Telegraph Transmission Theory

The shapes of telegraph signals were shown in Figs. 3_; 4, 6, and 9. An analysis of the transmission problems involved can be made on either a transient basis, or on a steady-state sine-wave basis in which the Fourier method of analysis is used.¹⁶ This method was indicated in the consideration of Fig. 3. The transient basis was used for many years and is still used, but the sinusoidal steady-state method¹⁷ has proved to be more practicable. When the sine-wave method is used, the theory of the transmission of telegraph signals is essentially the same as the transmission theory considered in preceding chapters.

The sending speed with Morse operation was discussed elsewhere. With teletypewriter operation the standard operating speeds are 40, 60, and 75 words per minute, although faster operation is used to a limited extent. Telegraph transmission speed is defined² as "the rate at which signals are transmitted, and may be measured by the equivalent number of dot-cycles per second, or by the average number of letters or words transmitted and received per minute." A given number of dot cycles per second, each of which consists of an on-off, or a mark-space, signal, is equivalent to twice that number of bauds,² a baud being one pulse per second. A speed of 60 words per minute requires 360 operations per minute and gives a dot frequency of about 25 cycles per second. If at least the third harmonic is transmitted to produce an approximately square wave (Fig. 3), then the theoretical band width required is about 75 cycles. In practice, band widths of about 100 cycles are provided for telegraph service. It is emphasized that the figures just given, and those on page 318 are approximate.