lec-15 (encoding decoding)

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Muhammad Waseem Iqbal

Lecture # 15

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Today’s Menu

Ϟ _{Encoding/Decoding}

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Encoding/Decoding

Ϟ _{Digital-to-Digital conversion or encoding/decoding is the}

representation of digital information by digital signal

Ϟ _{For example when we transmit data from computer to the}

printer, both original and transmitted data have to be

digital

Ϟ _{Encoding a digital signal is where 1’s and 0’s generated by}

the computer are translated into voltage pulses that can

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Encoding/Decoding

Ϟ _{A digital signal is a sequence of discrete, discontinuous}

voltage pulses, each pulse is a signal element

Ϟ _{Binary data are transmitted by encoding each data bit into}

signal elements

Ϟ _{In the simplest case, there is a one-to-one}

correspondence between bits and signal elements

Ϟ _{An example would be in which binary 0 is represented by}

a lower voltage level and binary 1 by a higher voltage level

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Encoding/Decoding

Types of Encoding

1. Unipolar 2. Polar 3. Bipolar

1-Unipolar:

Ϟ _{Encoding is simple , with only one technique in use}

Ϟ _{Simple and primitive}

Ϟ _{Almost obsolete today}

Ϟ _{Study provides introduction to concepts and problems involved}

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Unipolar Encoding

Ϟ _{It works by sending voltage pulses on the transmission}

medium

Ϟ _{The signal elements all have the same algebraic sign, that}

is, all positive or negative

Ϟ _{One voltage level stands for binary 0 while the other}

stands for binary 1

Ϟ _{It is called Unipolar because it uses only one polarity}

Ϟ _{This polarity is assigned to one of the two binary states}

usually a ‘1’

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Unipolar Encoding

Ϟ _{Figure shows the idea: 1’s are encoded as +ve values, and}

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Unipolar Encoding

Pros and Cons of Unipolar Encoding Pros

Ϟ _{Straight forward and simple} Ϟ _{Inexpensive to implement}

Cons

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Polar Encoding

2-Polar:

Ϟ _{Polar encoding uses two voltage levels, positive and} negative

Ϟ _{One logic state is represented by a positive voltage level,} and the other by a negative voltage level

It has 3 subcategories:

1. Non Return to Zero (NRZ)

ϞNRZL ϞNRZI

2. Return to Zero (RZ) 3. Biphase

ϞManchester

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Non Return to Zero (NRZ)

In NRZ, the level of signal is either positive or negative

NRZ-L (Non-Return-to-Zero-Level)

Ϟ _{Level of the signal depends on the type of bit it represents} Ϟ _{A +ve voltage usually means the bit is a 1 and a –ve voltage}

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Non Return to Zero (NRZ)

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Non Return to Zero (NRZ)

NRZ-I (Non-Return-to-Zero-Invert On One)

Ϟ _{The inversion of the level represents a 1 bit} Ϟ _{A bit 0 is represented by no change}

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Non Return to Zero (NRZ)

Problem with NRZ-I

Ϟ _{NRZ-I is superior to NRZ-L due to synchronization provided} by signal change each time a 1 bit is encountered

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Non Return to Zero (NRZ)

Ϟ _{The NRZ codes are the easiest to engineer and, in} addition, make efficient use of bandwidth

Ϟ _{The main limitations of NRZ signals are the presence of a} dc component and the lack of synchronization capability Ϟ _{Because of their simplicity and relatively low frequency}

response characteristics, NRZ codes are commonly used for digital magnetic recording

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Return to Zero (RZ)

Ϟ _{Any time, data contains long strings of 1’s or 0’s, receiver} can loose its timing

Ϟ _{In unipolar, we have seen a good solution is to send a} separate timing signal but this solution is expensive

Ϟ _{A better solution is to somehow include sync in encoded} signal somewhat similar to what we did in NRZ-I but it should work for both strings of 0 & 1

Ϟ _{One solution is RZ encoding which uses 3 values; Positive,} Negative and Zero

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Return to Zero (RZ)

Ϟ _{Like NRZ-L, +ve voltage means 1 and a –ve voltage means} 0, but unlike NRZ-L, half way through each bit interval, the signal returns to zero

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Return to Zero (RZ)

Problem with RZ

Ϟ _{The only problem with RZ encoding is that it requires two} signal changes to encode one bit and therefore occupies more bandwidth

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Biphase

Ϟ _{Best existing solution to the problem of synchronization} Ϟ _{Signal changes at the middle of bit interval but does not}

stop at zero

Ϟ _{Instead it continues to the opposite pole}

There are two types of biphase encoding

1. Manchester

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Manchester

Ϟ _{Uses inversion at the middle of each bit interval for both} synchronization and bit representation

Negative-to-Positive Transition = 1 Positive-to-Negative Transition = 0

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Differential Manchester

Ϟ _{Inversion at the middle of the bit interval is used for} synchronization but presence or absence of an additional transition at the beginning of bit interval is used to identify a bit

Ϟ _{A transition means binary 0 & no transition means binary} 1

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3-Bipolar Encoding

Ϟ _{Although the biphase techniques have achieved} widespread use in local-area-network applications at relatively high data rates, they have not been widely used in long-distance applications

Ϟ _{The principal reason for this is that they require a high} signaling rate relative to the data rate

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Bipolar Encoding

Ϟ _{An approach is to make use of some sort of scrambling} scheme

Ϟ _{The idea behind this approach is simple;}

Ϟ _{Sequences that would result in a constant voltage level} on the line are replaced by filling sequences that will provide sufficient transitions for the receiver's clock to maintain synchronization

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Bipolar Encoding

Ϟ _{Like RZ, it uses three voltage levels}

Ϟ _{Unlike RZ, zero level is used to represent binary 0}

Ϟ _{Binary 1’s are represented by alternate positive and} negative voltages

Ϟ _AMI

Ϟ _{Pseudoternary} Ϟ _B8Zs

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Alternate Mark Inversion(AMI)

Ϟ _{Simplest type of bipolar encoding}

Ϟ _{A binary 0 is represented by no line signal, and a binary 1} is represented by a positive or negative pulse

Ϟ _{The binary 1 pulses must alternate in polarity}

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Alternate Mark Inversion(AMI)

Pros and Cons:

Ϟ _{There will be no loss of synchronization if a long string of} is occurs

Ϟ _{Each 1 introduces a transition, and the receiver can} resynchronize on that transition

Ϟ _{A long string of 0s would still be a problem}

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Pseudoternary

Ϟ _{Inverse of AMI}

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Pseudoternary

Ϟ _{Two variations are developed to solve the problem of} synchronization of sequential 0’s

1. B8Zs (used in North America) 2. HDB3 (used in Europe & Japan)

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B8Zs

Ϟ Bipolar with 8-zeros substitution

Ϟ _{Difference between AMI and B8Zs occurs only when 8 or} more consecutive zeros are encountered

Ϟ _{Forces artificial signal changes called violations}

Ϟ _{Each time eight 0’s occur, B8Zs introduces changes in} pattern based on polarity of previous 1 (the ‘1’ occurring just before zeros)

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HDB3

Ϟ High-Density Bipolar-3 Zeros

Ϟ _{Alteration of AMI adopted in Europe and Japan}

Ϟ _{Introduces changes into AMI, every time four} consecutive zeros are encountered instead of waiting for eight zeros as in the case of B8Zs

Ϟ _{As in B8Zs, the pattern of violations is based on the} polarity of the previous 1 bit

Ϟ _{HDB3 also looks at the number of 1’s that have occurred} since the last substitution

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HDB3

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HDB3

Ϟ High-Density Bipolar-3 Zeros

Ϟ _{If the last violation was positive, this violation must be} negative, and vice versa

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