1. Chapter 4 Digital Transmission
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2. Lecture Outline Chapter 4: Digital Transmission
4.1 Digital-to-Digital Conversion ( From Book )
Line coding
4.2 Analog-to-Digital Conversion (From Lecture notes )
Pulse Code Modulation (PCM)
4.3 Transmission Modes (From Book )
Parallel Transmission
Serial Transmission

3. Line coding and decoding
4-1 DIGITAL-TO-DIGITAL CONVERSION
We can represent digital data by using digital signals.
The conversion involves three techniques: line coding, block coding, and scrambling.
Line coding is always needed.
Block coding and scrambling may or may not be needed.
Line coding and decoding

4. Signal element versus data element
Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite.

5. Example
A signal is carrying data in which one data element is encoded as one signal element ( r = 1).
If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1?
Solution
We assume that the average value of c is 1/2 . The baud rate is then

6. Built in Error Detection Immunity to Noise and Interference Complexity
Characteristics of Line Coding
DC components
Self Synchronization
Built in Error Detection
Immunity to Noise and Interference
Complexity

7. Line coding schemes

8. Unipolar NRZ scheme

9. Polar NRZ-L and NRZ-I schemes
level of voltage determines value of the bit
inversion or lack of inversion determines value of the bit
Both have an average signal rate of N/2 Bd.
Both have a DC component problem.

10. Transition at the middle is used for synchronization
Polar biphase: Manchester and differential Manchester schemes
Transition at the middle is used for synchronization
The minimum bandwidth is 2 times that of NRZ

11. Topics discussed in this section:
4-2 ANALOG-TO-DIGITAL CONVERSION
We have seen in Chapter 3 that a digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data. In this section we describe the common technique pulse code modulation
Topics discussed in this section:
Pulse Code Modulation (PCM)

12. Figure 4.21 Components of PCM encoder

13. The analog signal is sampled
Components of PCM encoder
The analog signal is sampled
The sampled signal is quantized
The quantized values are encoded as stream of bits

14. 1. Sampling
The first step in PCM is sampling. The analog signal is sampled every Ts sec, where Ts is the sample interval or period. The inverse of the sampling interval is called the sampling rate or sampling frequency as Fs = 1/Ts.
The sampling process is sometimes referred to as pulse amplitude modulation (PAM).
Note: the result of PAM is still an analog signal with nonintegral values.

15. According to the Nyquist theorem, the sampling rate must be
Note
According to the Nyquist theorem, the sampling rate must be
at least 2 times the highest frequency contained in the signal.

16. 2. Quantization
The result of sampling is a series of pulses with amplitude values between the maximum and minimum amplitudes of the signal.
In example, assume that we have a sampled signal and the sample amplitudes are between -20 and +20v. We decide to have L=8. This means our ∆ = Vmax –Vmin / L.
Step 1: Normalized PAM values (actual amplitude/∆)
Step2: Normalized quantized values (middle value between two levels)
Step3: Quantization error (difference between Step 1 and step 2 values)
Step 4:Quantization code for each sample
Step5: encoded words

17. Figure 4.26 Quantization and encoding of a sampled signal

18. 4. Original Signal Recovery
3. Encoding
The last step in PCM is encoding. After each sample is quantized and the number of bets per sample is decided, each sample can be changed to an n bit code word.
Note: the number of bits for each sample is determined from the number of quantization levels.
4. Original Signal Recovery
The recovery of the original signal requires the PCM decoder.

19. Topics discussed in this section:
4-3 TRANSMISSION MODES
The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.
Topics discussed in this section:
Parallel Transmission Serial Transmission

20. Figure 4.31 Data transmission and modes

21. Figure 4.32 Parallel transmission

22. Figure 4.33 Serial transmission

23. Note
In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.

24. Asynchronous here means “asynchronous at the byte level,”
Note
Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized; their durations are the same.

25. Figure 4.34 Asynchronous transmission

26. Note
In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.

27. Figure 4.35 Synchronous transmission