Four-Phase Data Systems in Combined Delay Distortion, Gaussian Noise, and Impulse Noise

A previous paper on digital computer simulation of a four-phase d a t a system 1 was concerned with performance in the presence of delay and attenuation distortion, and impulse noise. This paper extends those results to include Gaussian noise. Combinations of Gaussian noise, impulse noise, and delay and attenuation distortion are presented and discussed. The prime effect of delay and attenuation distortion is to reduce system margin against error. Adding Gaussian noise to t h e distorted signal can have one or both of two effects. Either errors occur directly due to the Gaussian noise alone, or the margin against other disturbances, in particular impulse noise, is decreased. On most telephone lines it is generally feasible to keep the direct errors produced at a very low level. T h e generally steep slope of a curve of signal to Gaussian noise ( S / N ) versus probability of error, (as much as a factor of 10:1 in error rate for one db change in S/N) makes the system particularly sensitive to changes in the a m o u n t of Gaussian noise. Thus, at least on land facilities, Gaussian noise is usually kept to a level where it enters basically as a 253