The Picturephone System: Crosstalk Considerations in the Transmission of Analog Signals on Paired Cable

Exchange cable pairs will provide the medium for transmission of the analog baseband Picturephone signal over subscriber loops and most local trunks.1 The unwanted coupling of energy, or crosstalk, between pairs within a cable places important limitations on the engineering of these equalized transmission lines. 1.1 Types of Impairments Three types of impairments in the Picturephone signal which can occur in the presence of crosstalk are considered in this article. They are listed in Table I along with the source of the impairment. Nearend and far-end crosstalk coupling paths are shown in Fig. I. 2 l.i.i Worst-Disturber Interference The first impairment is caused by interference from the single most prominent Picturephone interfering signal among all those present, and is called worst-disturber interference. The signal from the worst 427 428 THE BELL SYSTEM TECHNICAL JOURNAL, FEBRUARY 1971 disturber, when passed through a crosstalk coupling path, will appear differentiated and superimposed on the desired signal. Since the two signals are not synchronized, the interference will drift across the screen when the desired signal is displayed. For example, if the sync pulses are the most detrimental part of an interfering Picturephone signal, the interference will appear as two vertical lines, one black and one white, drifting across the screen. 1.1.2 Random Noise Interference Even if no signal is visible as a worst disturber, each of the interfering Picturephone signals will contribute unwanted energy to the desired signal.