SE467946B - Device for turning off adaptation in a calculation unit for echo cancellation - Google Patents

Abstract

According to the invention a device is proposed for use for the suppression of echoes which arise at the transition from four-core cables to twin-core cables. A signal processor and microprocessor are arranged for this, where the signal processor is fast and handles the direct signal processing. This contains a double talk detector 25 and a calculating device 17 with adaptive algorithm. The microprocessor 31 monitors the echo suppression and sets the step size of the adaptation. A direct route is arranged between the detection device 25 and the calculation device 17 so that when a condition with double talk is detected the adaptation is shut off completely in the calculation device 17. The microprocessor 31 which only periodically monitors the detection device 25 sets the step size for the adaptation to a reduced value when the microprocessor 31 finds, during a periodically recurring check, that the condition double talk exists. The adaptation with reduced step size is maintained for a relatively long predetermined period of time, during which the direct route between the detection device 25 and the calculation device 17 is shut off. <IMAGE>

Description

467 946 2 computing device, which uses an adaptive algorithm to calculate a signal which is subtracted from the signal which is present on the two output lines. The fact that the algorithm is adaptive means that it depends on at least one or more coefficients and these coefficients can be changed so that the result of the calculation will be as intended. The intended effect is that the signal, which is derived from the result of the algorithm, when subtracted from the signal on the two output lines, should give a signal level which is as close to zero as possible.

For this, an additional detection device for the signal level in the two output lines must be provided, which compares the signal level with the desired zero level and then sends the result of the comparison to the calculation device. When a signal is transmitted on the two outgoing lines in the four-wire line from the subscriber connected to the two-wire line, the said change of the coefficients in the adaptive algorithm must no longer continue, otherwise this signal on the outgoing two lines in the four-wire line would be perceived as an echo and give gives rise to a completely incorrect setting of the echo canceller device. The same applies when signals are received on the two incoming lines in the four-wire line and the two outgoing lines in the four-wire line; ie for double talk. In the following, therefore, the term double talk also includes the case where only a signal from the subscriber connected to the two-wire line is present but no signal is received on the two incoming lines in the four-wire line.

Examples of the prior art are given in US 4 005 277 and US 4 360 712, where the latter document shows circuits for switching off the adaptive algorithm.

DISCLOSURE OF THE INVENTION The present invention relates to a method of turning off or modifying the adaptive algorithm in double talk and this by means of the components which in many modern telephony systems are available in connection with hybrid circuits. The more detailed features and characteristics of the invention are otherwise apparent from the appended patent claims. Thus, in an echo canceller device according to the above including computing circuits, which utilizes an adaptive algorithm, a direct connection is provided between the device for detecting double talk and the computing circuits. This path goes directly into the circuits and shuts off the adaptation, ie makes it impossible to further change the coefficients, which are included in the adaptive algorithm. In this direct path, such a signal is transmitted to turn off the adaptation as soon as a state with double talk is detected. A microprocessor is further provided, which monitors the device for echo cancellation and which also monitors the double talk detector periodically at small time intervals. If the microprocessor finds that a state with double talk exists, the microprocessor shuts off the direct path between the device for detecting double talk and the calculation circuits, so that the adaptation, ie the modification of the variable coefficients in the algorithm, can be resumed. Furthermore, the microprocessor sends a signal to the calculation circuits, so that the adaptation takes place with a reduced step size compared to the normal step size or possibly with a step size equal to zero. Furthermore, this condition with a reduced step size adaptation or with a step size equal to zero may also suitably take place over a period of time extending over several times, when the microprocessor would normally have checked if the double talk detection device had detected that a condition with double talk was present.

DESCRIPTION OF THE DRAWINGS The invention will now be described in connection with the accompanying drawing, which schematically shows the device according to the invention. PREFERRED EMBODIMENT second two lines 5 for signals from the local subscriber. These two groups consisting of two lines each are shown in the figures with single lines as well as the two-wire line 1. The signals in both directions on the four-wire line 3, 5 are assumed to be transmitted digitally, while the signals on the line 1 to the local subscriber are transmitted analogously in the usual manner.

For connection of the two-wire line 1 to the four-wire line 3, 5, a digital-to-analog-to-analog converter 7 is therefore required, which is connected to the incoming two lines 3 in the four-wire line and converts the digital signals on the lines 3 into analog signals. In the same way, for the two lines 5 for outgoing signals in the four-wire line, an analog-to-digital converter 9 is connected, which converts incoming analog signals into digital signals, which are transmitted on the lines 5.

The output of the digital-to-analog converter 7 and the input of the analog-to-digital converter 9 are connected to a converter 11, which takes care of the actual connection of the four-wire line 3, 5 to the two-wire line 1. This conversion or transmission in the converter 11 is necessarily incomplete, so that signals incoming on a line group to it are transmitted on in both the other two connected line groups, albeit with considerably reduced signal strength. However, due to the digital-to-analog converter 7, no signals can be transmitted on the lines 3 from the converter 11. On the other hand, signals on the near-subscriber's two-wire line 1 are transmitted through the analog-to-digital converter 9 to the lines 5. The same also applies to signals incoming on the four-wire line in its incoming part 3, since the transition to the wire-wire line 1 of this signal is not perfect. This gives rise to an echo in the lines 5, i.e. a signal in the lines 5, which is a more or less accurate representation with reduced signal level of the signal received on the incoming lines 3G of the four-wire line. To eliminate the echo thus obtained in the lines outgoing two lines 5 in the four-wire line there is an echo canceller 13. The echo canceller usually generates a signal which is subtracted from the signal in the outgoing two-wire lines 5 at an interconnection point 15. The signal which is subtracted by the echo canceller 13 from the signal in the the outgoing lines 5 in the four-wire line in the node 15, are generated by a calculation device 17 which via a line 19 connected to the incoming lines 3 in the four-wire line receives signals on these incoming lines 3 and on the basis of the signal thus received calculates a signal , which is supplied via a line 21 to the node 15.

The calculation unit 17 further senses the signal which is then transmitted from the node 15 on the output lines 5 of the four-wire line via a line 23, compares the sensed signal with the desired value zero, and takes special measures if the absolute value of this signal level differs. more than a predetermined value, a threshold value from zero. If this value is below the threshold value, no change is made in the calculation circuit 17, but this calculates its result in the same way as before to give a suitable signal. As before, this signal value in the node 15 is subtracted from the output signal on the lines 5.

If the value of the signal level in the lines 5 emanating from the node 15 exceeds the threshold value, certain coefficients are modified in the calculation algorithm, which is the basis for the function of the calculation unit 17. This change of the algorithm takes place step by step and these steps have a predetermined absolute size. The sign of the change is suitably selected for each such change of the coefficients, so that the signal output on the line 21 from the calculation unit 17 imitates as faithfully as possible the echo which is expected to be transmitted on this line.

However, when a signal arrives on the wire line 1 and is allowed to pass on the lines 5, this signal is not an echo and the calculation anode 17 would give a completely incorrect result, if the adaptation were allowed to continue. The same condition occurs if both signals are received on the lines 3 and transmitted on the lines 5 in the four-wire line. This latter condition is called double talk and this also includes the case that only signal is received on the main wire line 1 and transmitted on the output lines 5 in the four wire line. * 'To detect this state with double talk there is a special detector 25. This emits on its output 27 a signal that a state with double talk exists. The detector 25 can 467 946 ~ 6 be a simple circuit which utilizes the signal levels in the incoming lines 3 and the outgoing lines 5 in the four-wire line. For this purpose, the detection device 25 thus has its two inputs connected partly to the incoming lines 3 and partly to the outgoing lines 5 in the four-wire line. The output of the detection device 25 is connected to the calculation device 17 via a line which passes through an AND gate 29.

The signal thus transmitted via the AND gate 29 from the double talk detection device 25 is received in this way in the calculating device 17, and if the signal indicates that a state with double talk is present, the adaptation is stopped immediately.

This termination of the adaptation is effected directly in the calculation circuits of the calculating device 17. Thus, in this case, the signal emitted from the calculating device 17 is calculated only by means of the coefficient values which were present when the double talk state signal was obtained. The device also contains a microprocessor 31, which monitors the echo canceller device 13. The microprocessor 31 monitors in particular if a state of double talk exists. For this purpose, the microprocessor 31 periodically checks a register 33, in which a signal flag is written by the detection device 25, if a state with double talk is present.The register 33 is thus connected to the output of the double talk detection device 25 and also to the microprocessor 31.

When the microprocessor 31, by reading the register 33, finds that a state with double talk exists, the microprocessor 31 closes the direct path between the output of the detection device 25 and the calculation device 17. This is done by means of the AND gate 29, so that an output from the microprocessor 31 is connected to an input of the AND gate. The microprocessor 31 thus emits at its output, which is connected to the AND gate 29, a logic low signal, a zero, when it has read by reading the register 33 that double talk exists, while in normal cases it emits a logic high signal , a one at this exit, so the direct road then inhte is blocked. The line between the microprocessor 31 and the AND gate may further contain a register 35, from which another unit, not shown, can read, if the microprocessor 31 is in a state, when it controls the adaptation in the calculation unit 17 to a state with reduced step size or step size equal to zero. This is done by placing a signal flag in the register 35. Thus, when the microprocessor 31 sends a logic blocking signal, a zero, to the AND gate 29, the direct connection between the output of the detection device 25 and the calculation unit 17 is broken.

Furthermore, the microprocessor 31 sends via a line 37, which is connected to an input of the calculating device 17, a signal that the adaptation should now take place with a reduced value, normally amounting to, for example, less than one hundredth of the normal value, or possibly that the step size must be set to zero. This state with reduced step size or step size equal to zero is then maintained for at least a predetermined period of time, extending over several times at which the microprocessor 31 would normally have read the register 33 to determine if a double talk state exists. .

When the said predetermined period of time has elapsed, the microprocessor 31 then reads the register 33 again, which is connected to the output of the double talk detection device 25. If a state with double talk now no longer exists, as indicated by the value entered in the register 33, the microprocessor sends a signal on its line 37 to the calculation unit 17, that the adaptation of the calculation algorithm in the calculation unit 17 should be resumed with the normal step size. Otherwise, of course, the state of reduced step size or step size is continued equal to zero in the adaptation, until the microprocessor 31 reads the register 33 again.

The various components and circuits included in the echo canceller 13 are inherently fast and consist of hardware components and circuits specially designed for their purpose. These can be included in a signal processor, which in many cases is arranged in modern telephony systems next to the transition between four-wire line and two-wire line. A microprocessor 31 is also provided next to this transition in modern systems, but the microprocessor is comparatively slow and can perform 467,946% - 8 complicated logical choices and especially time measurements, which is not easily possible with the circuits available. hands in a signal processor, especially not in its echo-clearing part.

Such a signal processor and microprocessor can control the transition between a plurality of two-wire lines 1 and four-wire lines 3, 5.

Claims (2)

9 - 467 94 PATENT REQUIREMENTS A device for switching off the adaptation in a calculation unit, which comprises an adaptive algorithm and is connected at the transition between a 4-wire line connected to a remote subscriber and a 2-wire line connected to a local subscriber and which is included in a circuit for eliminating echoes in the 4-wire line, the calculation unit being so arranged that the adaptive algorithm is dependent on at least one coefficient and that this is periodically changed depending on a step of a certain step size with a character determined at each time, when said circuit senses , that an echo is present, characterized in that a direct path is arranged between the device for detecting double talk and the calculation unit and that this path is arranged to switch off the adaptation immediately, when a state with double talk is detected and that a microprocessor is arranged to monitor the double talk detection device at specified time intervals and to detect at detection the gs device has detected a state with double talk in turn turn off the direct path between the detection device and the calculation device and change the step size of the adaptation in the calculation unit's algorithm to a small part of its previous value, ie to resume the adaptation with reduced step size, or to set the step size equal to zero. Device according to claim 1, characterized in that the microprocessor is arranged that after detection that the detector device has detected a state with double talk, the adaptation always continues with reduced step size or step size equal to zero for a predetermined time and during this whole time. keep the direct path closed between the detection device and the calculation unit.