THEORY OF TRANSMISSION AND INTERFERENCE Transmission.—The fundamental requirement of a telegraph system is a source of energy controlled in accordance with the code and the characters to be transmitted, which sends sufficient energy through a line wire to operate a suitable receiving or recording device at the distant end. Where some interference is present, as is usually the case, the received energy must be sufficiently greater than the interference to enable the signal to be distinguished. The sending source of energy usually consists of a direct current battery or generator, under proper control. From what follows, however, it will be evident that the transmission of telegraph characters requires the use of apparatus and lines capable of transmitting sine wave alternating currents of various frequencies, i.e., a band of frequencies, within a certain range. A communica tion circuit of any kind is thus fundamentally different from a circuit for transmission of energy for power or lighting, in which direct current or alternating current of only a single frequency need be transmitted.
In a telegraph system the signals sent, of course, are continually changing in character, and there will be times when several consecutive dots or reversals are sent at the highest fundamental frequency of transmission for the given circuit. The number of such complete reversals per second may be spoken of as the fundamental signalling frequency or the dot frequency. Com mon fundamental signalling frequencies for various circuits are tabulated Messenger call circuits . . . . 2 cycles per sec.
Key operated Morse . . . . . 6 to 18 Automatic telegraph systems . . . 15 to 6o „ „ „ Submarine cables, non-loaded . . . 4 to 15 ,, Submarine cables loaded . . . . 3o to 8o „ „ „ The alternating current characteristics of signal combinations may be most readily considered by examining a number of typical combinations of signals, such as in fig. 22, the combinations being assumed to repeat themselves again and again. Such combinations
may be analysed by the ordinary Fourier method into a series of sine wave components. Analyses of a considerable number of these recurring combinations show that all frequencies from zero to infinity—excluding even multiples of the fundamental sig nalling frequency—are present in the transmitted signals. In order to operate a receiving in strument at the distant end, it is not necessary that all of the above frequencies be transmitted through the line. In a submarine cable system, which ordinarily is operated quite efficiently, it is usually sufficient to receive frequencies from zero up to about 1.6 times the fundamental signalling frequency. Since the higher frequencies are more difficult to transmit through the line this condition facilitates operation. In a land line telegraph sys tem operated with less complex equipment it is preferable that frequencies from zero to about three times the fundamental sig nalling frequencies be received.
The voltages and currents originating at the transmitter may be modified by any impedance at the sending end of the line, by the line itself, and by the receiving equipment. The effect of the line is ordinarily to reduce the amount of higher frequencies transmitted, thus limiting the speed of transmission. This is particularly true in the case of the submarine cable, in which frequencies in the neighbourhood of the fundamental signalling frequency are received with only a small fraction of the magni tude of frequencies much lower than this value. In order partly to counteract this difference a condenser is customarily placed in series at the sending end of a submarine cable, thus reducing the magnitude of the lower frequencies and thus reducing distortion of the received signal. Various special devices are used at the receiving end of such a cable for the purpose of strengthening the higher frequencies and reducing lower frequencies in order to reduce distortion.