Factors Affecting Signal Intelligibility

Author: Edmund A. Laport

Signal intelligibility means the reception of communication signals in a form sufficiently unmutilated to provide complete reproduction, at the receiver, of the original intelligence. The amount of mutilation that can be tolerated depends upon the type of emission employed. For example, manual telegraphy can tolerate signal distortion and noise and spurious-signal interference to a greater extent than can automatic teleprinter operation. The amount of mutilation and interference that will provide solid communication is largely dependent upon the ability of each operator. The same is true of telephone working, where experienced operators can understand signals that would appear quite garbled to a casual listener.

Different standards of engineering are required for different classes of services. A teleprinter system must be designed to higher standards of performance than a manual telegraph circuit, and high-speed facsimile or multiplexed teleprinter circuits in turn require superior over-all performance to a single-channel printer circuit. A radiotelephone system intended for public correspondence requires better system performance than does a telephone circuit used only by professional operators as in air-line communication.

In this discussion there are so many factors involved that any remarks must of necessity be quite general. Many of the main factors that control signal intelligibility vary over wide limits for intervals that may be from a fraction of a second to a season or a year. However, these considerations form an important background for the design of antennas.

At the high frequencies, propagation is extremely complex. On any given space circuit, there is always a best working frequency, a best set of transmitting- and receiving-antenna characteristics, and a certain minimum transmitting power that will give the desired signal intelligibility in the presence of propagation variations and other signal and noise interference. One cannot engineer a circuit for such optimum performance because the optimum requirements may be different the following hour and for a high percentage of total working time. One cannot change working frequency, operating power, and antenna characteristics from minute to minute to follow these variations, even if one knows exactly what to anticipate. Accordingly, a circuit is engineered for compromise conditions. The choice of compromise is the essence of high-frequency communication engineering, and even experienced engineers will not evaluate circumstances equally and select the same compromises. In fact, much of the existing discordance of opinions on system details rests on the fact that there hardly is one optimum compromise, but rather a compromise of compromises.