Calculation of Multidisturber Crosstalk Probabilities - Application to Subscriber-Loop Gain

An important conccrn in speech transmission is the avoidance of crosstalk. When it is intelligible, it is a potential violation of the telephone subscriber's privacy; when not intelligible, it is nevertheless an annoyance, especially if syllabic in nature. In subscriber loop equipment, as a result of the permanent assignment of a pair to each customer, such crosstalk situations might tend to be dedicated to a specific disturbing talker and a specific disturbed listener, who might even be known to each other. Therefore, the random occurrences of intelligible crosstalk must be limited to a probability that is very small indeed. On long voice-frequency subscriber loops, it is often desirable to overcome excessive attenuation of the voice signal by applying some gain at the central office. The higher signal level, however, increases the probability that intelligible crosstalk will be heard. The existing gain limit of 6 dB was suspected to be already the maximum possible 875 without risking excessive crosstalk probabilities. Therefore, desired increases to as much as 9 dB were considered problematic, and this analysis was undertaken to evaluate it. Intelligible crosstalk results from a fortuitous combination of several random variables, such as the activity coincidence of disturbing and disturbed subscribers, disturber volume level, coupling path loss, listener acuity, and various noise sources. The ill-defined, stochastic nature of the problem has always necessitated various approximations.