METHOD TO TEST THE FUNCTION OF AN ADAPTIVE ECHO CANCELLER

Abstract: A method of testing an apparatus for adaptive echo cancellation with regard to faults in the included units. A signal is sent from the send side, while the remote signal to the apparatus is disconnected. The received signal (s2) originating from the send signal is sampled and cancelled by the echo canceller of the apparatus, and the thus obtained corrected sampled receive signal (s3) is compared with a positive and a negative threshold value (+T or -T). As a measure of the test is taken the number of samples per second which exceeds or falls below the threshold values.

Description: METHOD TO TEST THE FUNCTION OF AN ADAPTIVE ECHO CANCELLER. TECHNICAL FIELD The present invention relates to a method of testing an apparatus for adaptive echo cancellation. Such an apparatus is included in a transmission equipment for digital communication. BACKGROUND ART An apparatus for adaptive echo cancellation of the kind mentioned above is previously described in the Swedish patent application 8106444-6. As described in this application, echo cancellation of such as an incoming analogue signal takes place by correction signals from an adaptive balance filter being added to the signal, the parameters of the balance filter being continuously or stepwise updated from a correction unit. A usual method of functionally controlling digital equipment is to loop circuit the data flow from transmitter to receiver and carry out a bit error measure¬ ment. In this way the transmitter, receiver and four-wire to two-wire junction of the echo cancellation apparatus can be functionally controlled. DISCLOSURE OF INVENTION Verification of the function of the echo canceller included in the apparatus involves certain problems, since a measuring signal must be supplied to the two- wire side of the apparatus in such a case. This means that an access relay must be connected to the two-wire before the hybrid circuit for supplying the measuring signal to the echo canceller. The measuring signal must be line coded in the way performed in the send units of the apparatus with the object of simulating the remote signal. Finally, the echo is changed when the line is interrupted, which makes it theoretically possible for the echo from the measuring equipment to be inhibited, but not the echo from the line. In the above mentioned apparatus of the prior art, it is of importance to establish that the correction unit and the digital balance filter fulfil their functions within given error margins. This means that the balance filter shall supply correction values to the summator, which adds the correction values to incoming signals so that the value of the residue echo falls below a given limit. If the incoming signal is sampled and the correction signal consists of digital values at the addition, ' these values should be correct within given intervals. Incorrect values for the correction signals give poorer echo cancelling and can thus give rise to incorrect reception. As examples of causes may be mentioned: false supply values to the balance filter, deficient synchronism and fault in the memory cell stucture of the balance filter, e.g. a faulty ROM in the store unit. The object with the present invention is to provide a method of testing the function of an adaptive echo canceller without using an access relay on the line side. The method is characterized as will be apparent from the appended claims. BREF DESCRIPTION OF DRAWINGS The invention will now be described in detail with reference to the accompany- ing drawing, whereon Fig 1 is a block diagram of an apparatus for adaptive echo cancellation, in which the inventive method is utilized, and Fig 2 is a diagram of some signals which occur in the apparatus according to Fig 1. BEST MODES FOR CARRYING OUT THE INVENTION An apparatus for adaptive echo cancellation is illustrated in the block diagram of Fig 1, together with a circuit for carrying out the method in accordance with the invention. On the send side, the apparatus includes a send unit A for data with a coding unit B connected to the output of the send unit A for recoding send data to a suitable line code, e.g. biphase code. The output of the coding unit B is connected to a two-wire to four-wire hybrid F, here exemplified as a voltage bridge including the resistors Rl, R2 and R3 as well as an operational amplifier 3. In one of the two-wire paths a swith SI is connected to one terminal of a line transformer TR. In the reception path of the apparatus there is a filter circuit (a lowpass filter) C with its input connected to the ouput of the amplifier J for receiving and filtering the incoming line signal from the two-wire line. The output of the filter C is connected to a summator AD and further to a receiver detector D. The block E constitutes an adaptive echo canceller connected between the send and receive paths and to a summator AD. The echo canceller E conventionally sends a correction signal β. to the received and filtered signal s„, a signal s, being obtained, which is free from echo, as far as possible. A correction unit KE is conventionally connected between the detector D and the echo canceller E for adjusting the filter parameters in the balance filter included in the echo canceller E in dependence on the received and detected signal s,. The plus input of a first and a second comparator 31 and J2 may be connected to the output of the summator AD. The minus input of one comparator Jl is connected to a first threshold voltage +T and the plus input of the other comparator J2 is connected to a second threshold voltage -T. Each output of the comparators Jl and J2 is connected to a contact Kl and K2, respectively, included in a sampling circuit SK. The contacts Kl and K2 are controlled such that sampling of the output signal from the respective comparator is done in the same way as for the received signal s, in the block D, i.e. synchronous with the sampling rate in block D. An OR-circuit EK is connected to the output of the sampling circuit SK. When testing the echo canceller function according to the inventive method, the echo canceller E is connected in the normal way and the contact SI is closed. The remote end transmitter is shut out so that no line signal occurs over the two-wire 2W, apart from the one generated by the local sender (block A). A random bit flow, i.e. a bit flow in which "ones" and "zeros" occur randomly, is applied to the input dx of the data sender A. This bit flow is transferred via the units B, F, and C to the input of the summator AD where it occurs as a signal 82, and also to the echo canceller E which gives a compensation signal 8, , and for perfect echo cancellation s, = -s«. The signal 8 on the output of the summator AD gives the echo signal which has not been cancelled. This signal is taken to the inputs of both comparators Jl and 32 and is compared with the threshold values +T and -T. If this signal s, is positive then the comparison with +T is of interest, while the comparison with the threshold value -T is of interest if S is negative. After sampling in the circuit SK, there is obtained a "one" pulse or no pulse a "zero" on the output of the OR circuit EK according to the following conditions at the sampling instants: If S3 > +T, a "one" (8 > 0) is obtained -T < 83 < +T, a "zero" (83 ^ 0) is obtained, and S3 < -T, a "one" (S3 < 0) is obtained. The threshold values +T, -T have a value selected such that the lowest expected remote signal can be detected even if the residue echo has such a value of the amplitude. The compared signals are sampled in the same way as a received remote signal, (e.g. 4 times/bit). An error signal v is obtained from the output of the OR circuit EK and consists of a pulse each time the echo canceller E cannot cancel the echo (the signal s, in its entirety) to a value less than /T/. The signal v, which is thus "zero" when the compensative signal S3 is less than /T/ and "one" when Iz > HI can be utilized and evaluated. A quality limit is suitably set for the echo canceller E, measured in the number of error pulses ("ones") per second. It is also conceivable to carry out the measurement of the compensated signal S3 with the contact SI open or with a nominal load connected to this contact. Noise disturbances thus disappear from the line, but the echo characteristic is simultaneously changed. Fig 2 illustrates both signals s 2 and 83 in Fig 1, and the action of a given compensation from the echo canceller E. The filtered signal s 2 is here assumed to be essentially sinusoidal and is sampled at the instants t Q , t., t ? , t,, ... t,, t 7 , tg, t o , the echo canceller here sending at these instants a digital signal level β, for the compensation. For example, at t, there is sent a level -d^ from the echo canceller E to the summator AD, which lowers the level of 8« so that the signal 8 3 will come below the threshold +T. This is assumed to take place at the instant t, also. In a similar way, a compensation with +d 2 takes place at the sampling instants t β , t 7 so that the level of the signal S3 comes above the threshold -T. At the instant t β the compensation is assumed to lapse due to possible errors in such as one of the stored units in the echo canceller E, resulting in that the value of 83 becomes lower than -T. Correct compensation takes place once again at the instant tg. If the error returns a given number of times during a given time interval, e.g. during one second, this can be detected and measures taken. For example, the error may be due to a cell being faulty in the store unit. If the store unit is then addressed by a 7-bit word the error will occur on an average for every 128th sample. With the inventive method, the remote signal is thus disconnected and it is a condition that the signal received from the local send side shall be lower than a given absolute level /T/ for the echo cancellation to be acceptable.

Claims: CLAIMS 1 Method of testing the function of an apparatus for adaptive echo cancellation included in a transmission equipment for data communication, in which a send signal is applied to the send side (A,B) of the apparatus for transferring via a two-wire to four-wire hybrid (F) to a remote end, said send signal giving rise to a correction signal (s,) on the receiver side (C, D) of the apparatus for correction of the signal (s«) received from the hybrid (F), characterized in that the signal transferred from the remote end is dis¬ connected, and that after said correction signal (s,) has compensated the received signal (s 2 ) originating from the supplied signal, the compensated signal (83 thus obtained is compared with at least one threshold value (+T or -T), a quality limit for the compensated signal (S3 being determined such that the part of the compensated signal (S3) which is greater or less than than said threshold value during a given time interval is determined. 2 Method according to claim 1, characterized in that said compensation signal (S3) is compared with two threshold values (-t-T, -T) of opposite polarity. 3 Method as claimed in claim 1 or 2, characterized in that said quality limit is determined such that the signal obtained after comparison with a threshold value is sampled and the number of signal samples thus obtained which exceed or fall below said threshold value (+T, -T) during a given time interval are determined.

Inventors: DAHLQVIST INGEMAR ERIK, DAHLQVIST, INGEMAR, ERIK
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1984
Main classification: H04B3
IPC codes: H04B3