digital signal encoding formats

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TERMSData rate Defines the number of data elements(bits) sent in 1s. Unit is bps Signal rate number of signal elements sent in 1s. Unit is baud Bandwidth in bits per sec speed of bit transmission in channel or link Baseline wandering The incoming signal power is evaluated against the baseline to determine the value of the data element. A long string of 0 s and 1 s can cause a drift in the baseline, and make it difficult for the receiver to decode correctly.

LINE CODING Unipolar encoding uses only one voltage level (either 0 or 1) 1 encoded as a positive value and the 0 encoded as zero value. NRZ

Polar encoding uses two voltage levels (one positive, one negative) NRZ, RZ and biphase

Bipolar encoding uses three voltage levels: positive, negative, zero AMI and pseudoternary

Continued There are numerous techniques available to convert digital data into digital signals. Let s examine few: 1. Nonreturn to Zero-Level (NRZ-L) 2. Nonreturn to Zero Inverted (NRZI) 3. Multilevel (Bipolar AMI) 4. Manchester 5. Differential Manchester 6. B8ZS 7. HDB3

Unipolar encoding

Nonreturn to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits Negative voltage for one value and positive for the other Voltage constant during bit interval No transition to 0V For Example: Negative Voltage (-5V) use to represent binary 1 and Positive Voltage (+5v) use to represent binary 0

Nonreturn to Zero Inverted (NRZ-I) Nonreturn to zero and inverted on 1 Constant voltage pulse for duration of bit Data encoded as presence or absence of signal transition at beginning of bit time Transition denotes a binary 1 No transition denotes binary 0

NRZNRZ-L and NRZ-I both have an average signal rate of N/2 Bd. Fundamental difference exists between NRZ-L and NRZIWith NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1, With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1

NRZ pros and cons Pros Easy to engineer Make good use of bandwidth

Cons String of 0 s or 1 s leads a constant voltage over a period of time Loss of synchronization between transmitter & receiver, this problem is more serious in NRZ-L Baseline wandering is a problem for both schemes, but twice as severe in NRZ-L Used for magnetic recording Not often used for signal transmission

Return to Zero (RZ) The main problem with NRZ encoding occurs when the sender and reciever clocks are not synchronized, the solution is RZ scheme. Signals changes during the bit Disadvantages Requires two signal changes to encode a bit, therefore occupies greater bandwidth Complexity: RZ uses three levels of voltage, which is more complex to create. Scheme is not used today

Biphase Schemes Overcomes the limitations on NRZ codes Two biphase techniques are commonly used: Manchester Differential Manchester Heavily used in LAN applications

BIPHASE continued1. Manchester Transition in middle of each bit period Idea of RZ and NRZ-L are combined Mid-Bit transition serves for clocking and data Rule

Low to high represents binary 1 High to low represents binary 0

Enables effective clock signal recovery at receiver

Poor bandwidth utilization Effective sending rate is cut in half Used by IEEE 802.3 (ethernet) standard for baseband coaxial cable and twisted-pair CSMA/CD bus LANs

BIPHASE continued

In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.

BIPHASE continued1. Differential Manchester Combines the ideas of RZ and NRZ-I Mid-Bit transition serves only for clocking Rule If there is a 0 , there will be a transition (start of a bit period) If there is a 1 , there will be no transition (start of a bit period) Used by IEEE 802.5 token ring LAN, using shielded twisted pair

Modulation Rate

Modulation rate for Manchester and Differential Manchester is twice the data rate inefficient encoding for long-distance applications

Biphase Pros and Cons Cons At least one transition per bit time and possibly two Maximum baud rate is twice NRZ Requires more bandwidth Signal rate is double that for NRZ Pros Synchronization on mid bit transition (self clocking) No baseline wandering Error detection

Big difference between NRZ and Manchester codes: For long strings of 0-bits, NRZ codes generate signal that does not change over long time period Manchester codes always produce signal change during every bit transmission. Manchester codes are called self-clocking codes, because they provide a guaranteed voltage change (a clock signal ) in the middle of every bit received

Why do we care about self-clocking codes? Transmitter / receiver clocks are not perfectly synchronized to tick at same rate (too expensive). NRZ-L or NRZ-I cannot be used at high data rates or long distances unless a separate clock signal is sent on another wire. Manchester codes can be used at high data rates or long distances, because receiver continuously gets feedback on sender clock rate.

BIPOLAR ENCODING It uses three voltage levels: positive, negative and zero. The zero level is used to represent binary 0, while the 1s are represented by alternating positive and negative voltages Same signal rate as NRZ

Bipolar-AMI Encoding Scheme The bipolar-AMI (Alternate Mark Inversion) encoding scheme is unique among all the encoding schemes because it uses three voltage levels When a device transmits a binary 0, a zero voltage is transmitted When the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted Which of these is transmitted depends on the binary 1 value that was last transmitted

Used for long distance communication, No loss of sync if a long string of ones (zeros still a problem) Lower bandwidth Easy error detection

Pseudoternary Variation of AMI encoding I bit is encoded as zero voltage 0 bit is encoded as alternating positive and negative voltages.


Scrambling Technique Used for long distance transmission (WAN)

Solve the problem of bipolar AMI code where sequence of zero was problem. Provide synchronization Used to replace sequences that would produce constant voltage Avoid long sequences of zero level line signal No reduction in data rate Error detection capability Two commonly used techniques are: B8ZS, and HDB324

Bipolar With 8 Zeros Substitution (B8ZS) Based on bipolar-AMI Used in North America Eight consecutive zeros level voltages are replaced by the sequence of 000VB0VB V denotes violation; this is a non zero voltage that breaks an AMI rule of encoding B denotes bipolar, which means a non zero level voltage in accordance with the AMI rule Scrambling in this case does not change bit rate25

High Density Bipolar 3 Zeros (HDB3) HDB3 similar but based on 4 zeros Based on bipolar-AMI String of four consecutive zero level voltages are replaced with a sequence of 000V or B00V The reason for two different substitutions is to maintain the even number of non zero pulses after each substitution if no. is odd- substitution pattern-000V if no. is even- substitution pattern-B00V26



Recap of Digital Signal Encoding Formats0 NRZL NRZI Bipolar-AMI Manchester Diff Manchester(always a Transition in the middle of interval)

1 Low level transition +ve line signal Transition from low to high in the middle of interval No transition at start of interval

High level No transition at start of interval No line signal Transition from high to low in the middle of interval Tran at start of interval


Same as bipolar-AMI, except that any string of four zeros is replaced by a string with one code violation Same as bipolar-AMI, except that any string of eight zeros are replaced by a string of two code violations