GB2160745A - A device for adaptation of a decimal or multifrequency code dialling keyboard to a conventional telephone set - Google Patents

Abstract

A device is provided for adapting a telephone set to either decimal or multifrequency dialling, which device may be substituted for a conventional dialling block and comprises an electronic element intended to be placed in series with the transmission device of the set. This element is adapted for operating as a switch so as to place the transmission device out of circuit during decimal dialling and for operating as a series modulator during multifrequency dialling. <IMAGE>

Description

SPECIFICATION A device for adaptation of a decimal or multifrequency code dialling keyboard to a conventional telephone set The invention relates to a device for adapting a decimal ormultifrequencyemission keyboard,which device may be combined with a keyboard forforming an assembly which may be substituted forthe usual rotary dialling block a telephone set.

Most of the telephone sets in useatthepresenttime are of the rotary dial type. Dialling the number causes transmission overthe line of pulses generated during the return of the dial. Keyboard blocks have already been proposed for substitution purely and simply for the dialling blocks of existing sets. In most cases, the substitution blocks comprise an electromechanical relay for inhibiting the voice transmission part of the set during dialling, by short circuiting. This solution is very costly, the electromechanical relay being a high priced part. A decimal emission keyboard block has been proposed (French Patent No. 2,520,569) comprising an electronic circuit which isolatesthetransmis- sion device ofthe set during dialling. This electronic circuit is specific to a decimal emission keyboard.It cannot be transported to multifrequency code dialling because of its construction. In particular, this circuit uses a current generator placed in parallel with the device which not only excludes adaption to a multifrequency keyboard, but also has unfavorable consequences on the operation because of the change of power supply conditions. Other previously proposed systems also exist, such as disclosed for instance in British specification No. 2,016,869, which are not adapted for direct substitution.

The invention is intended to provide an interface device which may be equipped eitherwith a digital keyboard or with a multifrequency keyboard at will for forming an assembly which may be substituted for the dialling blocks a conventional set.

Before defining the means used by the invention to arrive at th is result, it may be useful to show, with a particular example formed by the telephone set S63 of the French post office, twenty million examples of which are in use at the presenttime, what are the problems posed by going overfrom conventional number dialling to calling using a digital ormultifrequency emission keyboard.

Figure 1 gives a simplified representation ofthe set S63, intended to be connected to the two wires L1 and L2 of a telephone pair. The set comprises a bell 10 connected between the wires, in series with an RC circuit, upstream of two line contacts 12.1 and 12.2 open when the hand set is laid on its support, closed when the hand set is raised. The transmission device 14, comprising the hand set, is connected to the terminals of lines L1 and L2when the hand set is lifted, through the dialling block 16.

The dialling block 16 is connected to the circuit which has just been describd byfourterminals, conventionally identified bythe Figures I, 11, III, IV. It comprises: -a galvanic contact 18 for short circuiting the transmission device 14, adapted for being closed during the whole of the dialling phases so as to prevent dialling pulsesfrom passing into device 14; - a contact 20 inhibiting the bell, fulfilling the same role as contact 18 in so far as the bell is concerned; - a dialling contact 22, coupled to the dial, closed when the dial is at rest and which opens and closes at the rate ofthe return rotation of the dial so as to emit a number of pulses corresponding to the figures dialled by the user.

Afirst problem which arises when it is desired to substitute an electronic circuit for switch 18 is the excessive impedance of such a circuit when it is closed for short circuiting device 14. This impedance is such that the ear phone ofthe hand set receives excessive sound pulses when the set comprises a decimal dialling keyboard. To get overthis difficuity, it has already been proposed, instead of short circuiting device 14,to open its connection with the dial, but in a form which does not correspond to practical requirements.

The invention proposes providing a device of this type which, contrary to those already known, may be associated not only with a digital emission keyboard but also with a multifrequency keyboard or even with a keyboard providing both functions. For this, the invention provides a device which comprises an electronic element placed in series with thetransmis- sion device of the set and adapted for operating as a switch so as to place the device out of circuit for decimal emission and to operate as a series modulator for multifrequency emission.

With such an arrangement, and providing that means are also provided which, during multifrequency emission, introduce an impedance in parallel across the device, a return towards the earphone of the multifrequency signal emitted towards the line may be obtained at a suitable level, at the same time as correct rejection ofthe microphone signal.

The invention will be better understood from reading the following description of a particular embodiment, given by way of example. The description refers to the accompanying drawings, in which: Figure 1, already mentioned, is a simplified diagram of a conventional telephone set, of passive type, Figure 2 is a general diagram for showing the method of inhibiting the transmission device during the calling phase, in the case of a decimal keyboard, Figure 3 is a black diagram showing the general construction of a device ofthe invention, and Figure 4 shows a possible construction of a part of the device of Figure 3.

Aswas mentioned above, thefirst problem which arises in the construction of a device able to form an interface between a conventional set and a decimal dialling keyboard consists in inhibiting thetransmis- sion device 14 during a call.

Figure 2, where only the elements are shown through which line current flows. comprises electron nic switch over means shown schematically by contacts 24 but in fact formed by transistor circuits. In the state shown in the Figure, contacts 24 allow the line currrent and the modulated current supplied by the transmission device 14to pass between land II, through a rectifier bridge 26 (required because the circuit must operate whateverthe polarity ofthe terminals of lines land II). In the opposite situation, shown with broken lines, the switching means 24 separate device 14from the line and loopthis latter through a rectifier bridge28whose role isthe same as that of bridge 26.In the loop thus formed is interposed the switch ofthe decimal emission keyboard, connected so that the current flows between land IV solely during closure phases. Aswill be seen further on, this circuit may be readily formed with a single high voltage transistorfor each of the situations, the switching transistor through which the telephone currents flow being of a low voltage type.

Foran adaption device to be able in addition to receive a multifrequency emission keyboard (often designated bythe abbreviation DTMFfor"dual tone multi-frequency",) when this keyboard uses a series modulatorthefollowingfunctions must be fulfilled during dialling: - short circuiting ofthe transmission device by switching means (which will in general comprise two elements one of which is effectively used, so as to take intoaccountthetwo possible polarities atthetermin- als of the transmission device), with an impedance such thatthere is a return to the earphone of a sufficient multifrequency signal level, forming the call return;; switching in ofthetransmission device 14, through a very low series impedance during a conversation phase, the load impedance of the line being limited then practically to that ofthe transmis sion device.

Thesefunctions may be fulfilled by disposing between land II and in place ofthe galvanic switch 22 of Figure 1, an electronic circuit which may operate as a switch for presenting a low impedance during the conversation phase and as a series modulatorfor allowing multifrequencyemission. Such a circuit may be readilyformed as a programmable impedance series modulator. This series modulator may be provided for presenting, during the multifrequency emission phase, an impedance of about 600 Ohms at the voice frequency.

As will be seen furtheron with reference to Figure 4, these circuits may be formed by simple addition, to the circuit in any case necessary for adapting a decimal keyboard, of a differential amplifier stage.

The device of Figure 3, in which the parts already shown in the preceding Figures bearthe same reference number, may be regarded as comprising a common assembly (rectifier block 30, decimal or multifrequencygeneration control system 32, and systemforsupplying with power and controlling possible external members 34), an assembly of means used for adapted a decimal keyboard 36 (switches 38 and 40) and an assembly used for adapting a multifrequency keyboard 42 (switches 44 and 46 and switches 48 and 50 one of which, 48 for example, plays the roleof series modulator during multifrequency emission). The lattertwo assemblies are controlled from the control system 32, by sending orders two the inputs labelled C.

The rectifier block 30 comprises the diode bridges 26 and 28 already mentioned, as well as two threshold diodes connected head to tail between terminals Ill and lVforforming a switch interrupting the ringing supplywhen the applied voltage exceeds a given value.

The purpose of the control system isto enable or, on the contrary, to disable the means assigned eitherto decimal emission orto multifrequency emission, depending on the signals which it receives. This system comprises an input DPforconnection to the integrated circuit ofthe decimal keyboard 36 and an input mute DEC which is energized when entering the calling phase, possibly by depressing a particular push-button when the set is provided for calling without lifting the hand set. System 32 comprises, in a symmetrical way, two terminals E and S for connection with the integrated circuit of a multifrequency keyboard 42, and a terminal mute MFwhich fulfills the same role as the terminal mute DEC, i.e. switching into and outofthe "call" phase, butverydifferently.

System 32 comprises, in addition to its control outputs, a multifrequency modulation output towards switch 48 (or switch 50 if it isthis latterwhich plays the role of modulator). The system for supplying with powerand controlling external members may be relatively conventional and will therefore not be described. It may be provided for energizing a light emitting diode at the rate oftransmission of call pulses. The system allows a buffer supply battery to be connected and comprises a circuit for stabilizing the supplyvoltage of the other assemblies. These latter, except four switch 48, are formed by transistors which the control system 32 causes to be alternately brought to the saturated condition and to cut-off condition.

Operation during the conversion phase, i.e. when the connection is established between the set and the correspondent, is the same whatever the type of keyboard which is connected to system 32. The mute DEC and mute MF inputs are in logic state 1. In response, system 32 opens switches 38,40,44 and 46 (which is in fact achieved by cutting off transistors).

The line currentthen flows through rectifier 26, switch 48, the supply system 34, switch 50, rectifier 26 and the transmission device. The transistors of switches 48 and 50 are then under saturated conditions.

The call, when the device is equipped with a decimal keyboard 36, takes place in two steps.

The first step, caused by lifting the hand set or pressing a control button, causes the mute DEC input to pass to state 0, which causes switches 48 and 50 to be disabled so as to switch outthe transmission device. System 32 at the same time controls switches 38 and 40 so as to cause them to pass to saturated operating conditions. At this stage, the transistors playing the role of switches 44 and 46 remain in the disabled state.

The DC line current may then flow through the loop comprising rectifier 28, switch 38, the supply system 34, switch 40 and, again rectifier 28.

The emission of decimal pulses causesthe input DP to pass periodically to state 0 (the mute DEC input remaining in state 0). Switching of DP to 0 causes switches 38 and 40 to be disabled while reducing the line currentto a value less than 200A. Switches 48 and 50thus inhibitthetransmission device 14.

When the device is equipped with a multifrequency keyboard 42, the call also takes place in two steps.

During the first step, the mute MF input passes toO.

which causes switches 44 and 46 to pass to the saturated state, transforms switch 48 into a series modulator and leaves the transistor of switch 50 in the saturated state. The transistors forming switches 38 and 40 remain cut off. Switches 44 and 46 then close a circuit inhibiting the transmission device 14. The line currentthistimeflowsthrough switch 44 or 46, depending on its polarity (a single one of diodes 45 and 47 being enabled).

The multifrequency signal supplied by the circuit of keyboard 42 is applied bythe control system 32 to a transistor of modulator 48 operating under linear conditions, so as to modulate the line current.

The device may be equipped with a keyboard comprising a memory circuit for repeating a recorded call number.

Figure 4 shows a possible construction of the series modulator48and of the elements which are associated therewith, the other components being able to be of a conventional kind. This modulator comprises a low voltage bipolar PNPtransistor54connected in a common collector circuit. It is associated with the differential stage operating as operational amplifier and with a generator delivering a constant current i having a shunt regulator. The common supplyforthe different components is taken at the terminals of the shunt regulator which supplies, when associated with a decimal keyboard, the circuit generating the signals which cause the loop opening by separating the transmission device and inhibiting it.

Operation ofthe device is as follows.

During the conversation phase, the bipolartransistor 54 is operating undersaturated conditions.

During multifrequency code dialling, the transistor operates under linear conditions: a circuit comprising transistor 64 applies thereto a moduiated transmission voltage. Voltage Vce is controlled by the mean current injected, by a current mirror consisting of transistors 66, into the counter feedback resistor R-2.

The required impedance ZA (600 Ohms) is obtained by an appropriate selection of resistors R1, R2 and r, which are relatedtoZbythe relationship: Z= r.R2/R1.

During multifrequencysignal generation, the high voltage NPN 58 and PNP 56 transistors remain saturated.

So as not to disturb the impedance for the AC currents and so to avoid distorsions, the biasing currentforthe differential amplifier included inthe system 32 must remain constant despite the variations ofthe line current. With the real (as opposed to imaginary) impedance being equivalent to R2 in parallel across r.R2/R1, R2 must be large as compared to 600 Ohms. This result is reached by giving to R2 a value of some tens of kOhms.

The circuit of Figure 4forms only one example, suitable for construction from discrete components.

For large scale manufacture, an integrated circuit construction is more economic and may be entirely made from high voltage bipolartransistorsforforming a chip which may be added to a printed circuit board carrying circuits specific to each telephone set manufacturer.

Claims (5)

1. Adeviceforadapting a decimal and multifrequency emission keyboard to a telephone set, in place of a dialling block, characterized in that it comprises an electronic element (48) intended to be placed in series with a transmission device (14) ofthe telephone set, adapted for operating as a switch for placing the transmission device (14) out of circuit during decimal emission and for operating as a series modulator during multifrequencyemission.

2. The device according to claim 1, characterized in that it further comprises means controlled in response to passing overto a multifrequency emission calling phase forforming an impedance placed in parallel with the transmission device so as to obtain a return towards the ear phone of the multifrequency signal emitted towards the line, at a suitable level.

3. Device according to claim 1 or2, characterized in that it comprises means forming switches (48,50) intended to isolate the transmission device (14) in response to passing overto decimal emission calling conditions.

4. Device according to any one of the preceding claims, characterized in that the electronic components comprise bipoletransistors operating under alternately saturated and disabled conditions, except the one intended to function as a series modulator which is provided for operating alternately under disabled and linear conditions.

5. Device according to claim 1, constructed and arranged as shown in and as described with reference to Figures 2-4 ofthe accompanying drawings.