. A capacitor has low impedance at high frequencies and high impedance at low frequencies. Therefore, as shown in FIG. 4, at a frequency of about 300 Hz, the value of the capacitor C is almost infinite, and at a frequency of about 10 kHz, it becomes a short-circuit state of approximately OΩ. Therefore, by measuring the line impedance ZL at 10 kHz, the resistance R7 of the circuit can be determined.

R+ = l Zt l +oi Also, the line impedance Z1 at 300 Hz. is an impedance formed by connecting a resistor R4 and a resistor R2 in series. Therefore, the line impedance ZL at that time
The value obtained by subtracting the value of the resistor R1 becomes the value of the resistor R2.

R2-IZL 1300 R+ Next, the value of capacitor C in the line impedance ZL can be found from the line impedance Z1 at frequency 11 (Hz).As shown in Figure 4. , line impedance Z at a frequency of 1 kHz is for a parallel combination circuit of resistor R1, resistor R2, and capacitor C.Here, Z is the impedance of the parallel combination circuit of resistor R2 and capacitor C.

shall be. And the locus of impedance of this circuit is
The result is as shown in FIG.

Therefore, line impedance ZL can be expressed as follows.

Zt 1-(R+ →-ZC'CO3θ)2+(Zo
-5in θ)2 Then, by transforming this, the following equation is obtained.

Zl, l −R,” = 2・R,・
Zo −cos θ+Zo 2 On the other hand, since the resistor R2 and the capacitor C are connected in parallel, their impedance is expressed as follows.

1/Zc" = 1/Xc 2+1/R2"Here, Xc
=11/ω·C1.

Further, regarding the parallel circuit of resistor R2 and capacitor C, there is the following relationship.

θ"" ta, n −' R2/X c Therefore, the following relationship is derived.

cosθ−Xc/(Xc”+R2”)”
``Using this relational expression, we obtain the following.

Xc = (R2” ・ (ZL 2 R+ ”)/
(2・R2・R2+R2″−ZL2 10R, 2) l′/2 And since ω−2πf (f is the frequency), the value of the capacitor C can be obtained as follows.

C=1/2πf・(R22・(Zt,” R+2
)/(2・R1・R2+R2”−ZL”+R,2) l I/2 In this equation, the line impedance Z1 at the frequency of 1kHz (f=1kHz) mentioned above is added. By substituting the value of , the value of capacitor C in the bridge circuit can be found.

In this way, line impedance ZL can be determined.

Adjustment of the impedance ZSH of the telephone internal circuit Next, unless the impedance of the telephone internal circuit 12 is within a predetermined range, the impedance ZBN in the balance adjustment circuit 36 and the other circuits cannot be matched. The impedance ZSN of this internal circuit of the telephone may be set to, for example, about 62 ohms. In this case, it is a good idea to insert a resistor at a predetermined location in the phone's internal circuit and adjust its value.

Adjustment of balance adjustment circuit ZBN When the line impedance Z1 and the impedance ZSN of the internal circuit 12 are determined in this way,
In accordance with this, the impedance Z of the balance adjustment circuit 14
Adjust BN. That is, the balance adjustment circuit 14 is
As shown in Figure 6, variable resistance element R, □, RI 1%
It has a ROBs capacitor C131 and a variable capacitance diode (varicap diode) C132, whose resistance value and capacitance value are changed by commands from the control circuit 34.

The capacitance value of the varicap diode Cl32 changes depending on the value of the reverse voltage applied thereto, and the capacitance value changes depending on the value of the reverse voltage applied by the variable DC power supply Ev. And this varicap diode CI
Due to the change in capacitance value of 3°, capacitor C43,
The capacitance value of the series connection of the varicap diode CI3□ and the varicap diode CI3□ is adjusted to a predetermined value. Note that the coil L1 is used to cut off the alternating current component and maintain the reverse voltage applied to the varicap diode C032 at a predetermined value.

Further, the resistance values of the variable resistance elements R1°, R, SR8, are set to predetermined values in response to commands from the control circuit 34, and, for example, those shown in FIG. 7 are employed.

In other words, what is shown in FIG. 7(A) uses an LED-CdS (light emitting diode cadmium sulfide element) as a variable resistance element. In this LED-CdS, the resistance value of the cadmium sulfide resistance element CdS changes depending on the amount of light emitted from the light emitting diode LED. Since the light emitting diode LED changes the amount of light emitted depending on the amount of current, the resistance value of the cadmium sulfide resistance element changes depending on the amount of current. Therefore, the resistance value can be controlled by changing the amount of current.

What is shown in FIG. 7(B) is a Lam as a variable resistance element.
p-Cd5 (lamp cadmium sulfide element). In this variable resistance element, the amount of light emitted by the lamp changes depending on the amount of current supplied to the lamp Lamp, and the resistance value of the cadmium sulfide element changes accordingly. Therefore, the resistance value can be controlled by changing the amount of current.

What is shown in FIG. 7(C) is a MOS-FET (Meta
l Oxide Sem1conductor Fie
ld Effect Transistor). That is, in MOS-FET,
The electrical resistance between the source and drain changes depending on the voltage applied to the gate. Therefore, the resistance value can be controlled by this gate applied voltage.

Furthermore, what is shown in FIG. 7(D) is a J-F ET (Junction-type F
This method uses a field effect transistor. In this example as well, the above MOS-
As in the case of FETs, the resistance value can be controlled by the voltage applied to the gate.

In this way, by employing the variable resistance elements shown in FIGS. 7(A) to 7(D), the resistance value can be controlled by electrical signals. Therefore, by forming this electrical signal in accordance with the signal from the control circuit 34, the resistance value of the variable resistance element can be set to a desired value. Furthermore, the capacitance of the varicap Cl32 can be controlled by adjusting the voltage value of the variable power source Ev using a signal from the control circuit 34.

In addition, the resistance R8I in the balance adjustment circuit 14], R
, , , R,2, as an example of the value of the capacitance C131, the telephone internal circuit 12 and the impedance Z.

In relation to this, the following values can be cited:

For example, the balance resistor R9, RIO is R9=47Ω R. -470Ω, 10 times the value, is adopted, and the resistance value of internal circuit 12 is 620Ω.
If set to Ω, the resistance Ro8 in the balance adjustment circuit 14 corresponding to the resistance in the internal circuit 12 will be: Ron=1. OX ZsN= 10 x 620Ω
It is sufficient to set it to -6,21cΩ.

Also, the resistances R, 2, R, of the balance adjustment circuit 14.

and capacitor CI3 each have a line impedance of 2.
．． It may be determined as follows, corresponding to the resistances R3 and R2, and the stray capacitance MC.

RI2=10 Therefore, by adjusting the impedance of the balance adjustment circuit 14, the bridge circuit 36 always maintains the balanced condition (ZL//ZSN) ・R4°=
It can be adjusted to satisfy ZBN'R9.

It is also understood that by appropriately selecting the values of the balance resistors R, , R, o, the resistance value in the balance adjustment circuit 14 can be easily adjusted. In the above embodiment, the balance resistor RO is used as the balance impedance element.

Although RIG is adopted, as is clear from the above equation, the same effect can be obtained if the impedance element has a constant ratio.

FIG. 8 shows sidetone prevention characteristics of a bridge circuit according to an embodiment of the present invention. In this figure, the horizontal axis represents signal level loss (line impedance) in the telephone line, and the vertical axis represents sidetone. From FIG. 8, it can be seen that according to the present invention, sidetone can always be effectively prevented even if the magnitude of the signal level loss (line impedance) of the telephone line changes. As shown in the figure, according to the conventional example, when the magnitude of signal level loss (line impedance) of the telephone line changes, good sidetone prevention can be achieved only at the - point.

Therefore, in the embodiments of the present invention, good sidetone prevention can always be achieved.

Note that this adjustment does not need to be performed all the time, and may be performed at a predetermined frequency other than when the telephone is installed.

[Effects of the Invention] As explained below, according to the sidetone prevention circuit of the telephone according to the present invention, even if the impedance of the telephone line changes, good sidetone prevention can always be achieved. .

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of a sidetone prevention circuit for a telephone according to the present invention, and Figs. 2 (A), (B), and (C) show examples of an impedance measuring circuit in the same embodiment. 3 is a circuit diagram showing an equivalent circuit of line impedance ZL, FIG. 4 is a characteristic diagram showing frequency characteristics of line impedance ZL, and FIG. 5 is an explanatory diagram showing the locus of line impedance ZL. Fig. 6 is a circuit diagram showing an example of a balance adjustment circuit, Fig. 7 is a circuit diagram showing an example of the configuration of a variable resistance element, Fig. 8 is a characteristic diagram showing sidetone prevention characteristics of the embodiment, and Fig. 9 is a conventional one. FIG. 2 is a configuration diagram showing an example of a sidetone prevention circuit of a telephone. 12... Internal circuit 14... Balance adjustment circuit 16... Receiver 18... Transmitter 32... Impedance measurement circuit 34...
Control circuit 36 ... Bridge circuit procedural amendment (method) Indication of the case 1988 Patent application No. 323868 Name of the invention Telephone side] Connection to the case of person who makes corrections to the on prevention circuit Patent applicant representative Sawara Research Department Agent 5゜Date of amendment order J-Initial supplementary letter E (spontaneous) Incident display tff ($11 f53 Patent! 111) J'l
No.:'(2:3a 6 g2, Name of the invention Telephone No.: 19j Stop circuit 4, generation

Claims (1)

[Claims] (1) An impedance measurement circuit that is connected to the connection end of a telephone line and measures the impedance on the telephone line side, and a circuit that connects the connection end to a transmitter and a handset, and that has a predetermined impedance. a balance adjustment circuit having a balanced impedance element, a variable resistance element, and a variable capacitance diode; a bridge circuit that balances impedance at the receiver connection end; A control circuit that controls impedance by changing the resistance value of the resistor element and the capacitance value of the variable capacitance diode, and by changing the impedance of the balance adjustment circuit according to the measured impedance of the telephone line. A sidetone prevention circuit for a telephone, characterized in that it satisfies the equilibrium conditions of a bridge circuit and prevents sidetone.