IL47682A - Echo trap - Google Patents

Abstract

1510162 Echo suppressors SIEMENS AG 27 June 1975 [24 July 1974] 27285/75 Heading H4R In an echo suppressor provided on the four wire side of a hybrid junction a normally transmissive blocking device 3, Fig. 6, is provided in the transmit path together with an attenuation element 4 in the receive path, with a switch 41 which is normally in a position to by-pass the attenuation element. A control circuit detects a signal in the receive path and, in the absence of signal in the transmit path, causes the blocking element 3 to block the transmit path. When a signal is detected in both paths operation of blocking element 3 is inhibited but the switch 41 is operated to insert the attenuation element, but is arranged so that as the signal in the transmit path increases relative the signal in the receive path the attenuation of the attenuation element is reduced and/or the switch 41 is allowed to revert and by-pass the attenuation element. As described the signal in the transmit and receive paths can be compared in comparator 8 and the output caused to increase the attenuation 43 in the control path of a compressor 42, 43, 4, or, Fig. 2 (not shown) the output of comparator 8 can be used to inhibit the operation of AND gate (47) controlling switch 41. Alternatively control of the attenuator 43 in the compressor 42, 43, 4, can be effected via a threshold device coupled either to the output of comparator 6, Fig. 4 (not shown) or the output of signal detector 62, detecting the signal in the transmit path, Fig. 5 (not shown). A further modification, Fig. 4, includes a variable gain amplifier 71 in the transmit path signal detector circuit and a variable attenuator 73 in the receive path signal detector circuit which are both controlled in accordance with signal level in the respective path via a storage control device 70 which is blocked by controller 60 during duplex operation. The arrangement varies the control sensitivity in accordance with the hybrid junction attenuation. A delay device 65 is provided in the receive path speech detector to compensate for the delay through the hybrid junction. [GB1510162A]

Description

E CHO ΪΒ 0. 5331 This invention relates to echo traps, and in particular to an echo trap for use in a four-wire transmission system which includes a blocking element in a transmitting path considered from an adjacent hybrid junction to a two-wire transmission path, an attenuation element in a receiving path considered from the hybrid junction, and a control arrangement which is responsive to speech levels occurring on the transmitting and receiving paths to control the blocking element and the attenuation element.

An echo trap of this type in which the attenuation element is designed as an attenuation four-pole with dynamic compression is known for example from German Specification No. 1 297 153. Such an echo trap is used in four-wire links which are subject" to transit time delays and serves to prevent speech signals which emanate from a subscriber and are reflected at the far end of the link from returning ia the form of an echo to the speaking subscriber. Referring to Figure 1 of the accompanyin drawings, which illustrates a telephone transmission system between two subscribers A and B. via a four-wire link between hybrid junctions 1 and l*,the mode of operation of such an echo trap will be explained. At each of the two ends of the four-wire link there is connected a respective echo trap 2, 2'. The echo trap 2 includes a normally transmissive blocking element 3 which is connected in the transmitting path considered from the adjacent hybrid junction 1, i.e. between lines S and SL. In order to suppress echoes, on the occurrence of speech levels on the receiving path the blocking element 3 is activated to. cut off the transmitting path.

In addition, the echo trap 2 contains an attenuation element 4, with an associated switch 41 which is normally in a position to by-pass the attenuation element 4, which is connected in the receiving path, i.e. between lines EL and E. The attenuation element 4 is activated by operation of the switch 41 to ensure adequate echo attenuation when there are speech levels simultaneously on the receiving path and on the transmitting path, this situation is hereinafter referred to as the duplex state.

The echo trap 2' is identical in construction and operation to the echo trap 2, and accordingly includes a blocking element 3' connected in the transmitting path between lines S' and SL' and an attenuation element 4' with, an associated switch 41' connected in the receiving path between lines EL* and EL, the transmitting path and the receiving path again being considered from the adjacent hybrid junction 1! .

As during the duplex state generally the attenuation elements are activated in the echo traps at both ends of the four-wire link, in this case a mean attenuation of e.g. 6 dB will suffice for each of the two attenuation elements 4 and 4' in order to attenuate sufficiently the speech currents which pass through the four-wire link in the outward and the return directions.

An attenuation of 6 dB is sufficiently low to allow a speaking subscriber to recognise that the other subscriber is speaking, but it is very difficult for both the subscribers to hear during the duplex state, on account of the effective attenuation in the receiving path. This difficulty can be alleviated if each ' attenuation element is designed as an attenuation four-pole which exhibits dynamic compression, as with this known measure only large speech levels are strongly attenuated (e.g. by 20 dB) whereas small speech levels are attenuated only slightly or not at all during the duplex state. Nevertheless this does not avoid the problem that the reception loudness for the subscriber who is speaking inbetween times fluctuates very considerably and the hearing of the partner is greatly impeded. Also at' the end of the duplex state, when the attenuation element is again bypassed, the sudden increase in loudness, which at high receiving speech levels can amount to up to 20 dB, is particularly unpleasant.

This invention seeks to provide an echo trap by means of which the above described disadvantages can be overcome or at least reduced.

According to this invention there is provided an echo trap for use in a four-wire transmission system which includes a blocking element in a transmitting, path" considered from an adjacent hybrid junction to a two-wire transmission path, an attenuation element in a receiving path considered from the hybrid junction, and a control arrangement which is responsive to speech levels occuring on the transmitting and receiving paths to control the blocking element and the attentuation element and which controls is when the speech level of the transmission path exceeds the speech level of the receiving path or exceeds the speech level which is reduced by the attentuation of the hybrid junction on the receiving path, characterised in that the control arrangement comprises an evaluation device which compares the speech level of the transmissing and receiving paths for the determination of the duplex state and for switching on the attenuation element, and that the control of the attenuation of the attenuation element is effected via a threshold value device.

The invention is based on the fact that a subscriber finds the echo of his own speech signals proportionately less disturbing the more this echo is covered by the.speech signals of his partner. This results in the advantage that generally for most of the duration of the duplex state only very small attenuations or no attenuations need to be effected in the receiving path, this giving rise to a significant 'improvement in the mutual speech contact, in particular greatly reducing the drastic leaps in loudness which otherwise occur at the end of each period in which the duplex state exists.

The invention also extends to a four-wire transmission system including one or more such echo traps.

The invention will be further .understood from the following description with reference to Figures 2 to 6 of the accompanying drawings, each of which figures schematically illustrates by way of, example an echo trap in accordance with a respective embodiment of the invention.

Each of Figures 2 to 6 illustrates an echo trap which for e.xample constitutes the echo trap 2 shown in Figure 1. Accordingly each echo trap includes a blocking element 3 which is connected in the transmitting path between the lines S and SL and a dynamic compression attenuation element 4 which is connected in the receiving path between the lines EL. and E. The dynamic compression function is indicated .by a rectifier 42 which is part of a control loop of the attenuation element 4. In fact as shown in each of Figures 2 to 6 the input of the attenuation element 4 is connected to the line EL via the switch 41 which is a change-over switch which in the normal or rest state shown conducts the incoming signal on the line EL of the receiving path directly to the line E, thereby by-passing the attenuation element 4.

Each echo trap also includes a gate 31 the output of which controls the blocking element 3, a gate 47 (Figure 2) or 45 (Figures 3 to 6) having an AND function and the output of which controls the change-over switch 41, a first speech detector consisting of an amplifier 51 and a following rectifier 52, a second speech detector comprising an amplifier 61 and a following rectifier 62, a third speech detector comprising an amplifier 63 and a following rectifier 64, and an evaluation device 6 (Figures 2, 3, 5 and 6) or 60 (Figure 4). The output of the- evaluation device 6 or 60 is connected to a blocking input of the. gate 31 and to an input of the gate 45 or 47. The output of the rectifier 52 of the' first speech detector, the input of the amplifier 51 of which is connected to the line EL of the receiving path, is connected to another input of each of the gates 31 and 45 or 47.

The output of the rectifier 62 of the second speech detector, the input of the amplifier 61 of which is connected to the line S of the transmitting path, is connected to an input of the evaluation device 6 or 60.

The output of the rectifier 64 of the third speech detector, the input of the amplifier -63 of which is connected to the line E of the receiving path, is connected to another input of the evaluation device 6 or 60, directly in the case of the evaluation device 6 and via a delay element 65 in the case of the evaluation device 60.

When the gate 31 produces no output signal the blocking element 3 is in the rest state shown, i.e. is transmissive , to interconnect the lines S and SL of the transmitting path. If the first speech detector 51, 52 detects a speech level on the line EL of the receiving path which exceeds a specific value for example of -31 dBmO, it produces an output signal which, unless the gate 31 is blocked by an output signal being applied to its blocking input from the output of the evaluation device 6 or 60 as is. the case in the duplex state as described below, is conducted via- the gate 31 to the blocking element 3 and causes the latter to adopt its blocking state in which it does not interconnect the lines S and SL of the transmitting path.

The evaluation device 6 or 60 is responsive to the outputs of the second and third speech detectors to produce an output signal if the speech level on the line S of the transmitting path as detected by the second speech detector is of such a magnitude in comparison to the speech level on the line E of the receiving path as detected by the third speech detector that it can not originate solely from the speech currents on the receiving path which have crossed via the adjacent hybrid junction, i.e. the hybrid junction 1' in Figure" 1. This output signal is thus produced in the duplex state and being conducted to the blocking input of the gate 31 is effected to prevent activation of the blocking element 3 so that this remains in its rest state shown.

The simultaneous occurrence of output signals from the first speech detector 51, 52 and the evaluation device 6 or 60, which represents a criterion for the existence of the duplex state, is detected by the gate 45 or 47 which consequently produces an output signal (assuming in the case of the gate 47 that this is not blocked by a signal applied to a further blocking input) which causes the change-over switch 41 to be switched over to its operative state in which it connects the line EL to the input of the attenuation element 4.

In each echo trap an- additional limitation is imposed in respect of the attenuation which is effected by the attenuation element 4. This is further described in the following with reference to the individual ones of Figures 2 to 6.

Referring to Figure 2, in addition to the devices described above the echo trap shown therein includes a further evaluation device 8 the output of which is connected to a blocking input of the gate 47, one input of which is connected to the output of the second speech detector, i.e. to the output of the rectifier 62, and another input of which is connected to the output of the first speech detector, i.e. to the output of the rectifier. 52. The further evaluation device 8 is arranged to produce an output signal when the speech level on the line S of the transmitting path as detected by the second speech detector exceeds the speech level on the line EL of the receiving path as detected by the first speech detector 51, 52, or when the former speech level exceeds the latter speech level reduced by the attenuation of the adjacent hybrid junction. This output signal serves to block the gate 47 so that the change-over switch 41 remains in its rest position shown so that the attenuation element 4 remains by-passed.

Instead of the gate 47 shown in Figure 2, the echo trap shown in each of Figures 3 to 6 includes the gate 45 but is additionally provided with a further controllable attenuation element 43 which is included in the control loop of the attenuation element 4. By means of this further attenuation element 43 during the duplex state in which the change-over switch is in its operative state the dynamic compression attenuation range of the attenuation element 4 is limited to an increasing extent the greater the speech level on the line S of the transmitting path is relative to the speech level on, the line EL of the receiving path. Thus the attenuation effected by the attenuation element 4 is reduced., in proportion to the increase in the controlled setting of the attenuation of the further attenuation element 43.

The echo trap shown in Figure 3 includes a threshold value device 44 whose threshold value defines that range of the ratio between the speech level on the line S of the transmitting path and the speech level on the line EL of the receiving path in which the attenuation element 4 is operative. The input of the threshold value device 44 is connected to the output of the evaluation device 6 and its output is connected to the control input of the further attenuation element 43 so that the latter is adjusted to higher attenua ions, and hence the attenuation element 4 is adjusted to lower attenuations, by the output signal of the threshold value device 44.

The echo trap shown in Figure 4 is identical tc that shown in Figure 3 in respect of the provision of the threshold value device 44 but differs from it in respect of the second and third speech detectors and the evaluation device, which are arranged along the lines described in German Patent Specification No. 2 104 280. To this end the second speech detector includes a control element 71 which is constituted by a gain-controlled amplifier which is disposed between the output of the amplifier 61 and the input of the rectifier 62, and the third speech detector includes a' control element 73 which is constituted by a controllable attenuation element which is disposed between the output of the amplifier 63 and the input of the rectifier 64. The control elements 71 and 73 are both controlled by a storing control device 70 which .has the property of adjusting the control elements 71" and 73 during times of blockage of the transmitting path in accordance with the relative speech levels on the transmitting and receiving paths in such' manner that the transmitting-path-side control element 71 is constantly adjusted to greater amplifications and the receiving-path-side control element 73 is constantly adjusted to higher attenuations. Thus during a suitable interval of time the constantly increasing higher evaluation of the transmitting path speech levels in comparison to the receiving path speech levels is set at an optimum degree of sensitivity by the evaluation device 60.

The storing, control device 70 is controlled, by the output signal of .the control element 73 via an additional attenuation element 75 having an attenuation of 6 dB and a following rectififer 74 and by the output signal of the control element 71 via a rectifier 72. The evaluation device 60 also has a controlling action on the storing control device 70 which is such as to block the regulation process in the state of duplex. This type of control ensures that during the aforementioned interval of time the echo trap automatically determines the hybrid junction attenuation from the receiving path E to the transmitting path S, and in the event of a higher junction attenuation than 6 dB, sets the control elements 71 and 73 in the direction of a higher evaluation of the speech levels in the transmitting path S in comparison >to those of the receiving path E and' thus towards' an optimum sensitivity for the speech levels of the subscriber who is speaking infrequently. Generally this gives rise to the advantage that the break-in sensitivity for short interjections by the currently listening subscriber is considerably increased, i.e. the state of duplex can be established more easily, more quickly and more surely, as a result of which short interjections can be switched through so quickly .that they reach the more frequently speaking partner without mutilation. In the echo trap shown in Figure 4 it is thereby ensured that the attenuation element 4 can be adjusted to lower attenuations even when the transmitting path speech level exceeds the receiving path speech level reduced by the automatically determined hybrid junction attenuation.

"A's mentioned above the output, of the rectifier 64 is in the echo trap shown in Figure 4 connected to the evaluation device 60 via the delay element 65 which delays the drop of the receiving-path-side control voltage at least by the end echo transit time in order to eliminate the camouflaging influence of the end echo transit time on the evaluation device 60 in respect, of the true instantaneous state of the conversation of the two subscribers. It is thus possible additionally to increase the break-in sensitivity for the currently listening subscriber so that in each phase of the conversation he is able to convey even very short inter ections in a no't particularly loud voice without mutilations to the currently speaking partner, and nevertheless a reliable protection from incorrect switch-over i,nto the duplex state is ensured.

The echo trap shown in Figure 5 corresponds to that shown in Figure 3 with the exception that the further attenuation element 43, and hence the attenuation element 4, is controlled by the output signal of a threshold value device 48 the input of which is connected to the output of t-he rectifier 62 of the second speech detector, this threshold value device replacing and corresponding in function to the threshold value device 44.

The echo trap shown in Figure 6 again corresponds to that shown in Figure 3 with the exception that instead of being controlled by a threshold value device the further attenuation element 43 is controlled by the output signal of a further evaluation device 8 the inputs of which are connected and which operates in exactly the same manner as described above with reference to Figure 2. By means, of this further evaluation device 8 it can in each case be directly established whether the transmitting path speech level differs from, e.g. exceeds, the receiving path speech level by a quite specific, predeterined value, in which case the further attenuation element 43 is controlled to have a higher attenuation and hence the attenuation element 4 is controlled to have a lower attenuation.

It will be appreciated that the' invention is not restricted to the particular arrangements described but also extends to such modifications and combinations of these arrangements as fall within the scope of the appended claims. 47682/2 7

Claims (14)

1. I. Echo trap for use in a four-wire transmission system which includes a blocking element in a transmitting path considered from an adjacent hybrid junction to a two-wire transmission path, an attenuation element in a receiving path considered from the hybrid junction, and a control arrangement which is responsive to speech levels occuring on the transmitting and receiving paths to control the blocking element and the attentuation element and which controls the attenuation element to smaller attenuations or deactivates is when the speech level of the transmission path exceeds the speech level of the receiving path or exceeds the speech level which is reduced by the attentuation of the hybrid junction on the receiving path, characterised in that the control arrangement comprises an evaluation device which compares the speech level of the transmissing and receiving paths for the determination of the duplex state and for switching on the attenuation element, and that the control of the attenuation of the attenuation element is effected via a threshold value device.

2. An echo trap as claimed in Claim 1 wherein the control arrangement includes a first gate an output signal from which serves to render the blocking element blocking, a second gate having an AND function an output signal from which serves to activate said switch, a first speech- detector having an input connected to said third line, and an output connected to an input of each of said first and second gates, an evaluation device having two inputs and an output which is connected to another input of the second gate and to a blocking input of the first gate and at which an output signal is produced which is dependent upon the relative magnitudes of signals supplied to said two inputs, a second speech detector having an input connected to said first line and an output connected to one of said two inputs of the evaluation device, and a third speech detector having an input connected to said fourth line and an output connected to the other of said two inputs of the evaluation device.

3. An echo trap as claimed in Claim 1 or 2 wherein the attenuation element is a dynamic compression attenuation element.

4. An echo trap as claimed in Claim 3 wherein a rectifier is connected between said fourth line and a control input of the attenuation element to achieve said dynamic compression.

5. An echo trap as claimed in any of Claims 2 to 4 wherein the control arrangement includes a further evaluation device having first and second inputs connected to the outputs of said first and second speech detectors respectively and an output connected to an additionally provided blocking input of said second gate whereby the control arrangement ceases to activate said switch when an output signal is produced at the output of said further evaluation device.

6. An echo trap as claimed in Claim 3 or 4 wherein the control arrangement is arranged to limit the dynamic compression attenuation range of the attenuation element to an increasing extent the greater the speech level on the transmitting" path is relative to the speech level on the receiving path.

7. An echo trap as claimed in Claims 4 and 6 wherein the control arrangement includes a further controllable attenuation element connected between the output of said rectifier and the control input of said attenuation element.

8. An echo trap as claimed in Claim 7 wherein the control arrangement includes a further evaluation device having first and second inputs connected to the outputs of said first and second speech detectors respectively 47682/2 and an output connected to a control input of said further attenuation element.

9. An echo trap as claimed in Claim 7, wherein the control arrangement includes a threshold value device having an input connected to the output of the -second speech detector and an output connected to a control input of said further attenuation element and an output signal at which serves to reduce the attenuation of said attenuating element.

10. An echo trap as claimed in any of Claims 2 to 10 wherein said evaluation device and the second and f third speech detectors are arranged to be such that the transmitting path speech level is evaluated to be higher than the receiving path speech level by a fixed value v/hich is not greater than the minimum attenuation from the receiving path to the transmitting path produced by the adjacent hybrid junction. 47682/2

11. An echo trap as claimed in any of Claims to 10 wherein the second and/or third speech detectors include one or more control elements and the control arrangement includes controlling devices therefor, which control elements and controlling device therefor are arranged to effect a higher evaluation of the speech levels of the transmitting path in comparison to those of the receiving path in the event of rising attenuation from the receiving path to the transmitting path of the adjacent hybrid junction.

12. An echo trap as claimed in any of Claims 2 to 12 wherein the control arrangement includes a delay element connected in series with or forming part of the third speech detector, which delay element serves to delay the signal supplied by the third speech detector to -the evaluation device at least by the transit time from the receiving path to the transmitting path via the adjacent hybrid junction.

13. An echo trap, for use in a four-v/ire transmission system, substantially as herein described with reference to any of Figures 2 to 6 of the accompanying drawings .

14. - A four-wire transmission system including one or more echo traps each as claimed in any of the preceding Claims.