GB2043398A

GB2043398A - Telephone circuitry

Info

Publication number: GB2043398A
Application number: GB7906838A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1979-02-27
Filing date: 1979-02-27
Publication date: 1980-10-01
Also published as: GB2043398B

Abstract

In a telephone set using loop-disconnect dialling, it is desirable to mute the receiver Rx during dialling, and to do this in an electronic set, electronic circuitry is to be preferred for this function as relays tend to be costly. In the present arrangement the receiver Rx is connected to the transmission circuit Tx CCT via two diodes D6, D7. When the set is in the speech mode, direct current flows in the receiver and in these two diodes, which latter are then in their low dynamic impedance state. Hence they have little or no effect on the efficiency of the instrument, unlike certain known circuits. For a period which embraces the loop disconnect impulses of each digit, the direct current is cut off, via Transistor T3 controlled by IC1 so that the diodes D6 and D7 are then in their high dynamic impedance states, so that the receiver is electrically isolated from the transmission circuit, and is therefore muted. <IMAGE>

Description

SPECIFICATION Telephone circuit This invention relates to an electronic circuit for use in a telephone subscriber's instrument.

When dialling or otherwise transmitting wanted number information from such an instrument it is necessary to disable (mute) the receiver, i.e. the ear piece, from the transmission circuit, so that the subscriber does not suffer from acoustic shock due for instance to loop disconnect signalling. In a conventional telephone instrument this disablement is effected by the dial off-normal contacts. In certain modern instruments, the dial is replaced by an electronic unit controlled, for instance, by push-buttons.

In such a case it is preferable for the off-normal contacts to be replaced by electronic circuitry, as this is cost-effective. However, in some such circuits the electronic circuitry which replaces the off-normal contacts may impair the performance of the transmission circuit when the instrument is in the speech mode.

An object of the invention is to provide a circuit in which the above-mentioned disadvantage is minimised or overcome.

According to the present invention there is provided an electronic circuit for use in a telephone subscriber's instrument, in which the telephone receiver is connected to the instrument's transmission circuit via diode means so poled that when the receiver is in use a direct current flows in that receiver and in the diode means, in which when the said direct current flows the diode means is in its low impedance condition such that the low dynamic impedance of the diode means has little or no effect on the receiving level, and in which when the subscriber dials or otherwise transmits wanted number information said direct current flow is interrupted, such that the diode means has a high dynamic impedance, so that the receiver is electrically isolated from the transmission circuit.

An embodiment of the invention will now be described with reference to the accompanying drawing, which is a circuit diagram of a telephone subscriber's instrument embodying the present invention.

In the instrument whose circuit is shown, the conventional dial is replaced by a push-button set, the operations of the button for digit sending controlling an integrated circuit IC1, which produces trains of correctly-timed break impulses in response to the push-button operation. The integrated circuit unit used in a Siliconix DF320, although it will be appreciated that other units which provide the same functions may be used. This DF320 unit contains the logic and timing circuitry needed to interface a conventional push-button set to a telephone system responsive to loop-disconnect signalling. Timing is derived from an on-chip oscillator using a 3.58 MHz crystal X1, and the circuit can store up to 20 digits.

The unit has a dial pulsing output DP for controlling the impulsing and a masking output M1 for controlling the muting function. Solid-state switching is preferred because of the high cost of relays.

In the drawing we see the circuit of a telephone set embodying the present invention, which is connected to the line via hook switch contacts HS1, HS2, a resistor R1 and a suppressor ZD1 for the suppression of line transients and a diode bridge D1 to D4 to ensure that the circuit is insensitive to changes in line polarity. The muting circuit includes transistor T3, resistor R7 and diodes D6 and D7.

When the hook switch contacts HS1, HS2 are closed on off-hooking, direct current initially flows via a diode D5, transistor T1, resistor R4 and the transmission circuit, referenced Tx CCT. Base cur rentfrom IC1 turns the transistor T1 on sothatT2 receives base drive and is therefore turned on. At the same time the capacitor C1 is charged to (Vz - 0.7 ) volts, where Vz is a voltage determined by the zener diode ZD2. When C1 is charged to the operating voltage of the unit IC1, both its output DP and M1 become low. Hence T1 and T2 remain on, and T3, in the muting circuit, is turned on.

With transistor T3 on, some of the line current flows through T3 and the resistor R7. This current is then divided so as to flow via D7 and via the series combination of the receiver Rx and diode D6. However, all the line current flows through the transmitter Tx. The dynamic impedance of diodes D6 and D7, which is inversely proportional to the forward current, is low, so that the receiver Rx is active and is in normal usage. In this condition the presence ofthe diodes D6 and D7 has very little effect on the operation of the receiver.

When the first keyed digit is reconised, the unit IC1 causes the output M1 to go to its logic 1 state, which turns the transistor T3 off. Hence as the line current no longer flows in D6 and D7, the dynamic impedance of these diodes becomes very large. Hence the receiver Rx is electrically isolated from the transmission circuit, so that the receiver is muted.

The impulsing occurs through the DP output, which turns T1, and thereafter T2, off for each break impulse to be sent. During the break period, rapid discharge of C1 via ZD2 is prevented by the diode D5. The value of C1 is so chosen as to ensure that its voltage does not fall below 2.5 volts during the break period. When "dialling" is completed, the circuit returns to the speech mode with transistors T1, T2 and T3 on.

Such a circuit outpulses successfully with a line current as low as 11 mA, below which value the feed bridge relay at the telephone exchange fails to operate. At the upper end, the limiting factor is the dissipation of ZD2, which can take a maximum forward current of 101 mA at an ambient temperature of 45"C.

With 60 volts, and a 2 x 200 ohm feed, the line current is 90mA at zero line.

The transmission circuit is not described in detail as it follows conventional principles.

1. An electronic circuit for use in a telephone subscriber's instrument, in which the telephone receiver is connected to the instrument's transmission circuit via diode means so poled that when the receiver is in use a direct current flows in that receiver and in the diode means, in which when the said direct current flows the diode means is in its low

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Telephone circuit This invention relates to an electronic circuit for use in a telephone subscriber's instrument. When dialling or otherwise transmitting wanted number information from such an instrument it is necessary to disable (mute) the receiver, i.e. the ear piece, from the transmission circuit, so that the subscriber does not suffer from acoustic shock due for instance to loop disconnect signalling. In a conventional telephone instrument this disablement is effected by the dial off-normal contacts. In certain modern instruments, the dial is replaced by an electronic unit controlled, for instance, by push-buttons. In such a case it is preferable for the off-normal contacts to be replaced by electronic circuitry, as this is cost-effective. However, in some such circuits the electronic circuitry which replaces the off-normal contacts may impair the performance of the transmission circuit when the instrument is in the speech mode. An object of the invention is to provide a circuit in which the above-mentioned disadvantage is minimised or overcome. According to the present invention there is provided an electronic circuit for use in a telephone subscriber's instrument, in which the telephone receiver is connected to the instrument's transmission circuit via diode means so poled that when the receiver is in use a direct current flows in that receiver and in the diode means, in which when the said direct current flows the diode means is in its low impedance condition such that the low dynamic impedance of the diode means has little or no effect on the receiving level, and in which when the subscriber dials or otherwise transmits wanted number information said direct current flow is interrupted, such that the diode means has a high dynamic impedance, so that the receiver is electrically isolated from the transmission circuit. An embodiment of the invention will now be described with reference to the accompanying drawing, which is a circuit diagram of a telephone subscriber's instrument embodying the present invention. In the instrument whose circuit is shown, the conventional dial is replaced by a push-button set, the operations of the button for digit sending controlling an integrated circuit IC1, which produces trains of correctly-timed break impulses in response to the push-button operation. The integrated circuit unit used in a Siliconix DF320, although it will be appreciated that other units which provide the same functions may be used. This DF320 unit contains the logic and timing circuitry needed to interface a conventional push-button set to a telephone system responsive to loop-disconnect signalling. Timing is derived from an on-chip oscillator using a 3.58 MHz crystal X1, and the circuit can store up to 20 digits. The unit has a dial pulsing output DP for controlling the impulsing and a masking output M1 for controlling the muting function. Solid-state switching is preferred because of the high cost of relays. In the drawing we see the circuit of a telephone set embodying the present invention, which is connected to the line via hook switch contacts HS1, HS2, a resistor R1 and a suppressor ZD1 for the suppression of line transients and a diode bridge D1 to D4 to ensure that the circuit is insensitive to changes in line polarity. The muting circuit includes transistor T3, resistor R7 and diodes D6 and D7. When the hook switch contacts HS1, HS2 are closed on off-hooking, direct current initially flows via a diode D5, transistor T1, resistor R4 and the transmission circuit, referenced Tx CCT. Base cur rentfrom IC1 turns the transistor T1 on sothatT2 receives base drive and is therefore turned on. At the same time the capacitor C1 is charged to (Vz - 0.7 ) volts, where Vz is a voltage determined by the zener diode ZD2. When C1 is charged to the operating voltage of the unit IC1, both its output DP and M1 become low. Hence T1 and T2 remain on, and T3, in the muting circuit, is turned on. With transistor T3 on, some of the line current flows through T3 and the resistor R7. This current is then divided so as to flow via D7 and via the series combination of the receiver Rx and diode D6. However, all the line current flows through the transmitter Tx. The dynamic impedance of diodes D6 and D7, which is inversely proportional to the forward current, is low, so that the receiver Rx is active and is in normal usage. In this condition the presence ofthe diodes D6 and D7 has very little effect on the operation of the receiver. When the first keyed digit is reconised, the unit IC1 causes the output M1 to go to its logic 1 state, which turns the transistor T3 off. Hence as the line current no longer flows in D6 and D7, the dynamic impedance of these diodes becomes very large. Hence the receiver Rx is electrically isolated from the transmission circuit, so that the receiver is muted. The impulsing occurs through the DP output, which turns T1, and thereafter T2, off for each break impulse to be sent. During the break period, rapid discharge of C1 via ZD2 is prevented by the diode D5. The value of C1 is so chosen as to ensure that its voltage does not fall below 2.5 volts during the break period. When "dialling" is completed, the circuit returns to the speech mode with transistors T1, T2 and T3 on. Such a circuit outpulses successfully with a line current as low as 11 mA, below which value the feed bridge relay at the telephone exchange fails to operate. At the upper end, the limiting factor is the dissipation of ZD2, which can take a maximum forward current of 101 mA at an ambient temperature of 45"C. With 60 volts, and a 2 x 200 ohm feed, the line current is 90mA at zero line. The transmission circuit is not described in detail as it follows conventional principles. CLAIMS

1. An electronic circuit for use in a telephone subscriber's instrument, in which the telephone receiver is connected to the instrument's transmission circuit via diode means so poled that when the receiver is in use a direct current flows in that receiver and in the diode means, in which when the said direct current flows the diode means is in its low impedance condition such that the low dynamic impedance of the diode means has little or no effect on the receiving level, and in which when the subscriber dials or otherwise transmits wanted number information said direct current flow is interrupted such that the diode means has a high dynamic impedance, so that the receiver is electrically isolated from the transmission circuit.

2. A circuit as claimed in claim 1, in which each end of the receiver is connected to the transmission circuit via one of two diodes, which two diodes from said diode means, in which a connection extends via the emitter-collector path of a transistor from one side of the line to the junction between one of said diodes and the receiver, in which the transistor is normally conductive with the diodes in their low impedance state, and in which during each impulse which is sent the transistor is non-conductive with the diodes in their high impedance states.

3. A circuit as claimed in claim 1 or 2, and in which the transmitter is connected across the line in series with a transistor circuit which is normally conductive but is rendered non-conductiveforthe duration of each break impulse to be sent.

4. Anelectroniccircuitforatelephonesub- scriber's instrument substantially as described with reference to the accompanying drawing.