onal changes in the cable loss, time etc. will 20.10 Hypothetical reference system not be greater than 1dB. Unwanted modulation products from low frequencies (50Hz Hypothetical reference systems (CCITT, 1985, G.212 - G.222 & etc.) will not be greater than 48dBmO for a 280km homo- G.229) have been constructed to define network building and how geneous line section. A similar requirement is placed on the the various degradation factors are apportioned and planned. The multiplex equipment. Companding 20/17 20.11 Companding Input level (dBmO) Companding (compression/expansion) enables a noise advantage by compressing the dynamic range of the transmitted voice channel and performing the opposite expansion of the signal at the receiving end. Companders are used on long circuits where the noise additions Compressor have rendered the circuits unacceptable, or where a noisy trans- mission medium can be made viable. 20.11.1 Compander characteristics The Compander parameters (Bell, 1971) are Compression Ratio and Unaffected Level, where compressor and expandor power output Expandor are given by Equations 20.36 and 20.37. Compressor output (dBm) Compressor power input (dBmO) (20.36) Expandor power output (dBmO) = K ( Expandor power input (dBmO)) - KA (20.37) Figure 20.16 Idealised compressor and expandor K is normally 2 (the Compression Ratio) and KA equal 5 (the characteristics Unaffected Level). Compander characteristics are as in Figure 20.16. The values for K and KA are limited by practical considerations. Increasing these values will improve the noise reduction but at the Using the new value for L (compressed) of -11.48dBmO new expense of other transmission properties. For instance increasing values for Pcq (compressed) can be calculated from Equation 30.24. the compression ratio has the disadvantage of expanding any gain For example a 4MHz 960 channel system has an overload require- change or level error within the transmission. ment of +26.62dBmO with normal -15dBmO single channel load- ing. With compression the overl