Hill. McGraw-Hill. Wallace, A.D., Humphrey, L.D. and Sexton, M.J. (1991) Anal- Jain, A.K. (1981) Image data compression: a review, Proc. IEEE, ogue/digital conversion, Chapt.6 in Flood, J.E. and Cochrane, P. 69, pp. 349-389. (eds.) Transmission systems, Peter Peregrinus. Johnson, F.B. (1968) Calculating delta modulator performance, Williams, M.B. The characteristics of telephone circuits in relation IEEE Trans., AU-16, pp. 121-129. to data transmission, Post Off. Electr. Engrs. Jour. 59, .151. Frequency division multiplexing David Lockstone Bell Northern Research Contents FDM principles 20/3 History 20/3 FDM hierarchy 20/3 Frequency translation 20/5 Carriers 20/7 Pilots 20/8 Noise contributions 20/9 20.8 Measurement of noise contributions 20/14 20.9 Overload 20/15 20.10 Hypothetical reference system 20/16 20.11 Companding 20/17 20.12 Through connections 20/18 20.13 Transmultiplexers 20/18 20.14 Repeatered cable line equipment 20/20 20.15 References 20/22 This page intentionally left blank FDM principles 20/3 FDM principles Previously all telephony transmission was at audio frequency and the move to FDM was inspired for economic reasons and the Frequency Division Multiplex (FDM) is the frequency translation pressure on cable utilisation. From then onwards the drive was for of a number of individual standard telephony channels so that they lower loss and higher bandwidth cables to support ever higher can be stacked side by side and form a single wide band signal. This bandwidth systems. principle is illustrated in Figure 20.1. The period during the 1960's to the mid 1970's saw the peak of Two identical telephony channels C, and C, (0.3 to 3.4kHz) are FDM systems in service and the development of transistorised each mixed with a carrier frequency and combined. Only the lower systems to 60MHz. However during this period developments of sideband is selected from the mixing process and thus the channel digital systems started to fully occupy the R&D budgets and pene- frequencies are inverted on the combined signal. The