JPH01233867A - Side tone prevention circuit for telephone set - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Summary] Regarding a sidetone prevention circuit for a telephone, the purpose is to prevent sidetone during dialing and depending on the condition of the communication line, and the circuit is operatively connected to the communication line. A field effect transistor is connected to a side opposite to one side of the balanced bridge circuit, and a control circuit is connected to the gate of the field effect transistor. The resistance value of the field effect transistor is made small, and after dialing is completed, the resistance value of the field effect transistor is set to the value in the balanced state of the balanced bridge circuit in the communication state.

[Industrial Application Field] The present invention relates to a sidetone prevention circuit for telephones, which effectively prevents sidetone especially during dialing, and also performs sidetone prevention appropriately depending on the line condition of the telephone line. This invention relates to a sidetone prevention circuit for telephones.

[Prior art and problems to be solved by the invention]

If the audio from the handset leaks to the handset of that phone,
Since it is difficult to hear the other party's call, and in severe cases, it can cause howling, an electrical circuit is installed in the telephone to prevent sidetone, the sound based on the leaked signal from the transmitter. The sidetone prevention circuit eliminates the electrical coupling between the transmitter and the handset.

FIG. 5 shows a side sound protection circuit for a conventional electronic telephone.

The side sound protection circuit basically consists of a bridge circuit 10'.

The bridge circuit lO' connects the telephone line (
LINE) is operatively connected, and the other party's phone is further connected. The other bridge sides b-c, c-d, and d-a each have a resistor 13 (its resistance value R13=18Ω), a resistor 17 (its resistance value R1?=180Ω), and a balance circuit m
1l (its impedance Z□) is connected. This bridge circuit 10' estimates the impedance Z of the bridge side a-b, which changes depending on the communication line distance with the other party's telephone, to a standard value, and under that condition, the bridge circuit 10'
′ is kept in equilibrium.

A resistor 15 (
The transistor 16 to which the transmitter 21 is connected is connected to the resistance value R, , =24Ω).

A capacitor 41 and a resistor 42 are connected to the terminal d of the bridge circuit 10' to cut the DC component and pass the AC audio component.
An amplifier 43 is connected to the series circuit of the receiver 22, and the output of the amplifier 43 is connected to the receiver 22.

The bridge circuit 10' detects a direct current and makes the potential difference between terminals a and C zero under bridge equilibrium conditions, thereby eliminating the electrical coupling between the transmitter 21 and the receiver 22, that is, the sidetone. However, since the impedance ZL of the bridge sides a-b changes depending on the other party's telephone, the electrical coupling does not always completely disappear, but the electrical coupling attenuates around the estimated impedance.

However, the sidetone prevention circuit of FIG. 5 has the following problems.

First, the first problem is that side noise prevention is performed by fixing the impedance ZL of the communication line to a standard value, so the side a prevention amount changes depending on the distance of the communication line, and the sidetone prevention is not sufficient. There is.

Next, as a second problem, in addition to the problem of preventing sidetone during a call, there is a problem that sidetone cannot be prevented during dialing. When the telephone is dialed off-hook, the telephone line is in an open state. Therefore, the bridge sides a-b of the bridge circuit 10' in FIG. 5 are open, and the impedance of the bridge sides becomes very large. Therefore, the bridge circuit lO' is completely out of equilibrium, and sidetone prevention becomes ineffective.

FIG. 6 shows another conventional sidetone prevention circuit. This sidetone prevention circuit solves the first problem mentioned above. That is, a plurality of balance circuits 'fI4ttat to 11.3 are provided in parallel on the bridge side d-a of the bridge circuit 10'', and a switch ub is provided to select one of them.

Furthermore, a line condition detection circuit 30' is provided to measure the amount of leakage on the receiver 22 side and select a balanced circuit network that minimizes the amount of leakage.

This makes it possible to appropriately prevent sidetones during calls.

However, even in this sidetone prevention circuit, the above-mentioned second
issue has not been resolved. Therefore, there remains the problem that howling occurs, especially in loudspeaker telephones.

Furthermore, the following problem is unique to the sidetone prevention circuit shown in FIG.

First, multiple balancing networks and switches must be provided,
There are problems in that the circuit is complex and expensive.

Another problem is that it takes several seconds or more to detect line conditions and select a balanced network, resulting in poor responsiveness.

Therefore, it is desired to be able to effectively prevent sidetones during a call with a simple circuit and with good response, and to also be able to effectively prevent sidetones when dialing. What about this invention?
It is intended to solve the problems that arise.

[Means for Solving the Problems and Effects] FIG. 1 shows a block diagram of the principle of a sidetone prevention circuit for a telephone according to the present invention.

In FIG. 1, the bridge circuit 10 has bridge sides a-
A telephone line is operatively connected to b, a fixed resistor 13 is connected to bridge side b-c, and one balancing circuit Xll is connected to bridge side d-a. Further, a field effect transistor 12 is connected to the bridge side c-d opposite to one side of the balanced bridge circuit to which the speech line is operatively connected.

Further, a control circuit 30 is connected to the gate of the field effect transistor 12.

In the above configuration, the field effect transistor 12 has a function as a variable resistance value whose drain-source resistance value changes in response to a gate signal.

On the other hand, the control circuit 30 has a function of detecting dialing.

The control circuit 30 reduces the resistance value of the field effect transistor when dialing the telephone, and after dialing is completed, the resistance value of the field effect transistor is set to the value in the balanced state of the balanced bridge circuit in the talking state. This makes it possible to prevent sidetones during dialing with a simple circuit configuration.

Next, the control circuit 30 has a function of detecting the line state.

The control circuit 30 changes the resistance value of the field effect transistor depending on the line condition of the communication line. This makes it possible to prevent sidetone during calls with a simple circuit configuration.

According to the present invention, the control circuit 30 can perform the above-described operation during dialing and operation during a call in combination. This makes it possible to prevent sidetones both when dialing and during a call.

Embodiment C] As a first embodiment of the present invention, a sidetone prevention circuit during dialing will be described with reference to FIG.

The configuration of the bridge circuit 10 in FIG. 2 is the same as in FIG. Resistor 13 with resistance value R+3=18Ω, side a-
A resistor 15 having a resistance value R, .=24Ω and a transistor 16 are connected between the resistor 15 and the transistor 16. However, the bridge side c-
A field effect transistor (F
ET) 12 are connected.

A microcomputer (μC) is installed at the gate of FET12.
) is connected to the control circuit 30a.

In this embodiment, II C in the control circuit 30a is
The PE 712 is controlled to detect dialing and prevent sidetone during dialing.

Fig. 3 shows a control circuit 30 that prevents sidetone during dialing.
The operation flowchart of a is shown.

In FIG. 3, the control circuit 30a is configured so that the telephone
Step 01: 5Q to detect whether a signal has been received or not
L) If received, an incoming call flag is set (302).

Next, it is checked whether it is an off-footer (SO3). If it is not an off-footer, the process returns to step O1.

After the off-footer is detected, it is checked whether the incoming call flag is set (SO4). If the incoming call flag is not set,
Since dialing is in progress, a control signal is output to FET 12 to set the corresponding resistance value of FET 12 (S0
5). The resistance value of FET 12 at this time is, for example, 18Ω
It is. This brings the bridge circuit 10 into an equilibrium state when the speech line becomes open. The above state is maintained until the end of dialing (SO6).

After dialing is completed or when the incoming call flag is set, the control circuit 30a outputs a control signal to the FEET 12 so that the resistance value of the FET 12 is in a balanced state during the call, for example, 180Ω. (SO7)

As described above, side noise can be prevented even during dialing.

FIG. 4 shows a circuit for appropriately preventing sidetone during a call as a second embodiment of the present invention.

The bridge circuit 10 is similar to that in FIG.

However, the control circuit 30b is configured to detect line conditions. The control circuit 30b is also realized by μC.

The control circuit 30b directly measures the amount of leakage of the transmitted voice on the receiver side, for example, the current flowing through the terminal d, and changes the FET 12 in the range of, for example, 180Ω±90Ω according to the amount of leakage.

Thereby, sidetone prevention during a call can be appropriately performed depending on the distance of the call line.

In FIGS. 5 and 6, the bridge circuit IO and others may be completely the same, and only the control circuit 30a or 3pb may be changed.

Incidentally, in order to appropriately prevent side noise during dialing and during a call, the above-mentioned control circuits 30a and 30b may be integrated as a control circuit.

〔Effect of the invention〕

As described above, according to the present invention, sidetone prevention during dialing and/or
Appropriate prevention of sidetone during a call becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a principle block diagram of a sidetone prevention circuit for a telephone according to the present invention, Fig. 2 is a diagram of a sidetone prevention circuit during dialing as a first embodiment of the present invention, and Fig. 3 is a control circuit in Fig. 2. FIG. 4 is a sidetone prevention circuit diagram during a call as a second embodiment of the present invention, and FIGS. 5 and 6 are conventional sidetone prevention circuit diagrams. (Explanation of symbols) IO... Bridge circuit, 12... Field effect transistor, 30... Control circuit, 30a... Control circuit for dialing detection, 30b...
・Control circuit for detecting track conditions.

Claims (1)

[Scope of Claims] 1. In a balanced bridge type sidetone prevention circuit provided between a transmitter and a handset to prevent sidetone to the handset, a balanced bridge circuit to which a speech line is operatively connected is provided. A field effect transistor (12) is connected to the side opposite to one side, and a control circuit (30) is connected to the gate of the field effect transistor. A side tone of a telephone set, characterized in that the resistance value of the field effect transistor is made small during dialing, and the resistance value of the field effect transistor is set to the value in the balanced state of the balanced bridge circuit in the talking state after dialing is completed. prevention circuit. 2. The sidetone prevention circuit according to claim 1, wherein the control circuit (30) is further configured to change the resistance value of the field effect transistor depending on the line condition of the speech line.