 spacing of the tration of digital into the trunk transmission network rapidly over- carrier dictates the spacing between the combined channels and is took the FDM analogue network. normally 4kHzi.e. f2-f1=4kHz. = The received path at B undergoes the opposite process. As the carrier is not transmitted with the signal then f and are FDM hierarchy separately generated at B. Any frequency difference between the carriers at A or B will impose a frequency shift on the recovered 20.3.1 General considerations channel of The function of filters Fal and Fa2 is to select the wanted lower A number of different transmission media are available, such as sideband and suppress the unwanted carrier signal, upper sideband open wire cable, coaxial cable, radio or satellite systems, all with etc. Fb1 and Fb2 filters select the required set of frequencies from different bandwidth capability. The multiplexing schemes use a the broadband line frequency spectrum for demodulation. hierarchy of building blocks to construct systems to the required In the higher orders of translation it is not normally economic to bandwidth. Many stages of translation may be required with the design Fb filters with sufficient selectivity, such that only the re- final stages of modulation only being specific to the particular quired band of frequencies are selected. The signal presented to the transmission medium. demodulator in this case can contain some information from the The building blocks are optimised for cable transmission. The adjacent channels. These unwanted signals are removed by the dimensions of the various blocks are largely historic and based on greater selectivity of the lower order demodulating equipments. the economics of filter design. An additional requirement for Fb filters is to limit the total band of frequencies applied to the demodulator and thus keep the inter- 20.3.2 Channel bandwidth modulation noise generated to a minimum. Fa and Fb filters are normally the same design. Channel spacing is standardised on 4kHz. This provides en