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

In all these cases instead of t ransmitting f t we t

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: in Fig. 5.11b. T he r esulting signal is a b inary P CM signal. T he analog signal f (t) is now converted t o a ( binary) digital signal. A b inary d igit is called b it for convenience. This contraction of binary digit by b it has become a n i ndustry s tandard a bbreviation. T he a udio signal b andwidth is a bout 15 kHz, b ut s ubjective tests show t hat signal a rticulation (intelligibility) is n ot affected if all t he c omponents above 3400 Hz a re suppressed. 3 t Since t he objective in telephone communication is intelligibility r ather t han h igh fidelity, t he c omponents above 3400 Hz are eliminated by a lowpass filter. T he r esulting signal is t hen s ampled a t a r ate o f 8000 samples per second (8 kHz). This r ate is i ntentionally kept higher t han t he Nyquist sampling r ate of 6.8 kHz t o avoid unrealizable filters required for signal reconstruction. Each sample is finally quantized into 256 levels (L = 256), which requires a group of eight binary pulses t o e ncode each sample (2 8 = 256). Thus, a digitized telephone signal consists o f 8 x 8000 = 64000 or 64 k bits/s d ata, requiring 64,000 b inary pulses p er second for its transmission. T he c ompact disc (CD) is a recent application of P CM. T his is a high-fidelity s ituation r equiring t he audio signal bandwidth of 15 kHz. Although t he Nyquist sampling r ate is only 30 kHz, a n a ctual sampling r ate of 44.1 kHz is used for the t Components b elow 300 Hz may also b e s uppressed w ithout affecting t he a rticulation. (a) Digit Binary equivalent Pulse code wavefonn 0 0000 I 0001 2 0010 • ••• • •• • 3 0011 4 0100 5 0101 6 0110 7 Olll 8 1000 9 1001 10 1010 11 1011 12 1100 13 1101 14 1110 15 l 1ll •• • • • • •• • ••• • ••• • •• • • • •• • • •• • •• • • ••• • ••• • • •• •• • • • •• • • ••• F ig. 5 .11 Analog-to-digital (A/D) conversion of a signal. (b) 334 5 Sampling reason mentioned earlier. T he signal is q uantized into a r ather large number of levels (L = 6 5,536) t o reduce quantizing error. T he binary-coded samples are now recorded on t he CD. Advantages o f Digital Signals Some o f t he advantages of digital signals over analog signals are listed below: 1. T ransmission of digital signals is more rugged t han t hat of analog signals because digital signals can withstand channel noise and distortion much b etter as long as t he noise a nd t he d istortion are within limits. T he digital (binary) message i n Fig. 5.12a is d istorted by t he channel, as illustrated in Fig. 5.12b. Yet if t he d istortion remains within a limit, we can recover t he d ata w ithout error because we need only t o make a simple binary decision as t o w hether the received pulse is positive o r negative. Figure 5.12c shows t he s ame d ata with channel d istortion a nd noise. Here again t he d ata can be recovered correctly as long as t he d istortion a nd t he noise are within limits. Such is n ot t he case with analog messages. Any distortion o r noise, no m atter how small, will distort the receive...
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