Pulse communication

A brief review is presented of the present state of development of pulse communication, its application, theoretical basis and circuit-techniques. It is divided into three parts. In Part 1, the four basic types of pulse modulation, namely amplitude, length, phase and frequency modulation, are briefly described, and the history of the subject is touched upon. Time-allocation multiplex is discussed, and a broad technical comparison with frequency-allocation multiplex is made. The more important specific problems of the technique, such as the best pulse shape and repetition frequency, signal/noise ratio and privacy, are discussed. The application to a point-to-point very-short-wave radio system is considered, with special reference to the pros and cons of pulse modulation compared with c.w. modulation. The possibilities of its use on cables and in omni-directional radio systems, including broadcasting, are examined, as are the anti-fading properties of pulse transmission on longer radio waves for long-distance work. In a Section on future trends of development it is stressed that more quantitative data on actual working systems is required; it is submitted that new applications may arise for which pulses may be particularly suitable; lines of development for time-allocation multiplex are indicated; and the likely evolution of pulses as a complementary technique to other systems of veryshort-wave communication is envisaged. In Part 2, the results of theoretical and experimental analysis of some specific problems of the technique are presented graphically and are discussed. The spectrum of modulated pulses is given, and utilized to estimate the distortion and intelligibility of received speech. The minimum repetition frequency for intelligible transmission is determined experimentally. A consideration of noise problems introduces a comparison of the merits, from the point of view of signal/noise ratio, of typical pulse-modulation systems as applied to single-channel and - multiplex communication. The pulse systems are compared amongst themselves and with conventional c.w. amplitude- and frequency-modulation systems. The noise performance below threshold has an effect on the conclusions. A note on a theoretical investigation of a pulse-synchronization system is added. The preferred length of transmitted pulses is discussed from the point of view of best use of transmitted power. An investigation of the adjacent-channel radio interference leads to theoretically best shapes of transmitted pulses and a simplified specification of the pulse shape is suggested and discussed. In Part 3, a brief survey is made of the way in which circuit elements are combined to produce pulse-communication systems. Some of these circuit elements are already well known in the art and have been previously described with reference to radar circuit design, whilst others have been developed expressly for communication purposes. The detailed operation of systems is fully described in the supporting papers, and in this Part an attempt has been made to present a fairly complete picture of the methods which are known to have been used up to the present, and to give an opinion on their relative merits A number of functions are common to most equipments, such as pulse modulation, demodulation, channel spacing, slicing and ringing, etc., and the techniques developed to carry out these operations are enumerated and criticized. Finally, suggestions are made as to which are the most promising lines for further development.