A METHOD OF CONVERGING AN ECHO CANCELLER

Abstract: The invention relates to a method of converging an echo canceller at the start of a telephone call. A converging of the echo canceller (4A-D) integrated into a telephone exchange (1, 2) is initiated by an internal command of the telephone exchange given by a call control (2) of the telephone exchange as soon as the call control (2) has observed an off-hook state of a called subscriber connection, said command causing the echo canceller (4A-D) to transmit a test signal to the echo path for converging the echo canceller before appearance of the speech signal.

Description: A method of converging an echo canceller Field of the Invention The invention relates to a method of converging an echo canceller at the start of a telephone call. Background of the Invention End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed for instance in the case of normal speech, when a signal is reflected from the far end of a con¬ nection back to the talker. An echo canceller is a device for processing a signal, such as a speech signal, so as to reduce echo by subtracting estimated echo from the echo (signal) occurring in a connection. The echo canceller can be either digital or analog. Echo cancelling devices are at present realized by digital signal processing, by means of which it is possible to model echo paths including considerably long transit time delays. A digital adaptive filter is then used, in which the coefficients of the filter modelling the echo path are updated on the basis of a correlation of the speech signal and the returning echo signal. Since the echo path in principle is different at each call, it is necessary to apply to the echo canceller a method which always adapts itself to a new echo path at the start of a call. It takes a little time to converge the echo canceller, i.e. to create a model of the echo path after the speech has begun in the connection, and therefore, echo occurs momentarily at the start of the call before the echo canceller has been converged. Disclosure of the Invention The object of the invention is a method which makes it possible to converge an echo canceller as soon as possible at the start of a telephone call. This is achieved according to the invention by means of a method of converging the echo canceller at the start of a telephone call, which method is char¬ acterized by initiating a converging of the echo can¬ celler integrated into a telephone exchange by an internal command of the telephone exchange given by a call control of the telephone exchange as soon as the call control has observed an off-hook state of a called subscriber connection, said command causing the echo canceller to transmit a test signal to the echo path for converging the echo canceller before appearance of the speech signal. The basic idea of the invention is to integrate an echo canceller -into a telephone exchange in such a way that a converging of the echo canceller can be controlled by the call control of the telephone ex¬ change by utilizing the internal call status data of the exchange. This makes it possible to start con¬ verging the echo canceller and to perform it for in¬ stance as soon as a subscriber outside the telephone exchange acting as the called subscriber answers (off-hook) and before the called subscriber manages to get the handset to his ear and starts talking, due to which the calling subscriber of the telephone ex¬ change does not observe any echo, not even at the start of the call. Because the speech channel does not yet convey any speech signal, by means of which the echo canceller could be converged to the echo path, the echo canceller transmits a particular test signal, such as an impulse, to the echo path for this purpose. The invention relates also to another method of converging an echo canceller at the start of a tele¬ phone call, which method is characterized in that the echo canceller integrated into a telephone exchange transmits a test signal to the echo path in the di¬ rection of the calling subscriber between ringing tones at a moment controlled by a call control of the telephone exchange for converging the echo canceller by means of said test signal. In some cases, converg- ing can also be performed by means of the very out¬ going ringing tone. Also this method is based on integrating the echo canceller into the telephone exchange in such a way that a converging of the echo canceller can be controlled by the call control of the telephone ex¬ change by utilizing the internal call status data of the exchange. This method aims to prevent an echo from occurring at the start of a telephone call also in a situation when a subscriber of the telephone exchange is the called subscriber and the intention is to avoid the test signal being heard disturbingly in the calling subscriber's ear. Then the test signal can be transmitted to the echo path in the direction of the calling subscriber at a moment controlled by the call control of the telephone exchange immediate¬ ly after the ringing tones going to the calling sub¬ scriber, due to which the test signal will probably not be observed. Brief Description of the Drawing The invention will be described in the follow¬ ing in greater detail by means of an embodiment with reference to a drawing, in which Figure 1 shows a block diagram of an arrangement according to the in- vention, in which arrangement echo cancellers are integrated into an telephone exchange. Detailed Description of the Invention The arrangement according to the invention can be applied in connection with any type of telephone exchange and adaptive echo canceller. Figure 1 shows a telephone exchange comprising four near end lines Al to A4, to which the subscribers of the telephone exchange are connected, and four far end lines, pre- ferably 2,048 Mbit/s PCM lines according to the CCITT recommendation G.703, each line having 32 chan¬ nels, 30 of which are telephone channels. Between the channels of the lines Al to A4 and the channels of the lines Bl to B4, it is possible to switch trans- mission connections (calls) through a digital switch¬ ing unit 1 in a manner known per se. The operation of the telephone exchange is controlled and monitored by a call control computer 2. The telephone exchange naturally also includes devices and software for af- fecting the signalling to be used each time. To each PCM line Bl to B4 of the near end is connected respective echo cancelling unit 4A to 4D, which carries out the echo cancellation of each PCM line channel separately, the channels being 31 in number in the example case. Each echo cancelling unit 4A to 4D can thereby include a separate echo cancel¬ ler for each channel. The operation of the echo can¬ cellers 4A to 4D, especially switching on and off, is controlled by a computer 3 for the control of the echo cancellation via a common control channel 5 within the telephone exchange. The operation of the computer 3 is controlled by the call control computer via a control channel 6. The control computer 3 thus acts as an interface between the call control 2 and the echo cancelling units 4A to 4D. The control com- puter 3 delivers the commands from the call control computer 2 to the echo cancelling units 4A to 4D in the corresponding control time slots of the time- shared control channel 5, the commands including e.g. commands for enabling or disabling the echo cancella¬ tion and the commands for starting converging. On the other hand, the call control computer 2 produces the commands for the control of the echo cancellers on the basis of a received echo cancellation signalling and on the basis of other call status information used by the call control, such as off-hook state (an¬ swering) of the called subscriber. The purpose of the echo cancellers 4A to 4D is to eliminate the echo, originating from the near end and being caused by a speech signal originating from the far end, from reaching the far end. For this, the echo canceller creates a model of the echo path in the direction of the near end at the start of each call. The procedure of creating the model of the echo path is called converging. In the following is explained how a converging of an echo canceller is started under control of the call control computer 2. It is assumed that a call is coming from outside the telephone exchange in one of the channels of the PCM lines Bl to B2 to a sub¬ scriber of the telephone exchange, which is connected to one of the channels of the PCM lines Al to A4. In other words, the called subscriber is a subscriber of the telephone exchange and the calling subscriber a subscriber from outside the telephone exchange. The call control 2 of the telephone exchange handles the call in a normal manner and transmits a ringing sig¬ nal, such as a ringing voltage, to the line of the called subscriber and a ringing tone (a ringback tone) to the direction of the calling subscriber. When the called subscriber answers the phone by pick¬ ing up the handset off the hook or by activating the subscriber line in some other way, the call control 2 causes the echo canceller of respective telephone channel to start converging by an internal command of the telephone exchange over the control computer 3 and the control channel 5. Then the echo canceller transmits a particular test signal to the echo path in the direction of the calling subscriber. On the basis of this test signal the echo canceller is con¬ verged, i.e. it creates a model of the echo path on the basis of the transmitted test signal and the sig¬ nal returned from the echo path. Due to this, the echo canceller is ready to cancel the echo immediate- ly at the beginning of the speech so well that no echo is observed by the called subscriber. Another case is assumed, in which a subscriber of the telephone exchange calls in one of the tele¬ phone channels of the lines Al to A4 outside the te- lephone exchange in one of the telephone channels of the lines Bl to B4. The calling subscriber is then a subscriber of the telephone exchange and the called subscriber an external subscriber. Establishing the call in the telephone exchange proceeds again in a normal way. When another telephone exchange, to which the outside called subscriber is connected, starts transmitting a ringing tone back to the calling sub¬ scriber, the echo canceller connected to respective telephone channel transmits between the ringing tones a test signal to the echo path in the direction of the calling subscriber at a moment controlled by the call control and is adapted to the echo path by means of this test signal. Consequently, as soon as the called subscriber answers and the speech begins, the echo canceller is ready to cancel the echo so well that no echo is observed by the called subscriber. After the call, the call control 2 commands the echo canceller back to normal state for next call. The figures and the description relating to them are only intended to illustrate the present in¬ vention. As to the details, the method of the inven¬ tion can vary within the scope of the enclosed claims.

Claims: Claims : 1. A method of converging an echo canceller at the start of a telephone call, c h a r a c t e r - i z e d by initiating a converging of the echo can¬ celler (4A-D) integrated into a telephone exchange (1, 2) by an internal command of the telephone ex¬ change given by a call control (2) of the telephone exchange as soon as the call control (2) has observed an off-hook state of a called subscriber connection, said command causing the echo canceller (4A-D) to transmit a test signal to the echo path for converg¬ ing the echo canceller before appearance of the speech signal. 2. A method of converging an echo canceller at the start of a telephone call, c h a r a c t e r ¬ i z e in that the echo canceller (4A-D) integrated into a telephone exchange transmits a test signal to the echo path in the direction of the calling sub- scriber between ringing tones at a moment controlled by a call control (2) of the telephone exchange for converging the echo canceller by means of said test signal. 3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that said test signal is a test pulse.

Inventors: LAEHDEMAEKI HEIMO, SIPPOLA JAAKKO, LAEHDEMAEKI, HEIMO, SIPPOLA, JAAKKO
Applicants: NOKIA TELECOMMUNICATIONS OY
Application year: 1992
Main classification: H04B3
IPC codes: H04B3, H04M3