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DIGITAL-TO-DIGITAL CONVERSION

DIGITAL-TO-DIGITAL CONVERSION. Line Coding Line Coding Schemes Block Coding Scrambling. Signal Encoding Techniques. Digital Data, Digital Signal. d igital signal discrete, discontinuous voltage pulses each pulse is a signal element binary data encoded into signal elements. Terminology.

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DIGITAL-TO-DIGITAL CONVERSION

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  1. DIGITAL-TO-DIGITAL CONVERSION Line Coding Line CodingSchemesBlock Coding Scrambling

  2. Signal Encoding Techniques

  3. Digital Data, Digital Signal • digital signal • discrete, discontinuous voltage pulses • each pulse is a signal element • binary data encoded into signal elements

  4. Terminology • unipolar – all signal elements have the same sign • polar – one logic state represented by positive voltage and the other by negative voltage • Bipolar -- A binary 0 is encoded as zero volts as in unipolar encoding. A binary 1 is encoded alternately as a positive voltage and a negative voltage. • data rate – rate of data ( R ) transmission in bits per second • duration or length of a bit – time taken for transmitter to emit the bit (1/R) • modulation rate – rate at which the signal level changes, measured in baud = signal elements per second. • mark and space – binary 1 and binary 0

  5. Line coding schemes

  6. Digital Signal Encoding Formats

  7. Nonreturn to Zero-Level (NRZ-L) • easiest way to transmit digital signals is to use two different voltages for 0 and 1 bits • voltage constant during bit interval • no transition (no return to zero voltage) • absence of voltage for 0, constant positive voltage for 1 • more often, a negative voltage represents one value and a positive voltage represents the other(NRZ-L)

  8. Non-return to Zero Inverted (NRZI) • Non-return to zero, invert on ones • constant voltage pulse for duration of bit • data encoded as presence or absence of signal transition at beginning of bit time • transition (low to high or high to low) denotes binary 1 • no transition denotes binary 0

  9. NRZ Pros & Cons • lack of synchronization capability • NRZ-L and NRZ-I both have a DC component problem. • used for magnetic recording • not often used for signal transmission

  10. Binary Bipolar-AMI • use more than two signal levels • Bipolar-AMI • binary 0 represented by no line signal • binary 1 represented by positive or negative pulse • binary 1 pulses alternate in polarity • no loss of sync if a long string of ‘1’s occurs • no net dc component • lower bandwidth • easy error detection In bipolar encoding, we use three levels: positive, zero, and negative.

  11. AMI encoding

  12. Multilevel Binary Pseudoternary • binary 1 represented by absence of line signal • binary 0 represented by alternating positive and negative pulses • no advantage or disadvantage over bipolar-AMI and each is the basis of some applications

  13. Theoretical Bit Error Rate The multilevel binary signal requires approximately 3 dB more signal power than a two-valued signal for the same probability of bit error.

  14. Polar biphase: Manchester Encoding • transition in middle of each bit period • midbit transition serves as clock and data • low to high transition represents a 1 • high to low transition represents a 0 • used by IEEE 802.3

  15. Polar biphase: Manchester and differential Manchester schemes

  16. Differential Manchester Encoding • midbit transition is only used for clocking • transition at start of bit period representing 0 • no transition at start of bit period representing 1 • this is a differential encoding scheme • used by IEEE 802.5

  17. Biphase Pros and Cons

  18. Spectral Density of Various Signal Encoding Schemes

  19. Normalized Signal Transition Rate of Various Digital Signal Encoding Schemes Table 5.3  

  20. 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 goals • have no dc component • have no long sequences of zero level line signal • have no reduction in data rate • give error detection capability

  21. AMI used with scrambling

  22. B8ZS (Bipolar with 8-zero substitution) scrambling technique (USA) a. Proceeding pulse is positive; 8 zero are coded as 000+-0-+; b. Proceeding pulse is negative; 8 zero are coded as 000-+0+-;

  23. HDB3 Substitution Rules (Europe)

  24. Note Multilevel schemes: increase the number of bits per baud. In Multilevel (mBnL) schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m≤ Ln.

  25. Multilevel: 2B1Q (2 binary, 1 quaternary) scheme, used in DSL

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