Signal Processing and Linear Systems-B.P.Lathi copy

57c we wonder whether it is possible to recover or

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: el by interweaving them. Figure 5.10 shows t he T DM of two PAM signals. In this manner, we c an multiplex several signals o n t he same channel by reducing pulse widths. Another method of t ransmitting several baseband signals simultaneously is frequency-division multiplexing (FDM), discussed in Sec. 4.8-4. In FDM various signals are multiplexed by sharing t he c hannel bandwidth. T he s pectrum of each message is s hifted t o a specific b and n ot occupied by a ny other signal. T he information o f various signals is l ocated in nonoverlapping frequency bands of t he channel (Fig. 4.45). In a way, T DM a nd F DM a re t he duals of each other. 332 5 Sampling 5.1 T he S ampling Theorem 333 Pulse Code Modulation (PCM) P CM is t he m ost useful a nd widely used of all t he pulse modulations mentioned. Basically, P CM is a m ethod of converting a n analog signal into a digital signal ( AID conversion). A n a nalog signal is c haracterized by t he fact t hat its amplitude can take on any value over a continuous range. Hence, analog signal can take on an infinite number of values. In contrast, a d igital signal amplitude can take on only a finite n umber of values. An analog signal can be converted into a digital signal by means of sampling a nd q uantizing (rounding off). Sampling an analog signal alone will n ot y ield a digital signal because a sampled analog signal can still take on any value in a continuous range. I t is digitized by rounding off its value t o one of t he closest permissible numbers (or q uantized l evels), as i llustrated in Fig. 5.l1a. T he a mplitudes of t he analog signal f (t) lie in t he range ( -V, V ). T his range is p artitioned i nto L subintervals, each of magnitude ~v = 2 V / L . Next, each sample amplitude is a pproximated by t he m idpoint value of t he s ubinterval in which the sample falls (see Fig. 5 .l1a for L = 16). I t is clear t hat each sample is approximated t o one of t he L n umbers. Thus, t he signal is digitized with quantized samples taking on a nyone of t he L values. Such a signal is known as a n L -ary d igital s ignal. From a practical viewpoint, a binary digital signal ( a signal t hat c an t ake on only two values) is very desirable because of its simplicity, economy, a nd ease of engineering. W e can convert a n L -ary signal into a binary signal by using pulse coding. Figure 5 .l1b shows such a code for t he case of L = 16. T his code, formed by binary representation of t he 16 decimal digits from 0 t o 15, is known as the n atural b inary c ode ( NBC). O ther possible ways of assigning a binary code exist. Each of t he 16 levels t o b e t ransmitted is assigned one binary code of four digits. Thus, e ach sample in this example is encoded by four binary digits. To t ransmit t his b inary d ata, we need to assign a distinct pulse shape t o each of t he two binary s tates. O ne possible way is t o assign a negative pulse t o a b inary 0 and a positive pulse t o a b inary 1 so t hat each sample is now t ransmitted by a group of four binary pulses (pulse code), as depicted...
View Full Document

This note was uploaded on 04/14/2013 for the course ENG 350 taught by Professor Bayliss during the Spring '13 term at Northwestern.

Ask a homework question - tutors are online