GB2110905A

GB2110905A - Handsfree telephone facility

Info

Publication number: GB2110905A
Application number: GB08230509A
Authority: GB
Inventors: Frank Bahr; Werner Hubner; Hans-Gunther Luhmann; Gunter Schulz
Original assignee: Deutsche Telephonwerke und Kabelindustrie AG
Current assignee: Deutsche Telephonwerke und Kabelindustrie AG
Priority date: 1981-12-03
Filing date: 1982-10-26
Publication date: 1983-06-22
Also published as: DK534682A; IT1154586B; FR2517911A1; IT8224414A1; GB2110905B; FR2517911B1; GR77765B; BE894144A; DE3148305C2; IE53672B1; IE822583L; IT8224414A0; NL8203010A; DE3148305A1; LU84322A1

Abstract

A handsfree facility in a telephone arrangement has voice-controlled voice-direction identification whose control criteria for changing over from transmitting to receiving operation are derived from the voice voltage levels. A hybrid set (GA) has only one transformer with the transmitting and receiving directions interchanged relative to traditional voice circuits (Fig 2 not shown). Fixed level increases are also provided using only three damping values which are produced by two switching amplifiers (SV1, SV2) in the transmitting amplifier and two damping members (DG1, DG2) in the receiving amplifier. The voice levels are monitored by means of regulating amplifiers (RV1, RV2, RV3) and level detectors (PD1, PD2, PD3) and the change-overs between rest, transmitting and receiving operations, are effected by the level detectors, a subsequent gate circuit (G1-G3) locking the respective operating state. Change-overs from one type of operation into another are influenced by the time constants of the level detectors. <IMAGE>

Description

SPECIFICATION A circuit arrangement for providing a handsfree telephone facility The invention relates to a circuit arrangement for providing a handsfree facility for a telephone arrangement having a two - wire four - wire hybrid set, a voice - controlled voice - direction identifier and a switch - over device for transmitting and receiving amplifiers, whose control criteria for switching on and off damping members are derived from voice voltage levels.

A handsfree facility is intended to permit speaking on a telephone without using the handset. To this end, the listening circuit of the telephone is replaced with a receiving amplifier and loudspeaker, and the speaking circuit with a transmitting amplifier and microphone. The transfer from four - wire transmission in the transmitting and receiving sections to two - wire transmission of the subscribed line takes place in the hybrid set. Both signal paths are decoupled by the hybrid transfer damping. The hybrid transfer damping depends on the matching of a balancing to the subscriber line and thus on the respective connection.

Feedback howling can occur due to the requirement not to exceed a certain value of the transmitting reference damping e.g. +12dB, and to achieve as high as possible a volume in the receiving direction, and conditional on the amplifications necessary therefor in the transmitting and receiving channel of the handsfree facility. Since an increase in the hybrid transfer damping by matching the balancings to the connection - dependent line impedance can be achieved only with difficulty, voice controls are used, which open or close the transmitting and receiving channels of the handsfree facility in opposite directions.

In the event of complete connection and disconnection of the signal paths of the handsfree facility, the opportunity for duplex voice communication, as one is accustomed to with the handset, is lost and a purely simplex voice communication is produced.

This is found to be disturbing particularly by the participant without the handsfreefacility. He has the impression of speaking into a "dead line". By the same token, the participant without the handsfree facility hears an echo of his own voice via the air path damping when the transmitting and receiving channels are completely open simultaneously.

Circuit arrangements for voice-controlled handsfree facilities are known. DE-AS 24 39 740, for example, discloses an arrangement for voice-controlled handsfree devices, which derives the control criteria for connecting and disconnecting the damping members from the voice voltage levels and guarantees partial switching of the damping members. A plurality of damping members are required for each voice channel and voice direction in order to achieve damping increases in small steps.

DE-AS 27 14 132 describes a loud - speaking telephone device comprising a switch with several switch positions for damping the transmitting channel, which also permits the adjustment of defined damping increases.

It can also be seen from DE-AS 27 10 816 that individual - channel control amplifiers are used for switching damping members, an overriding of the loudspeaker and therefore also a change - over from receiving to transmitting operation being prevented.

These known arrangements necessitate high expenditure on damping members and adjustment members, which are to be operated partly during an existing connection. Moreover, rapid recognition of a voice signal, reliable identification of the voice direction, permanent through - connection during a conversation and a delay - free change - over in the event of a direct exchange of conversation between the participants and low operating thresholds of the voice control are necessary. Because of the incomplete decoupling of the transmission paths of the handsfree facilities of the known type, no clear voice direction signal is immediately apparent when the operating threshold is exceeded. On the one hand, the transmitting signal reaches the receiving section via the hybrid transfer damping and there exceeds the operating threshold.On the other hand, the receiving signal emitted from the loud - speaker and a voice signal of the user can not be distinguished from the transmitting section in respect of levels.

According to the present invention there is provided a circuit arrangement for providing a handsfree facility for a telephone arrangement having a two - wire four - wire hybrid set, a voice - controlled voice - direction identifier and a switch - over device for transmitting and receiving amplifiers, whose control criteria for switching on and off damping members are derived from voice voltage levels, the circuit arrangement being such that when it is in use a transmitting signal is fed via a resistance into a secondary winding of a hybrid transformer, there being a tap for a receiving signal coupled in the cross branch of a bridge of the hybrid transformer, and in use there being provided, to determine the three operating states of transmitting operation, rest position and receiving operation, constant level increases and drops which are produced by two switching amplifiers of the transmitting amplifier and two damping members of the receiving amplifer, which are controlled by regulating amplifiers and level detectors, and in use a time sequence of the switching processes for the voice direction identification being fixed by means of the level detectors, which control a locking-in of the respective operating state via gates, the release of a locking being influenced by the time constants of the level detectors.

A preferred embodiment of the present invention may provide a handsfree facility, which suppresses the echo to a large extent, permits reliable identification of the voice direction, and guarantees a practically delay - free change - over in the event of a change in voice direction and a steady level pattern in the connected - through state of the handsfree facility.

For a better understanding of the invention and to show how it may be put into effect reference will now be made by way of example to the accompanying drawings in which: Fig. lisa block circuit diagram including a circuit arrangement providing a handsfree telephone facility; and Fig. 2 is the circuit arrangement of the hybrid set of Figure 1.

The level increases in the transmitting and receiving directions are crucial for subjective hearing impression. Three defined level increases or respectively operating states are provided for the handsfree facility, the levels given below also being able to comprise other values, but always three fixed values: During transmission, the transmitting amplification increases relative to the rest amplification of the transmitting section by 9 dB, whereas the receiving amplification drops relative to the rest amplification of the receiving section by 12dB. The increase of the transmitting amplification is then deliberately kept lower than the drop in receiving amplification. A possible increase in the transmitting amplification of the same intensity as that of the drop in receiving amplification would be distinctly audible by the other participant in the telephone conversation.

The receiving state of the handsfree facility is characterised by the increase in receiving amplification by 12dB with a simultaneous drop in transmitting amplification by 12dB. Thus, for the other participant in the conversation, the echo is damped constantly by a specific amount, e.g. 12db, when change - over is effected from receiving operation into the rest state.

The transmitting and receiving operations are initiated from the rest position after a voice level and voice direction have been identified by the voice control, as described below in greater detail with the use of the block circuit diagram according to Fig. 1.

During a pause in conversation, the handsfree facility adopts the rest position, in which the transmitting amplification is lowered by 9dB to the rest value and the receiving amplification takes on the value of 2dub. Here, the transmitting amplification of the handsfree facility which is reduced by 9dB, is sufficient for communication with the other participant, so that the distance between the participant with the handsfree facility and the microphone M of the handsfree facility can be variable without loss of voice contact. A direct transfer from transmitting operation to receiving operation is given only with direct exchange of conversation.

The transmitting amplification is undertaken by two switching amplifiers SV1, SV2, whose amplifications can be exchanged between two values by binary control signals: the first switching amplifier SV1 between the two values V1 and V1 - 12dB and the second switching amplifier SV2 between the values V2 + 9dB and V2.

The regulating amplifiers RV1, RV2, RV3 operate as level regulators with a limited level increase. With suitable dimensioning, the operating threshold, pick-up time constant and hold time constant of these stages can be adjusted independently of each other.

The second regulating amplifier RV2 serves to balance damping in receiving operation. The level increase amounts to 20dB, for example. The pick - up time constant is selected to be relatively large, in order to prevent closing of the receiving channel by short peak levels.

With a large hold time constant of e.g. lows, an even level pattern is produced in the receiving direction.

To adjust the receiving volume, the amplification of the adjustment amplifier EV can be varied infinitely within a certain range or in several stages by a switching amplifier controlled by a key.

The transmitting signal for the voice control is tapped behind the first switching amplifier SV1. In receiving operation, the first switching amplifier SV1 damps this signal by 1 2dB. The first regulating amplifier RV1 serves to evaluate background noise in the transmitting direction. If there is any loud background noise on the side of the participant with the handsfree facility, the operating threshold of the first regulating amplifier RV1 is exceeded and the amplification lowered. There results therefrom a reduced response sensitivity of the voice control in the transmitting direction. As a result of large time constants in the regulating circuit, the stage forms a mean value of the room noise.In order also, with a regulated first regulating amplifier RV1, to guarantee connection through into transmitting operation, the level increase is limited to 9dB.

When the first damping member DG1, respectively the second damping member DG2 are activated in the receiving amplifier, the levels are dropped by 1 2dB each. On the receiving side, the voice control processes two signals damped in different manners in the signal path. The first receiving signal is derived behind thefirst damping memberDG1 and corresponds in its processing to the signal on the transmitting side. The third regulating amplifier RV3 regulates the reaction sensitivity of the voice control in the receiving direction in dependence on the received level. The increase in level is limited here to 12dB.

The second receiving signal is tapped behind the final stage ES. Its level is thus dependent on the position of the volume potentiometer of the adjustment amplifier EV and on the switching state of the second damping member DG2.

In the block circuit diagram according to Fig. 1,the voice control is symbolised by the level detectors PD1, PD2, PD3 and the gates G1, G2, G3.

As already mentioned in the introduction, rapid identification of a voice signal, the voice direction, permanent th rough - connection during a conversation and a delay - free change - over in the event of direct speech exchange, are the most important demands made on the voice control.

In order to guarantee reliable identification of the voice direction, a time sequence for the switching processes of the level detectors PD1 to PD3 is determined and a two - way locking of the operating states of the handsfree facility is undertaken. In the voice control, the transmitting signal is fed to the first level detector PDl,the first receiving signal to the second level detector PD2 and the second receiving signal to the third level detector PD3. The level detectors PD1 to PD3 produce binary output signals. After the threshold voltage has been exceeded, a logical "1" appears at the output of the respective level detector PD. The corresponding level detector then reacts in a delayed manner, the delay being dependent on its pick-up time constant tA.If the threshold voltage is fallen short of, then a logical "0" appears at the output of the level detector PD after a delay time determined by the hold time constant. The pick - up time constants tA of the level detectors PD1 to PD3 are determined in such a manner that they conform to the inequation tA3 < tA1 < tA2.

In the rest position, without the presence of a transmitting or receiving signal, a logical "0" appears at the output of each level detector PD1 . . PD3. The control outputs S, E for the handsfree facility adopt the state S=0, E=0 (Fig. 1).

When the microphone M is exposed to sound waves, the transmitting signal exceeds the threshold in the first level detector PD1. At the same time, the voice signal passes over the hybrid set GA into the receiving section of the handsfree facility and there activates the second level detector PD2. In the rest position and in transmitting operation, the second receiving signal is damped by the damping member DG2 by 12dB. The operating threshold in the third level detector PD3 is sized in such a way that it can not be exceeded by the transmitting section even in the event of unfavourable hybrid transfer damping.

Thus, the first gate G1 remains permanently open in the case where reaction has been made to an operation.

In the event of simultaneous activation of the first and second level detectors PD1, PD2, the precedence in time of the first level detector PD1 given by tAl < tA2 effects a safe change over from the rest position S=0, E=0 into transmitting operation S=1, E=0. By locking the third gate G3, the transmitting operation remains independent of the output state of the second level detector PD2, until the output of the first level detector PD1 has a logical "0".

In transmitting operation, the receiving signals are damped by 12dB at the input of the receiving channel by the first damping member DG1. On the basis of this additional damping the transmitting level passing over the hybrid set GA into the receiving amplifier EV is no longer sufficient to activate the second level detector PD2. In the transmitting direction, therefore, distinction is made only between the operational states of transmitting operation and rest position with an amplification increase of 9dB.

The signal level of the other participant arriving via the hybrid set GA, on the other hand, exceeds the receiving - side threshold voltage of the second level detector PD2 and activates it even when the handsfree facility is in transmitting operation. Even brief interruptions in the output signal of the first level detector PD1 then lead, via the unlocking of the third gate G3, to the immediate change - over from transmitting operation S=1, E=Oto receiving operation S=0, E=1. A direct change of voice direction is also possible in the transfer from the receiving operation to transmitting operation. In principle, it runs in the same way. In that case, as long as the third level detector PD3 remains inactive, intermediate speech exchange is made more simple for both participants equally.

With good connections, the receiving signal emitted from the loudspeaker Land the voice signal of the participant having the handsfree facility reach approximately the same level values. The receiving signal can then activate the transmitting - side first level detector PD1 out of the rest postion. In this case of operation, an erroneous change - over into the receiving direction is prevented by third level detector PD3.

The sizing of the operating threshold of the third level detector PD3 is produced from the receiving level required to activate the first level detector PD1.

Because of the antihowling stability required for a handsfree facility (circuit amplification < 1) this level value can only be exceeded by receiving signals. The level detectors PD1 . . PD3 are then activated simultaneously. The change - over of the transmission channels is produced from the time sequence of the level detector signals. On the basis of the prece dence tA3 < tA1,first the first gate G1 and thus the signal of the first level detector PD1 are blocked. The second level detector PD2 then selects receiving operation S=0, E=1.

Since exceeding the operating threshold of the third level detector PD3 leads automatically to change - over into receiving direction, the other participant gains precedence over the participant having the handsfree facility.

The operating behaviour of the handsfree facility depends essentially on the level increases being retained in the transmission channels and the operating thresholds being kept within the voice control.

In order to permit feeding from the subscriber lines a, b, a substantial reduction in the rest current consumption of the whole handsfree facility is necessary at the same time. For this reason, low power operational amplifiers are used for the signal path. The change - over or regulation of the amplification of individual stages is controlled by N channel field effect transistors.

An integrated circuit is used as a final stage, which is distinguished by a low rest current and a low expenditure for the external circuitry. The level of the loudspeaker L is damped by 6dB behind the final stage output by a series - connected resistance.

When an additional loudspeaker is connected, an increase in volume for the whole room is gained of 6dB.

The hybrid set according to Fig. 2 supplied the handsfree facility from the connecting lines a, bvia the parallel - connected bridge - connected rectifiers G21, G22 with the operating voltage UB, which is limited to a certain value by a Z - diode G20. The supply voltage UV for the final stage ES of the handsfree facility is tapped at the positive half bridge of one bridge - connected rectifier G22. This parallel supplying of the final stage ES is undertaken in order to prevent couplings of the receiving signal at the input SE via the operating voltage line into the input of the transmitting amplifier.

Contrary to traditional voice circuits, the transmitting and receiving directions are exchanged: the transmitting signal SS is fed into the secondary winding of the transformer UE via a resistance R20. The voltage supply and the tap for the receiving signal SE lie in the cross branch of the bridge. The RC - combination R21, R22, C20 together with the resistance of the winding of the transformer UE, which is connected to the resistance R21, act as a complex line balancing network for the wave resistance. This circuit arrangement saves an additional transformer for uncoupling the receiving signal SE.

When the subscriber lines a, b are interrupted by reieasing the handsfree facility operating member, or during dialling, the hybrid set is not closed on the input side and the transmitting signal SS is coupled completely into the receiving amplifier, consisting of the adjusting amplifier EV, the final stage ES, the associated damping members DG1, DG2 and the regulating amplifiers RV2, RV3 of the handsfree facility. Conditional on the high storage capacities of the capacitors C21,C22, the positive operating voltages UV and UG drop only slowly. Thus, feedback howling can occur directly after the loop interruption. In order to avoid this, the build - up is suppressed by a field effect transistor at the input of the receiving amplifier.A supply gap signal SL, which arises in the event of a loop current through the hybrid set as a negative operating voltage UE at the bridge - connected rectifier G22, blocks the input transistor of the receiving amplifier. When the loop current stops, that is, in the event of an interruption of the subscriber lines a, b, the bridge - connected rectifier G22 loses its current and the input transistor short circuits the input of the receiving amplifier.

In general terms, there is provided for a handsfree facility in a telephone arrangement with a voice controlled voice - direction identification, whose control criteria for changing over from transmitting to receiving operation are derived from the voice voltage levels, a hybrid set (GA) with only one transformer, whilst the transmitting and receiving directions are interchanged, contrary to traditional voice circuits. Fixed level increases are also provided, which are based on only three damping values, and which are produced by two switching amplifiers (SV1, SV2) of the transmitting amplifier and two damping members (DG1, DG2) of the receiving amplifier. The voice levels are monitored by means of regulating amplifiers (RV1, RV2, RV3) and level detectors (PD1, PD2, PD3) and a time sequence of the change - overs between the types of operation of rest position, transmitting and receiving operation, is effected by the level detectors, a subsequent gate circuit locking the respective operating state, and change - over from one type of operation into another being influenced by the time constant of the level detectors.

Claims

1. A circuit arrangement for providing a handsfree facility for a telephone arrangement having a two - wire four - wire hybrid set, a voice - controlled voice - direction identifier and a switch - over device for transmitting and receiving amplifiers, whose control criteria for switching on and off damping members are derived from voice voltage levels, the circuit arrangement being such that when it is in use a transmitting signal is fed via a resistance into a secondary winding of a hybrid transformer, there being a tap for a receiving signal coupled in the cross branch of a bridge of the hybrid transformer, and in use there being provided, to determine the three operating states of transmitting operation, rest position and receiving operation, constant level increases and drops which are produced by two switching amplifiers of the transmitting amplifier and two damping members of the receiving ampli fier, which are controlled by regulating amplifiers and level detectors, and in use a time sequence of the switching processes for the voice direction identification being fixed by means of the level detectors, which control a locking - in of the respective operating state via gates, the release of a locking being influenced by the time constants of the level detectors.

2. A circuit arrangement according to claim 1, wherein the hybrid set comprises two parallel bridge - connected rectifiers to a first bridge - connected rectifier of which there is connected in parallel a Z diode for limiting the operating voltage to a value subjected only to minimal variations, and the second of which bridge - connected rectifiers supplies the supply voltage for a final stage of the handsfree facility circuit arrangement and a supply gap signal for blocking the receiving amplifier when the hybrid set is not closed - off on the input side.

3. A circuit arrangement according to claim 1 or 2, wherein the transmitting amplifier comprises two switching amplifiers, a regulating amplifier and a level detector.

4. A circuit arrangement according to any pre ceding claim, wherein the receiving amplifier com prises two damping members, two regulating ampli fiers, an adjusting amplifier, a final stage and two level detectors.

5. A circuit arrangement according to any pre ceding claim, wherein the regulating amplifiers are arranged to operate as level regulators with a limited increase, whose operating thresholds, pick - up time constants and hold time constants are independent ly adjustable.

6. A circuit arrangement according to claim 3, or claim 4 or 5 when appended to claim 3, wherein for the transmitting operation of the handsfree facility the transmitting signal is arranged to be tapped behind a first of said two switching amplifiers and controls locking of an associated gate via the regulating amplifier and the level detector of the transmitting amplifier.

7. A circuit arrangement according to claim 4, or claim 5 or 6 when appended to claim 4, which is such that a signal level approaching via the hybrid set and exceeding the receiving - side threshold voltage of a level detector of the receiving amplifier controls the locking of an associated gate and switches over the handsfree facility to receiving operation.

8. A circuit arrangement according to claim 7, which is such that exceeding the operating threshold of the other level detector of the receiving amplifier effects a switch - over of the handsfree facility to the - receiving direction, whilst an associated gate is blocked.

9. A circuit arrangement according to claims 3 and 4 combined, or any of claims 5 to 8 when appended thereto, wherein the pick-up time constants of the three level detectors conform to the expression tA3 < tA1 < tA2, where tAl, tA2 and tA3 are the pick - up time constants of the three level detectors in the order in which they are mentioned in claims 3 and 4.

10. A circuit arrangement according to any preceding claim, wherein low - power operational amplifiers are used for the signal processing of the handsfree facility.

11. A circuit arrangement according to any preceding claim, wherein N - channel field effect transistors are used to switch over or regulate the individual amplifier stages.

12. A circuit arrangement for providing a handsfree facility in a telephone arrangement, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.