Abstract: An electronic cross connect frame for an exchange or a multiplexor therefor is provided in which an analogue access matrix is provided for providing a limited number of spare cards or different cards to which any of the lines may be switched to provide a "sparing" or alternative services.

Abstract: A low frequency digital notch filter comprising two sub-filters - an all-pass-network filter (1) and a T-section filter (3) with a feedback connection from the T-filter to an input node (31), and feed forward connection from this input node to a terminal output node (33). The all-pass-network filter (1) has a transform function:A(Z)=[Z.sup.-1 +K]/[1+K.sub.1 Z.sup.-1]; andthe T-section filter (3) has throughput and tap transform functions:B(Z)=[(K.sub.3 +K.sub.2 .multidot.K.sub.4)Z.sup.-1 -1 ]/[1-K.sub.3 Z.sup.-1 ],and,C(Z)=K.sub.2 Z.sup.-1 /[1-K.sub.3 Z.sup.-1 ],where Z.sup.-1 is the unit delay operator and K.sub.1 to K.sub.4 are multiplier coefficients. The T-section filter 3 may be implemented using a combination of three multipliers (19, 21 and 23) and a delay element (25).

Abstract: A filter structure including a notch filter is designed to have a transmission zero at a frequency slightly displaced from one half of the lower sampling rate of the interpolator/decimator, and thus providing increase in attenuation at the half-rate frequency. The notch filter is comprised of two like all-pass-network filters and has feed forward and feedback connections, the latter connection including a coefficient multiplier. The feedback connection is made between a tapped output of the second of the network filters and an input node. The tapped filter is characterized by throughput and a tapped output transform functions X(Z) and Y(Z) given by the following expressions:X(Z)=[Z.sup.-1 -K]/[1-KZ.sup.-1 ; and,Y(Z)=.alpha.Z.sup.-1 /[1-KZ.sup.-1 ;where Z.sup.-1 is the unit delay operator, K the multiplier coefficient and, .alpha., a structure dependant constant.

Abstract: A digital interface between at least two subscriber line interface circuits and a processing unit is described. The interface is configured in the form of a parallel interface. The level of an input/output register and of a control register is allocated, as needed, to the connections of the processing unit. These connections can be operated selectively as inputs or outputs. They can also be operated in two modes, depending upon which type of subscriber line interface circuit (SLIC) is connected. The connections of the last-mentioned units are partially operable as inputs or outputs and partially operable only as inputs but in the input case, selectively for various types of input signals.

Abstract: A digital interface of an integrated subscriber line interface circuit is provided. The interface serves for the connection of a signal processor unit. Individual connections are used as signal inputs in signal entry operation of the interface, and as signal outputs in signal transmission operation. Other connections are used as signal inputs in both operating mode, but receive different signals in each operating mode. One group of connections includes two registers with respective associated decoders to enable switching from one to the other, so that the number of operating instructions that can be represented by means of binary signal value combinations applied to the connections is doubled.

Abstract: An interpolator or decimator filter structure operable between two sampling rates (F.sub.s and 2F.sub.2), and in which two branches (21 and 23) of the structure are divided into low and high frequency sections by sampling switches (27 and 29). The high frequency section of one branch (21) incorporates a composite filter comprises of a nesting filter (1') and a nested filter 1. Each of these filters 1, 1' includes a digital coefficient multiplier (5,5'). The coefficient (K.sub.2) of the coefficient multiplier (5), part of the nested filter (1), is preset to a value such that 3 dB attenuation occurs at a point offset in frequency from one-half the lower sampling rate (F.sub.s).

Abstract: A digital adaptive echo canceller for use in cancelling echo signals caused by trans-hybrid losses in two-wire to four-wire converters comprises first and second adaptive filters (20,22) each arranged to generate synthetic echo signals. Transfer control logic (23) compares difference signals relative to the difference between the synthetic echo signal generated by the first filter (22), and a real echo signal and the synthetic echo signal generated by the second filter (20) and the real echo signal, and if the synthetic echo signal from the first filter (22) is closer to the real echo signal causes transfer of the coefficients of the first filter (22) to the second filter (20) such that the synthetic signal mixed with transmitted speech signals in a mixer (24) is updated when necessary.