PCM coder and decoder having function of two-wire/four-wire conversion

摘要

A CODEC including a coder and decoder to construct the subscriber's circuit of a digital telephone switching system or the like, wherein an analogue balancing circuit is provided between the output terminal of a post-filter and the input terminal of a pre-filter in order to effectively eliminate a return signal in the case of two-wire/four-wire conversion, and return signals not eliminated by the analogue balancing circuit are further eliminated by a digital balancing circuit. Especially in the present invention, the analogue balancing circuit is so constructed that its characteristics are independent of frequencies, and hence, the analogue balancing circuit and the digital balancing circuit are readily implemented as an LSI.