Encoding

1
Encoding

• There are four types of encoding possible.
• Digital Encoding of Digital Data
• Digital Encoding of Analog Data
• Analog Encoding of Digital Data
• Analog Encoding of Analog Data

2
Digital Encoding of Digital Data
Digital Data of 0s and 1s
Digital Encoding Schemes
Polar
Unipolar
Bipolar
3
Unipolar Encoding
Unipolar Single voltage - one value
Zero voltage another value
AMPLITUDE
V
0 V
TIME
UNIPOLAR (positive voltage 1 zero voltage 0)
4
Polar Encoding Schemes
POLAR
RZ
NRZ
BIPHASE
DifferentialManchester
NRZ-L
NRZ-I
Manchester
5
0
0
0
0
1
1
1
0
1
AMPLITUDE
V
0 V
- V
TIME
POLAR NRZ-L (positive voltage 0 negative
voltage 1)
6
1
1
1
1
0
0
0
1
0
AMPLITUDE
V
0 V
- V
TIME
POLAR NRZ-I (change if next bit is a 1)
7
1
1
1
1
0
0
0
1
0
AMPLITUDE
V
0 V
- V
TIME
POLAR RZ (basically NRZ-L with return to zero at
midbit)
8
Manchester Code

• Transition in the middle of each bit period
• Transition provides clocking and data
• Low-to-high1, high-to-low0
• Used in Ethernet

9
1
1
1
1
0
0
0
1
0
AMPLITUDE
V
0 V
- V
TIME
BI-PHASE Manchester - shift to opposite pole at
midbit
(neg-to-pos 1 while a pos-to-neg 0)
10
Differential Manchester

• Midbit transition is only for clocking
• Transition at beginning of bit period0
• Transition absent at beginning1
• Has added advantage of differential encoding
• Used in token-ring

11
1
1
1
1
0
0
0
1
0
AMPLITUDE
V
0 V
- V
TIME
BI-PHASE Differential Manchester - shift to
opposite pole at midbit
(transition 0 and no transition 1)
12
Bipolar Encoding Schemes
BIPOLAR
AMI
HDB3
B8ZS
AMI - Alternate Mark Inversion B8ZS - Bipolar 8
Zero Substitution (North America) HDB3 - High
Density Bipolar 3 (Europe Japan)
13
Encoding

• Digital Encoding of Digital Data
• Digital Encoding of Analog Data
• Analog Encoding of Digital Data
• Analog Encoding of Analog Data

14
Analog to Digital Encoding

• Pulse Amplitude Modulation (PAM)
• sample analog signal (measure amplitude of signal
  at equal intervals)
• uses sample and hold technique
• generate pulses based on sampling
• First step in PCM - Pulse Code Modulation
• Quantize PAM Pulse - assign an integral value in
  a specific range to sampled instances.

15
Sampling Rate

• How much should you sample?
• The sampling theorem (Nyquist Theorem) If a
  signal is sampled at regular intervals of time
  and at a rate higher at least twice the
  significant signal frequency, the samples contain
  sufficient information to ensure accurate
  reproduce of the original signal.
• 8000 samples/sec sufficient for 4000hz

16
Encoding

• Digital Encoding of Digital Data
• Digital Encoding of Analog Data
• Analog Encoding of Digital Data
• Analog Encoding of Analog Data

17
Analog Encoding of Digital Data

• Bit Rate - The number of bits transmitted in 1
  second
• Baud Rate - The number of signal units per second
  required to represent those bits
• Baud rate is less than or equal to bit rate
• Carrier Signal ?

18
Methods of Modulation

• amplitude shift keying (ASK)
• frequency shift keying (FSK)
• phase shift keying (PSK)
• differential phase shift keying(DPSK)
• quadrature amplitude modulation (QAM)

19
ASK Illustration
0
0
1
1
20
FSK Illustration
1
1
0
1
21
PSK Illustration
0
0
1
1
22
Complex Modulations

• Combining modulation techniques allows us to
  transmit multiple bit values per signal change
  (baud)
• Increases information-carrying capacity of a
  channel without increasing bandwidth
• Increased combinations also leads to increased
  likelihood of errors
• Typically, amplitude and phase modulation are
  combined

23
Quadrature Amplitude Modulation (QAM)

• the most common method for quadbit transfer
• combination of 8 different angles in phase
  modulation and two amplitudes of signal
• provides 16 different signals, each of which can
  represent 4 bits

24
Quadrature Amplitude Modulation Illustration
90
135
45
amplitude 1
0
180
amplitude 2
225
315
270
25
Bit Baud Rate Comparison
26
Encoding

• Digital Encoding of Digital Data
• Digital Encoding of Analog Data
• Analog Encoding of Digital Data
• Analog Encoding of Analog Data

27
Methods of Modulation

• amplitude modulation (AM)
• frequency modulation (FM)
• phase modulation (PM)

28
Codec

• Coder/Decoder
• converts analog signals into a digital form and
• converts back digital data to analog signals
• e.g., hi-fi music, television pictures, the
  output of copying machine, videoconferencing