System for preventing intelligible crosstalk in TDM switching systems

Apparatus is disclosed for protecting against intelligible crosstalk in a PCM-TDM system. Such crosstalk may arise when there is a failure in the system causing a mixing of signals in a first channel with signals from a second incoming channel. When the first channel is silent (sign bit zero and amplitude train zero) signals from the second channel may be added to the zero train causing the receiving subscriber of the first channel to receive undesirable crosstalk. To combat this situation, the invention employs two code conversion devices in series in a line. The first device inverts coded combinations having a sign bit 0 while transmitting without modification those with sign bit 1. The second device adjacent the output cancels the inversion of the first device to provide normal combinations to the called line.