Abstract: A PCM coder and decoder circuit for use in a subscriber line interface circuit of a telephone communication system has a digital balancing network for removing any echo signal. The digital balancing network is formed by series-connecting a first balancing circuit having characteristics corresponding to the fixed characteristics of a coder, decoder, etc. and a second balancing circuit having characteristics corresponding to variable characteristics of an external circuit including two-wire transmission line which is connected to the coder and decoder circuit. Thus, a replica of an echo signal is precisely produced, and the circuit configuration is simplified.

Abstract: Disclosed is a PCM coder-decoder having a construction such that a digital filter contained originally in the PCM coder-decoder is utilized on the time division basis in order to fold back a digital reception signal to a digital signal transmission side and thus to accomplish interruption, communication exchange between three parties, gain control, fold-over test of the PCM signal, and so forth, in addition to the coding and decoding functions inherent to the PCM coder-decoder.