Method of and circuit arrangement for improving the transmission

Abstract

When telecommunications systems with analog and digital circuit technology are interconnected, the two-wire/four-wire transitions are achieved with hybrid circuits in which mismatches cannot be avoided. They result in signal reflections in the form of interfering echoes which can considerably reduce speech intelligibility. The known methods of echo compensation for hands-free facilities are not suitable for compensating line echoes, since the signal level and the delay times of lines can be very different. A compander (3.7) is used to suppress the effect of line echoes, whose transmission characteristic, in addition to being controlled by a coupling factor between the transmit path and the receive path, and by a control signal determined by the noise level, is also controlled in the invention with the speech level that is received from the distant subscriber. Furthermore, the amplification in the linear starting range of the compander characteristic is made dependent on the ratio or comparison between the speech level and the noise level on the transmit path. The invention can be used in conjunction with hands-free facilities in communications terminals, in addition it can be spatially assigned to the hybrid circuits, or it can find central applications in base stations of radio exchange systems or in switching centers.