Interference in Telephone Circuits
This chapter provides an introduction to the problem of interference in telephone circuits. A common source of interference induced on local telephone loops is interference resulting in cases where the twisted pairs are routed in parallel and in proximity to active electrical conductors. When an alternating field is associated with the electrical conductor, electromagnetic coupling occurs and voltages are induced into each wire of the cable. One of the most common types of low-frequency interference encountered on telephone circuits is periodic interference, which originates from either a three-phase or single-phase power line. The chapter describes the coupling mechanisms for low-frequency interference and also focuses on the power-line interference problem, which is one of the most common types of noise appearing on a telephone network. A method for rating the subjective effects of the interference on voice band telephony is to use the so-called psophometric weighting characteristic.