Abstract: In the future ISDN network, up to eight subscriber terminals for speech, data and image traffic can be connected to the network terminating circuit (NT). The interface between the subscriber terminals and the network terminating circuit - the S-interface - is specified in CCITT Recommendation I.430. The transmitter in the S-interface must satisfy many, sometimes conflicting, requirements. This is satisfied to an optimum extent by providing the transmitter with a bridge circuit formed by semiconductor switching elements which are controlled by a digital control circuit. According to the invention, the output current of the transmitter is maintained substantially constant with the aid of a differential amplifier connected to the bridge circuit.

Abstract: A telephone signalling receiver comprising speech detectors for respective voice-frequency signalling components of push-button (TDK) signals which are in a frequency band divided into lower and upper frequency subgroups. Each detector calculates the Discrete Fourier Transform (DFT) of the corresponding signal frequency component, the frequency characteristic of such detector having a main lobe for such signalling frequency component and two side lobes. Such calculation employs DFT product coefficients calculated for the lower and upper frequency subgroups and which are stored in memories, such coefficients being for particular Kaiser-Bessel window functions and for pairs of frequencies in the lower and higher subgroups. The coefficients are selected so that the peaks of the side lobes are lower, such as 6 dB below, the peaks of the associated main lobes.