A comparison of the merits of phase and frequency modulation for medium speed serial binary digital data transmission over telephone lines

The telephone line is by no means an ideal channel for the transmission of serial binary digital data. It is important therefore to choose the method of transmission which is least affected by the various signal impairments which may be encountered. This paper indicates briefly why phase and frequency modulation are the best alternatives for general use and then examines in detail the relative merits of these two methods of modulation. The conclusions drawn from this study have been substantiated by measurements of error rate carried out on a number of p.m. and f.m. receivers working under conditions where comparisons could validly be made; the more important results are quoted in this paper. It is clearly shown that phase modulation has significant advantages over frequency modulation in the data transmission application.