A pseudo-random, pseudo-ternary pulse train, which is representative of a PCM signal, is applied to one end of a cable pair. At the other end of the cable pair, the received pulse train is equalized and amplified to obtain a pulse train output in which pulses have very nearly the same peak amplitude as each transmitted pulse, plus a characteristic which is similar but not identical. A variable amplitude interfering tone is combined with the equalized and amplified received pulse train. If no errors are determined by an error detector with a 0 amplitude interfering tone, the amplitude is increased until errors occur. The interfering amplitude control is calibrated in terms of eye degradation factor. The magnitude of the eye degradation factor is determinative of the capability of the cable pair to transmit the PCM signal with an acceptable error rate.