 which the signal-to-distortion ratio is compara- tively constant, about 26 dB. For A-law companding, an S/D = 37.5 dB can be expected (A = 87.6). And for u-law companding, we can expect S/D = 37 dB(u= 255) (Ref. 4). Turn now to Figure 6.5, which shows the companding curve and resulting coding for the European E1 system. Note that the curve consists of linear piecewise segments, seven above and seven below the origin. The segment just above and the segment just below the origin consist of two linear elements. Counting the collinear elements by the origin, there are 16 segments. Each segment has 16 8-bit PCM codewords assigned. These are the codewords that identify the voltage level of a sample at some moment in time. Each codeword, often called a PCM "word," consists of 8 bits. The first bit (most significant Segment 1111XXXX 1110XXXX 1101XXXX 1 (V) 1100XXXX 16 1 (V) 1011XXXX 1010XXXX 1001X X X X 1000XXX X 0000XX X X 0001XX X X 0010XX X X 0011XXX X 0100XXX X 0101XX X 011XXXX Figure 6.5 The 13-segment approximation of the A-law curve used with E1 PCM equipment. 6.2 BASIS OF PULSE CODE MODULATION bit) tells the distant-end receiver if the sample is a positive or negative voltage. Observe that all PCM words above the origin start with a binary 1, and those below the origin start with a binary 0. The next 3 bits in sequence identify the segment. There are eight segments (or collinear equivalents) above the origin and eight below (23 = 8). The last 4 bits, shown in the figure as XXXX, indicate exactly where in a particular segment that voltage line is located. Suppose the distant end received the binary sequence 11010100 in an E1 system. The first bit indicates that the voltage is positive (i.e., above the origin in Figure 6.5). The next three bits, 101, indicate that the sample is in segment 4 (positive). The last 4 bits, 0100, tell the distant end where it is in that segment as illustrated in Figure 6.6. Note that the 16 steps inside the segment are linear. Figure 6.7 shows an equivalent logarithmic curve for the No