1. Field of the Invention
The present invention relates to a method and a circuit arrangement for compensating crosstalk and/or echo signals, and more particularly to a method and a circuit arrangement in which such signals are compensated in a digital communication system using an adaptive crosstalk and/or echo compensation circuit.
2. Description of the Prior Art
In telecommunications systems, in the transmission of signals, linear signal disturbances can occur, for the elimination of which compensation circuits can be used. Such disturbances can be caused, for example, in telecommunications systems in which, for the conversion of two-wire lines into four-wire lines sections and vice-versa, hybrid circuits (bridge circuits) are provided, by virtue of the fact that a hybrid circuit of this type would have to contain an exact balance of the input impedance of the two-line wire in order to completely decouple the (receiving) arm of the four-wire line section, outgoing from the hybrid circuit, from the (transmitting) arm of the four-wire line section, incoming to the hybrid circuit, for which purpose the balancing line must be individually adapted to the two-wire line by accurate adjustment. This is accomplished with the assistance of a compromise balance, which is frequently used in practice and by way of which it is attempted to take into account various line impedances, the hybrid circuit, which then exhibits a finite blocking attenuation dependent upon the line impedance, is able to mutually decouple the transmitted and received signals only to a limited extent. On the other hand, even with optimum compensation of the hybrid circuit, echoes of its own transmitted signals which are caused by reflection points on the two-wire lines cannot be readily suppressed. Finally, in addition to the interference signals mentioned above, linear disturbance distortions produced in the transmission channel can also be compensated with the assistance of an adaptive equalizer.
In order to eliminate, or at least reduce crosstalk and/or echo signals, it is known, for example from the publication AGARD Conf. Proc. No. 103 (1972), pp. 12-1 . . . 12-16, and in particular to FIG. 12 and FIG. 13, and from FREQUENZ, Vol. 29, No. 5, 1974, pp. 118-122 and 155-161, and 122, and from Der Fernmelde-Ingenieur Vol 31, No. 12, 1977, pp. 1-25 and 21, to provide, between the four-wire line arm incoming to a hybrid circuit and the four-wire line arm outgoing from the hybrid circuit, an adaptive crosstalk and/or echo compensation circuit having a variable, non-recursive transversal filter and having a logic linking element which input-couples the compensation signal into the outgoing four-wire line arm, and in which the filter coefficients are adjusted in a regulating circuit which is basically known from, for example, the publication NTZ, Vol. 24, No. 1, 1971, pp.18-24 or from the book Bocker: Datenubertragung, Vol. 1, Chapter 5.3.2, in accordance with the residual signal which occurs following compensation in the outgoing four- wire line arm, so that this residual signal is minimized.
Such an adjustment of the adaptive filter, which is independent of the state of a receiving circuit which follows the outgoing four-wire line arm and which generally contains an adaptive equalizer connected before a decision device, and in which the compensated received signal, i.e. the receiving circuit input signal (or even only the sign thereof) serves as a regulating signal, in itself facilitates a reliable run-up of the compensation circuit to the status of the highest-attainable compensation accuracy. Since, however, the useful signal emanating from the two-wire line is superimposed upon the compensation error signal (residual signal), in order to ensure that the run-up takes place with the desired degree of accuracy, the adjustment value for the filter coefficients must be selected to be very small, which results in a relatively long run-up time and a long coefficient word length.
In addition, in order to eliminate, or at least reduce, crosstalk and/or echo signals, it is known from the Bell System Technical Journal Vol. 58, No. 2, 1979, pp. 491-500 and 493, in a compensation circuit of this type having a non-recursive digital filter having variable coefficients, to use the decision error of the decision device contained in the receiving circuit, i.e. the difference between the (compensated) input signal and the output signal of the decision device located in the outgoing four-wire line arm, as regulating signals for the adjustment of the filter coefficients and simultaneously for the adjustment of an adaptive equalizer which is provided with decision feedback.
Here, with a given synchronization so that no faulty decisions occur, it is only the compensation error signal (residual signal) which acts as a regulating signal, which results in a rapid run-up of the compensation circuit together with a relatively short coefficient word length. On the other hand, in the absence of synchronization, faulty decisions occur which of themselves result in long adjustment times and stability problems.