Abstract: Echo-cancellation with the aid of a number of cascade-connected adaptive filters, wherein each filter forms an output signal for cancelling its respective part of an echo signal appearing on a line included in a four-wire loop. The filters will preferably be able to converge rapidly, at least for relatively short impulse responses, and to provide effective echo-cancellation for different kinds of impulse response. Those filters which are required for the echo cancellation at that moment in time are ascertained by calculating a special quality measurement for each filter. There is formed an echo-reduced signal which is equal to the difference between the original, echo-included signal and the sum of the output signals from those filters that are needed for the echo-cancellation. These filters are updated with the echo-reduced signal, whereas the remainder of the filters are updated with signals formed as the difference between the echo-reduced signal and the output signal of respective filters.

Abstract: Apparatus for reducing the risk of undesirable drifting of the parameters, so-called bursting, of an adaptive echo filter which is coupled between a receiving branch and a transmitting branch included in a four-wire loop. A difference signal (E) which contains no echo from the receiving branch is generated in the transmitting branch in a known manner. The apparatus includes an adaptive predictor filter which is coupled between the transmitting branch and the receiving branch and which functions to generate a compensation signal from the difference signal (E) when transmitting narrowband signals from a near-end. The compensating signal is subtracted from a signal which appears in the receiving branch and the predictor filter is updated in a manner to minimize the correlation between the difference signal (E) and the input signal (X') of the echo filter.

Abstract: Filtering a signal with an adaptive digital filter includes updating the filter by performing an approximate division of a constant number by a variable number in binary form. The variable number represents a magnitude of energy of the signal to be filtered, and consists of a character bit and a plurality of bits which define the absolute value of the number. The variable number is first transformed by substituting with logic zeros any logic ones that have a lower significance than the most significant logic one of the bits. The first transformed number is then further transformed by reading the character bit of the variable number as a character bit and by reading the bits in the digital word in a reversed order. The resulting value is applied to adjust the adaptive digital filter which filters the signal.

Abstract: A method and an arrangement for performing an approximate division of a constant number by a variable number in binary form. The variable number is presumed to consist of a character bit and a plurality of bits which state the absolute value of the number. The number is converted by first forming a digital word, by substituting with logic zeroes any logic ones that have a lower significance than the most significant logic one of the bits. There is then formed a new number in binary form, by reading the character bit of the variable number as a character bit and by reading the bits in the digital word in a reversed order. The arrangement may mainly comprise a single gate network.

Abstract: An adaptive digital filter including a non-recursive part and a recursive part, and which can be updated in a simple and reliable manner. The recursive part of the filter has a plurality of separate, permanently set recursive filters (13-16) with different impulse responses, and a linear combination is formed with adaptive weighting factors (W0-W3) from the output signals of the recursive filters (13-16). The filter is updated by a single (e(n)) being utilized for updating the non-recursive part (11) of the filter and the adaptive weighting factors (W0-W3) in the recursive part of the filter.