1.  Field of the Invention
The invention relates to a circuit for full-duplex data transmission over a two-wire circuit.  The arrangement comprises a one-way transmit channel connected to a baseband data transmitter, a one-way receive channel connected to a baseband data receiver, a hybrid junction for interconnecting the transmit and receive channels and the two-wire circuit, an echo canceller comprising an adaptive filter with adjustable filter coefficients and an adjusting circuit for the filter coefficients, and a combining circuit inserted in the receive channel, the adaptive filter generating an approximated echo signal from signals in the transmit channel, the combining circuit differentially combining the approximated echo signal with incoming signals of the receive channel to generate a residual signal and the adjusting circuit, in response to signals in the transmit channel and the residual signal in the receive channel, adjusting the adaptive filter coefficients to minimize a predetermined function of the residual signal. 
2.  Prior Art
A similar arrangement is known from the article by P. J. van Gerwen et al.  in IEEE J. Select, Areas Commun.,  Vol.  SAC-2, No.  2, March 1984, pp.  314-323. 
For the full-duplex data transmission use is often made of the circuits of the public telephone network.  In the local network the circuits are usually of the two-wire type.  Data transmission is then subject to four main types of impairments: echo signals, intersymbol interference, cross-talk from ever present signals in adjacent wire-circuits and noise (background noise and impulse noise).  Many of the arrangements available now for full-duplex data transmission over two-wire circuits comprise an echo canceller with an adaptive filter to considerably reduce the disturbing effect of echo signals, the adaptive filter coefficients being usually adjusted iteratively according to a criterion for minimizing the mean square value of the remaining echo in the residual signal.  The data receivers utilized in these arrangements further comprise the current arrangements for strongly reducing the disturbing effect of intersymbol interference, ever present crosstalk and background noise.  However, in the arrangements known thus far no specific precautions have been taken to reduce the influence of impulse noise consisting of concentrated pulse-like disturbances occurring at irregular intervals and usually far apart.  The influence of this impulse noise is particularly felt in longer circuits operating at higher data rates. 
A possibility to combat the effect of impulse noise known per se is the insertion of a smearing filter at the transmit end and a complementary smearing filter (also referred to as desmearing filter) at the receive end of the circuit.  The two filters generally have a flat amplitude characteristic, but their group delays vary in the prescribed frequency band in a contrary fasion, the sum of the group delays being made as flat as possible.  In the ideal case, a data signal passing through the two filters merely experiences a delay.  A noise impulse injected into the channel, however, passes only the complementary smearing filter at the receive end so that the energy of such a noise impulse is smeared in time and, consequently, its effect on the received data signal may be decreased at any moment.  Further details about the design and implementation of efficient and yet simple smearing filters for data transmission can be found in the article by G. F. M. Beenker et al.  in IEEE Trans.  Comm.,  Vol.  COM-33, No.9, September 1985, pp.  955-963. 
In the arrangement for full-duplex data transmission over two-wire circuits, the effect of impulse noise can be combatted in the same manner by connecting the data transmitter via a smearing filter to the input of the one-way transmit channel and by connecting the data receiver via a complementary smearing filter to the output of the one-way receive channel.  This way of connecting the smearing filters implies that the signals in the transmit channel applied to the echo canceller are now formed by smeared data signals having a considerably greater number of significant signal values than would be the case without smearing filters.  This also results in a considerably more complicated implementation of the echo canceller as the signal samples to be applied to the digital echo canceller have to be represented with a far greater number of bits.  This is an undesired effect, more specifically in view of the expectation that smearing filters need only be used for a limited number of circuits in the local telephone network, so that the complexity of the echo canceller present in each two-wire circuit is of far greater importance than that of the smearing filters, which should be considered an option not applied relatively frequently.