Measurement of Phase Distortion

T an early date in the use of long loaded telephone circuits, certain disturbing effects at high frequencies were noticed which have been known as transients. 1 It was found that on such circuits, even when the attenuation was very carefully equalized within the transmitted range, these transient effects still persisted and were made worse. It was realized that these effects were due to phase distortion or delay distortion, t h a t is, the resultant effect of phase shift varying with frequency in a peculiar manner. It was also determined that these effects resulted largely from the loading associated with the circuit pairs and that the effect could be considerably reduced by using a much lighter loading for the circuits. 2 Lighter loading systems were applied to telephone circuits so that as a result these transient effects were minimized to such an extent as to make circuits commercial for telephone use. Recent developments in telephone transmission and in special services requiring the use of telephone circuits have emphasized these high-frequency effects due to phase distortion and have indicated a similar effect at low frequencies which results from the equipment associated with the circuit. The use of loaded cable circuits in place of open-wire circuits, with a corresponding increase in the number of repeaters, has increased the phase distortion considerably on telephone circuits. This is particularly true when very long telephone circuits in cable result so that these effects are quite disturbing.