Pulse communication on lines

A dispersive medium, i.e. one in which the waveform of an individual pulse changes its type at successive points during propagation, is not normally appropriate for pulse communication, since greater efficiency can be achieved through it in other ways. However, conditions can arise in which it becomes desirable to employ such a link, and it is necessary to discover the restrictions imposed by the medium on multichannel pulse communication, and the steps that can be taken to minimize these limitations. The paper briefly describes a theoretical and experimental investigation by the authors into the particular problems of pulse communication over a 50-mile open-wire line of the standard Army type. The major problem, of adjacent-channel interference caused by the progressive deterioration of the individual propagated pulse waveforms to a point where they appreciably overlap, is adequately solved by adapting and extending a technique employed for wide pulses in line telegraphy to the case of narrow pulses. This is the device of reducing the interfering effect of each trailing pulse response by transmitting, instead of a simple rectangular pulse, a controllable curbed pulse, i.e. a flexible combination of contiguous pulses of opposite sign. By this means satisfactory crosstalk results are achieved for multichannel pulse operation in one direction. For two-way operation the dominant problem now becomes the near-end interference arising from the irregular sequence of pulse reflections from various inhomogeneities in the line. Unless the line is sufficiently smooth, the crosstalk so occasioned may become intolerable.