114
DIGITAL NETWORKS
µ-law is plotted from the formula
F
µ
(x) =
ln
(1 + µ|x|)
ln
(1 + µ)
,
where
x is the signal input amplitude and µ = 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system (now outdated), and 255 for later North American (DS1) systems and the CCITT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 µ, for the respective com-
panding laws, determine the range over 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 µ-law companding, we can expect S/D = 37 dB(µ = 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
Figure 6.5
The 13-segment approximation of the A-law curve used with E1 PCM equipment.
Summary :
114 DIGITAL NETWORKS µ-law is plotted from the formula F µ (x) = ln (1 + µ|x|) ln (1 + µ) , where x is the signal input amplitude and µ = 100 for the original North American T1 system (now outdated), and 255 for later North American (DS1) systems and the CCITT 24-channel system (CCITT Rec.
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pcm,cure,system,segment,companding,signal,origin,alaw,just,below,elements,now,aboe