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
= 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 North American DS1 system.
It uses a 15-segment approximation of the
µ-law curve (µ = 255). The segments cutting the origin are collinear and are
counted as one. So, again, we have a total of 16 segments.
The coding process in PCM utilizes straightforward binary codes. Examples of such
codes are illustrated in Figure 6.5 and are expanded in Figure 6.6 and Figure 6.7.
The North American DS1 (T1) PCM system uses a 15-segment approximation of the
µ-law (µ = 255), shown in Figure 6.7. The segments cutting the origin are
collinear and are counted as one. As can be seen in Figure 6.7, similar to Figure 6.5,
the first code element (bit), whether a 1 or a 0, indicates to the distant end whether the
sample voltage is positive or negative, above or below the horizontal axis. The next three
elements (bits) identify the segment, and the last four elements (bits) identify the actual
quantum level inside the segment.
Concept of Frame
. As is illustrated in Figure 6.2, PCM multiplexing is car-
ried out with the sampling process, sampling the analog sources sequentially. These
sources may be the nominal 4-kHz voice channels or other information sources that have
a 4-kHz bandwidth, such as data or freeze-frame video. The final result of the sampling
and subsequent quantization and coding is a series of electrical pulses, a serial bit stream
of 1s and 0s that requires some identification or indication of the beginning of a sampling
The European E1 system, coding of segment 4 (positive).
More popularly referred to as T1.