Abstract: To check the bit-distortion ratio of a transmission line carrying bipolar PCM signals, converted from a binary to a balanced ternary code at the tramsmitting end and reconverted into binary signals at the receiving end, binary test words and identical reference words are simultaneously read out from storage at opposite ends of the line. The test words are converted into bipolar form, for transmission, and the received pulses are distributed according to polarity over a first pair of leads while the binary pulses of the reference words are distributed over a second pair of leads according to the conversion code used at the transmitting end. The pulse trains on corresponding leads are compared, bit by bit, in a deviation monitor whose output, in response to any mismatch, advances a reversible counter which is reverse-stepped by timing pulses of a cadence reduced by a frequency divider to a fraction of the bit rate.