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Chapter 5: Signal Encoding
Techniques
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Encoding Techniques
• Digital data, digital signal
• Analog data, digital signal
• Digital data, analog signal
• Analog data, analog signal
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Digital Data, Digital Signal
• Digital signal
—Discrete, discontinuous voltage pulses
—Each pulse is a signal element
—Binary data encoded into signal elements
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Terms (1)
• Unipolar
—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
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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
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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
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Encoding Schemes
• Nonreturn to Zero-Level (NRZ-L)
• Nonreturn to Zero Inverted (NRZI)
• Bipolar -AMI
• Pseudoternary
• Manchester
• Differential Manchester
• B8ZS
• HDB
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Nonreturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
—no transition, i.e. no return to zero voltage
—in general, absence of voltage for zero,
constant positive voltage for one
—More often, negative voltage for “ 1 ” value
and positive for the “ 0 ”
—This is NRZ-L
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Differential Encoding
• Data represented by changes rather than
levels
—More reliable detection of transition rather
than level
—In complex transmission layouts it is easy to
lose sense of polarity
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NRZ pros and cons
• Pros
—Easy to engineer
—Make good use of bandwidth
• Cons
—dc component
—Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
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Multilevel Binary
• Use more than two levels
• Bipolar-AMI
—“ 0 ” represented by no line signal
—“ 1 ” represented by positive or negative pulse
—“ 1 ” pulses alternate in polarity
—No loss of sync if a long string of “ 1 ”s (“ 0 ”
still a problem)
—No net dc component
—Lower bandwidth
—Easy error detection
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Pseudoternary
• “ 1 ” represented by absence of line signal
• “ 0 ” represented by alternating positive
and negative
• No advantage or disadvantage over
bipolar-AMI
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Biphase
• Manchester
—Transition in middle of each bit period
—Transition serves as clock and data
—Low to high represents one
—High to low represents zero
—Used by IEEE 802.3 (CSMA/CD, i.e. Ethernet)
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Manchester Encoding
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Biphase
• Differential Manchester
—Mid-bit transition is clocking only
—Transition at start of a bit period represents
zero
—No transition at start of a bit period
represents one
—Note: this is a differential encoding scheme
—Used by IEEE 802.5 (token ring)
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Differential Manchester
Encoding
BTW: does anything seem wrong here?
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Scrambling
• Use scrambling to replace sequences that would
produce constant voltage
• Filling sequence
—Must produce enough transitions to sync
—Must be recognized by receiver and replace with
original
—Same length as original
• No dc component
• No long sequences of zero level line signal
• No reduction in data rate
• Error detection capability
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B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+-
• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
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Data Encoding
- HDB3 - (High Density Bipolar 3) — Commonly used in Europe and Japan — Similar to bipolar AMI, except that any string of four zeros is replaced by a string with one code violation — Rules: - replace every string of 4 zeros by 000V - V is a code violation - this might result in DC components if consecutive strings of 4 zeros are encoded -- in this case the pattern B00V is used - B is a level inversion and - V is the code violation - general rule: use patterns 000V and B00V such that the violations alternate, thereby avoiding DC components CS420/520 Axel Krings Page 28 Sequence 5
B8ZS and HDB
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Bipol.
AMI
B8ZS
HDB
Test your understanding and see solutions on next slide
CS420/520 Axel Krings Page 32 Sequence 5 1 Bipolar-AMI B 8 ZS HDB 3 1 0 0 0 0 0 0 0 0 0 0 0 V B 0 V B 0 0 0 V B 0 0 V Figure 5. 6 Encoding Rules for B 8 ZS and HDB 3 B 0 0 V 1 1 0 0 0 0 0 1 0 B = Valid bipolar signal V = Bipolar violation (odd number of 1 s since last substitution)
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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 (PSK)
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Amplitude Shift Keying
Hal96 fig 2.
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Frequency Shift Keying
Hal96 fig 2. CS420/520 Axel Krings Page 38 Sequence 5
Frequency Shift Keying
• Frequency Modulation
—different carrier frequencies
—signal to be modulated
—spectrum
Hal96 fig 2.
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How does FSK work?
v (^) FSK ( t )=cos ω 1 t ⋅ vd ( t )+cos ω 2 t ⋅ vd '( t ) cos( 3 ) cos( 3 ) ...} 3
{cos( ) cos( )
cos 2
cos( 3 ) cos( 3 ) ...} 3
{cos( ) cos( )
cos 2
2 0 2 0 2 2 0 2 0 1 0 1 0 1 1 0 1 0 − − + + +
t t t t t t t vFSKt t t t ω ω ω ω ω ω ω ω π ω ω ω ω ω ω ω ω ω π ω Therefore we have: cos 3 ...)} 3
(cos
cos { cos 3 ...)} 3
(cos
() cos { 2 0 0 1 0 0
t t t vFSKt t t t ω ω π ω ω ω π ω The two carriers are ω 1 and ω 2 and (^) v (^) d '( t )= 1 − vd ( t ) CS420/520 Axel Krings Page 40 Sequence 5
Phase Shift Keying
Hal96 fig 2.
