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Attention is focused on the choice of a code for transmitting a PCM signal in the exchange plant via paper‐insulated cable pairs. Inter‐ and intrasystem crosstalk via near‐end coupling is the principal source of interference. A code and repeater structure for reconstructing this code to minimize the effect of crosstalk into the timing channel of a reconstructive repeater is emphasized. It is shown that the conventional self‐timed unipolar repeater is not suited to this environment. Several pseudo‐ternary codes are developed and surveyed for overcoming crosstalk interference. A bipolar code and a repeater for reconstructing this code are chosen for a variety of reasons. The repeater features nonlinear timing wave extraction and complete retiming and pulse width control, and its realization is discussed in detail in a companion paper by J. S. Mayo.3

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