DIGITAL-TO-DIGITAL CONVERSION
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Transcript of DIGITAL-TO-DIGITAL CONVERSION
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DIGITAL-TO-DIGITAL CONVERSION
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Line CodingLine Coding SchemesBlock CodingScrambling
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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
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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
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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.
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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+-;
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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.
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Multilevel: 2B1Q (2 binary, 1 quaternary) scheme, used in DSL
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