Data Encoding: Signal Encoding Techniques and Comparison of Encoding Schemes, Exams of Signals and Systems

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Data Encoding – Chapter 5

(part 1)(p )

COSC 3213

Instructor: U.T. Nguyen

9/27/2007 10:43 AM 1

Signal Encoding Techniques

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Encoding Techniques

• Digital data, digital signals (5.1)
• Analog data, digital signals (5.3)a og data, d g ta s g a s (5 3)
• Digital data, analog signals (5.2)
• Analog data, analog signals (5.4)

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Digital Data, Digital Signals (5.1)

• Digital signal —discrete, discontinuous voltage pulses —each pulse is a signal element —binary data encoded into signal elements

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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

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—Bandwidth —Encoding scheme

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

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—External clock —Sync mechanism based on signal

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

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—Some codes require signal rate greater than data rate

Encoding Schemes

• B8ZS
• HDB

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Nonreturn to Zero Inverted

• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bitCo sta t o tage pu se o du at o o b t
• Data encoded as presence or absence of signal transition at beginning of bit time
• Transition (low to high or high to low) denotes a binary 1
• No transition denotes binary 0

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• No transition denotes binary 0
• An example of differential encoding

NRZ

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Differential Encoding

• NRZI is an example of differential encoding
• Data represented by changes rather than levelsata ep ese ted by c a ges at e t a e e s
• 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

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g g

• Not often used for signal transmission

Bipolar-AMI and Pseudoternary

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Multilevel Binary Issues

• Synchronization needed for long runs of 0’s or 1’s —can insert additional bits (used in low-rate ISDN) —scramble data (later)
• Overcoming NRZ problems, but …
• Not as efficient as NRZ —Each signal element only represents one bit —In a 3 level system could represent log 2 3 = 1.58 bits R i di i i h b h l l

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—Receiver must distinguish between three levels (+A, -A, 0) —Requires approx. 3dB more signal power for same probability of bit error

Biphase

• Manchester — Transition in middle of each bit period — Transition serves as clock and dataT iti l k d d t — Low to high represents one — High to low represents zero — Used by IEEE 802.
• Differential Manchester — Midbit transition is clocking only T iti t t t f bit i d t

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— 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.

Manchester Encoding

• has transition in middle of each bit period
• transition serves as clock and data
• llow to high represents one t hi h t
• high to low represents zero
• used by IEEE 802.3 (Ethernet)

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Modulation Rate

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Scrambling

• use scrambling to replace sequences that would produce constant voltage
• these filling sequences must —produce enough transitions to sync —be recognized by receiver & replaced with original —be same length as original
• design goalsg g —have no dc component —have no long sequences of zero level line signal —have no reduction in data rate —give error detection capability 26

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 f

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• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all zeros

B8ZS and HDB

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