DIGITAL-TO-DIGITAL CONVERSION

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Line CodingLine Coding SchemesBlock CodingScrambling

Signal Encoding Techniques

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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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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 04

Line coding schemes

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Digital Signal Encoding Formats

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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 1more often, a negative voltage represents one

value and a positive voltage represents the other(NRZ-L)

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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 timetransition (low to high or high to low) denotes

binary 1no transition denotes binary 0

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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

Pros• easy to

engineer• make efficient

use of bandwidth

Cons• presence of a

dc component• lack of

synchronization capability

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Binary Bipolar-AMI

use more than two signal levels Bipolar-AMI

binary 0 represented by no line signalbinary 1 represented by positive or negative

pulsebinary 1 pulses alternate in polarityno loss of sync if a long string of ‘1’s occursno net dc component lower bandwidtheasy error detection

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In bipolar encoding, we use three levels: positive, zero, and negative.

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AMI encoding

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

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Theoretical Bit Error Rate

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

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

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Polar biphase: Manchester and differential Manchester schemes

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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 1this is a differential encoding scheme

used by IEEE 802.5

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Biphase Pros and Cons

Cons• at least one transition per

bit time and may have two• maximum modulation rate

is twice NRZ• requires more bandwidth

Pros• synchronization on midbit

transition (self clocking)• has no dc component• has error detection

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Spectral Density of Various Signal Encoding Schemes

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Normalized Signal Transition Rate of Various Digital Signal Encoding Schemes

Table 5.3  

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AMI used with scrambling

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B8ZS (Bipolar with 8-zero substitution) scrambling technique (USA)

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a. Proceeding pulse is positive; 8 zero are coded as 000+-0-+;b. Proceeding pulse is negative; 8 zero are coded as 000-+0+-;

HDB3 Substitution Rules (Europe)

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In Multilevel (mBnL) schemes, a pattern of m data elements is encoded as a

pattern of n signal elements in which 2m ≤ Ln.

Note

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Multilevel schemes: increase the number of bits per baud.

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

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