1. Lecture 6: Signal Encoding Techniques
2nd semester
By: Elham Sunbu

2. Outline
Encoding Techniques
Interpreting Signals
Encoding Schemes

3. Encoding Techniques Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal

4. Both Analog and digital data can be encoded as either analog or digital signals.
•Digital data, digital signal:
HDB3 coding scheme with CRC ON (ISDN lines).
•Digital data, analog signal:
dial-up modems and ADSL modems.
•Analog data, digital signal:
digitise voice and video for transmission. Pulse Code Modulation (PCM)
•Analog data, analog signal:
radio, amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM)

5. Digital Data, Digital Signal
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements

6. Terms (1) Unipolar Polar Data rate Duration or length of a bit
All signal elements have same sign
Polar
One logic state represented by positive voltage the other by negative voltage
Data rate
Rate of data transmission in bits per second
Duration or length of a bit
Time taken for transmitter to emit the bit

7. Terms (2) Modulation rate Mark and Space
Rate at which the signal level changes
Measured in baud = signal elements per second
Mark and Space
Binary 1 and Binary 0 respectively

8. Interpreting Signals Need to know
Timing of bits - when they start and end
Signal levels
Factors affecting successful interpreting of signals
Signal to noise ratio
Data rate
Bandwidth

9. Comparison of Encoding Schemes (1)
Signal Spectrum
Lack of high frequencies reduces required bandwidth
Lack of dc component allows ac coupling via transformer, providing isolation
Concentrate power in the middle of the bandwidth
Clocking
Synchronizing transmitter and receiver
External clock
Sync mechanism based on signal

10. Comparison of Encoding Schemes (2)
Error detection
Can be built in to signal encoding
Signal interference and noise immunity
Some codes are better than others
Cost and complexity
Higher signal rate (& thus data rate) lead to higher costs
Some codes require signal rate greater than data rate

11. Encoding Schemes Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3

12. (NRZ): NRZ-L and NRZ-I a. NRZ-L
Nonreturn to Zero-Level (no voltage transition during a bit interval)
1 = high level
0 = low level
Used on low speed links, e.g. serial ports.
Clock synchronization problem during long string of 0 or 1 bits.
Data rate = modulation rate

13. b. NRZI Nonreturn to Zero-Level Inverted (on 1s)
1 = change of signal level (high to low or low to high voltage)
0 = no change of signal level
used on fast ethernet
an example of differential encoding.
more reliable to detect transition in the presence of noise.
clock synchronisation problem during long string of 0 bits.

14. (Multilevel Binary): Bipolar-AMI and Pseudoternary
a. Bipolar-AMI
Bipolar-Alternate Mark Inversion
BAMI uses 3 signal levels: +ve, 0, and -ve
1 = alternating +ve and -ve voltages
0 = no signal (0 voltage)
no DC component because of alternative +ve and -ve voltages.
problem: loss of synchronisation during long string of 0 bits

15. b. Pseudoternary Same as BAMI except it reverses signaling.
1 = no signal (0 voltage)
0 = alternating +ve and -ve voltages

16. (Biphase): Manchester and Differential Manchester
a. Manchester
Voltage transition in middle of bit period.
1 = high to low voltage transition
0 = low to high voltage transition
used for 10Mbps ethernet over coax and twisted pair.
good for clock synchronisation and recovery.
no DC component
inefficient use of bandwidth (10Mbps ethernet uses a 20Mbps baud)
baud (modulation) rate = 2 x bit rate

17. b. Differential Manchester
Voltage transition in middle of bit period is for clocking.
1 = no transition at beginning of bit period
0 = transition at beginning of bit period (low-to-high or high-to-low, depending on previous output level)
used in IEEE (Token ring) at 4Mbps and 16Mbps.
inefficient use of bandwidth
Slightly better signal detection and clocking in noisy environment compared to Manchester encoding.
baud (modulation) rate = 2 x bit rate

18. (Scrambling): B8ZS and HDB3
a. B8ZS
Bipolar with 8 Zero Substitution
A scrambling scheme based on BAMI, modified to overcome problems of loss of synchronization due to long strings of zeros.
commonly used in US/Canada T1 (1.544MHz)
For every string of 8 zeros, bipolar code is substituted according to the following rules:
If an octet of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as
If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as
The receiver recognizes the pattern: two BAMI code violations and interprets the octet as a string of all zeros

19. b. HDB3 High Density Bipolar-3 Zeros
A scrambling scheme based on BAMI, modified to overcome problems of loss of synchronization due to long strings of zeros.
commonly used in Australia, New Zealand, Europe and Japan E1 (2.048MHz)
Coding is based on BAMI, substitution based on the above rule table.
Both B8ZS and HDB3 are highly suitable for high data rate transmission.

20. Digital Data, Analog Signal
Public telephone system
300Hz to 3400Hz
Use modem (modulator-demodulator)
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK)

21. Modulation Techniques

22. Amplitude Shift Keying
Values represented by different amplitudes of carrier
Usually, one amplitude is zero
i.e. presence and absence of carrier is used
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber

23. Binary Frequency Shift Keying
Most common form is binary FSK (BFSK)
Two binary values represented by two different frequencies (near carrier)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
High frequency radio
Even higher frequency on LANs using co-ax

24. Multiple FSK More than two frequencies used More bandwidth efficient
More prone to error
Each signalling element represents more than one bit

25. FSK on Voice Grade Line

26. Phase Shift Keying
Phase of carrier signal is shifted to represent data
Binary PSK
Two phases represent two binary digits
Differential PSK
Phase shifted relative to previous transmission rather than some reference signal

27. Differential PSK

28. Performance of Digital to Analog Modulation Schemes
Bandwidth
ASK and PSK bandwidth directly related to bit rate
FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies
(See Stallings for math)
In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK

29. Analog Data, Digital Signal
Digitization
Conversion of analog data into digital data
Digital data can then be transmitted using NRZ-L
Digital data can then be transmitted using code other than NRZ-L
Digital data can then be converted to analog signal
Analog to digital conversion done using a codec
Pulse code modulation
Delta modulation

30. Digitizing Analog Data

31. Pulse Code Modulation(PCM) (1)
If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal
(Proof - Stallings appendix 4A)
Voice data limited to below 4000Hz
Require 8000 sample per second
Analog samples (Pulse Amplitude Modulation, PAM)
Each sample assigned digital value

32. Pulse Code Modulation(PCM) (2)
4 bit system gives 16 levels
Quantized
Quantizing error or noise
Approximations mean it is impossible to recover original exactly
8 bit sample gives 256 levels
Quality comparable with analog transmission
8000 samples per second of 8 bits each gives 64kbps

33. PCM Example

34. PCM Block Diagram

35. Nonlinear Encoding Quantization levels not evenly spaced
Reduces overall signal distortion
Can also be done by companding

36. Effect of Non-Linear Coding

37. Typical Companding Functions

38. Delta Modulation Analog input is approximated by a staircase function
Move up or down one level () at each sample interval
Binary behavior
Function moves up or down at each sample interval

39. Delta Modulation - example

40. Delta Modulation - Operation

41. Delta Modulation - Performance
Good voice reproduction
PCM levels (7 bit)
Voice bandwidth 4khz
Should be 8000 x 7 = 56kbps for PCM
Data compression can improve on this
e.g. Interframe coding techniques for video

42. Analog Data, Analog Signals
Why modulate analog signals?
Higher frequency can give more efficient transmission
Permits frequency division multiplexing (Lecture 8)
Types of modulation
Amplitude
Frequency
Phase

43. Analog Modulation

44. Thank You