he dynamic range with little loss of fidelity, and expansion returns the signal to its normal condition. This is done by favoring low-level speech over higher-level speech. In other words, more code segments are assigned to speech bursts at low levels than at the higher levels, progressively more as the level reduces. This is shown graphically in Figure 6.4, where eight coded sequences are assigned to each level grouping. The smallest range rises only 0.0666 V from the origin (assigned to 0-V level). The largest extends over 0.5 V, and it is assigned only eight coded sequences. 6.2.3 Coding Older PCM systems used a 7-bit code, and modern systems use an 8-bit code with its improved quantizing distortion performance. The companding and coding are carried out together, simultaneously. The compression and later expansion functions are logarithmic. A pseudologarithmic curve made up of linear segments imparts finer granularity to low- level signals and less granularity to the higher-level signals. The logarithmic curve follows one of two laws, the A-law and the u-law (pronounced mu-law). The curve for the A-law may be plotted from the formula 05/01/11 15/x/s1, where A = 87.6. (Note: The notation In indicates a logarithm to the natural base called e. Its value is 2.7182818340.) The A-law is used with the E1 system. The curve for the +1.0V 111000 to 1111111 0.4666 110000 to 110111 0.1999 V 101000 to 101111 0.0666 V 100000 to 100111 6-bit coding Figure 6.4 Simple graphic representation of compression. Six-bit coding, eight six-bit sequences per segment. DIGITAL NETWORKS u-law is plotted from the formula where X is the signal input amplitude and u=100 for the original North American T1 system (now outdated), and 255 for later North American (DS1) systems and the CCITT 24-channel system (CCITT Rec. G.733) (see Ref. 3). A common expression used in dealing with the "quality" of a PCM signal is signal- to-distortion ratio (S/D, expressed in dB). Parameters A and u, for the respective com- panding laws, determine the range over