Effects of Interference

Crosstalk is usually easily controlled because, first, it occurs between circuits operating at about the same power level, and second, it is usually between lines of the same system, engineered and operated by the same organization. If crosstalk is noticeable, it not only interferes with telephone service but also destroys secrecy. Crosstalk interferes with normal telephone conversation by diverting attention from the conversation in progress to the one heard through induction from another circuit. There is, furthermore, a masking or interfering effect of considerable importance.

Induction from paralleling power lines offers a serious problem. One reason for this is the great difference in the voltage, current, and power magnitudes involved. If but an extremely small amount of the power in a power system is induced into a low-level communication circuit, it may be of the same order of magnitude as the useful telephone currents.

Induction from power systems may be of two types: first, low-frequency induction; and second, noise-frequency induction. (References 7 to 13 inclusive.) Low-frequency induction includes only induction at the fundamental frequency of the power system (usually 60 cycles), and ordinarily is bothersome only at times of abnormal power-line conditions. Noise-frequency induction is, however, due to the harmonics of the fundamental power frequency and is usually present at all times, although its effect is more pronounced under abnormal power-line conditions.

Both low-frequency and noise-frequency induction interfere with telephone service. Among the effects produced by low-frequency induction are¹¹ service interruptions, false signals, telegraph signal distortion, damage to plant, electric shock, and acoustic shock. Regarding this last effect, it is interesting to note that copper oxide varistors sometimes are connected across operators' telephone sets to reduce acoustic shock; also, that the non-resonant telephone receiver produces much less noise when a transient current passes through it. Noise currents tend to mask the useful speech or signal currents. A noisy circuit having a certain loss is equivalent from a telephone standpoint to a more quiet circuit having a greater loss. The added losses due to noise are known as noise transmission impairments, abbreviated N.T.I. Much more speech power must be supplied to the telephone user to override the induced and room noises.