Data Conversion Methods

Winter 2006 ECE 766Computer Interfacing and Protocols

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Data Conversion MethodsData Conversion Methods• Sending data from one place to the next

Transform data into signals• Formats of source vs. medium

– Format of the original data (analog/digital)– Format used by the communication hardware

(analog/digital)• 4 possible combinations

– Digital data / digital signal (computers over LAN)

– Analog data / digital signal (long distance phone)– Digital data / analog signal (computers over phone

lines)– Analog data / analog signal (radio broadcast)


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Data Encoding / ModulationData Encoding / Modulation• Baseband• Digitally Encoded• Resources shared by Time

Division Multiplexing

• Broadband• Analog Modulation• Resources shared by

Frequency Division Multiplexing

PSTN

Fre

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cy

Time

Should I have called the vertical axis bandwidth?


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TerminologyTerminology

• Data rate (bps)• Baud rate, “modulation rate”

(signal elements/sec)• Mark (1) and space (0) conditions

(from telegraphy)• Connection types

– Simplex: One way– Half Duplex: Two way, but only one way at a time– (Full) Duplex: Two way simultaneously


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Criteria for a Criteria for a Good Encoding SchemeGood Encoding Scheme• Signal Spectrum

– Minimize high frequency components– No DC components

• Synch capability (find bit positions)

• Signal error detection capability

• Signal interference and noise immunity

• Cost and complexity


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Absolute vs. Differential Absolute vs. Differential Encoding / Modulation SchemesEncoding / Modulation Schemes• Absolute:

– Each signal corresponds to a predetermined information unit

– The meaning of a signal sequence is fixed, not relative.

• Differential:– Information is encoded by difference between

current and previous signal element– The meaning of a signal sequence is relative,

not absolute.


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Digital Encoding SchemesDigital Encoding Schemes

• Digital information is converted to a sequence of voltage pulses that propagate over the link

• Three subcategories by voltage use:– Unipolar (Zero and Positive)– Polar (Negative and Positive)– Bipolar (Negative, Zero, and Positive)


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Unipolar EncodingUnipolar Encoding

• Uses zero and positive voltage pulses to encode binary data

• Not really “encoded” at all!

Amplitude

Time

0 1 0 1 1 1 0 0


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Polar EncodingPolar Encoding

• Polar encoding uses a positive and a negative voltage level to represent bits Solves the DC component problem(if balanced)

• Categories:– Nonreturn to Zero (NRZ)

• NRZ-L (L=Level)• NRZ-I (I=Inverted)

– Return to Zero (RZ)(as shown in book)

– Biphase• Manchester• Differential Manchester


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Nonreturn to Zero (NRZ)Nonreturn to Zero (NRZ)

• The voltage level is constant during a bit interval, i.e., no returns to zero

• Absolute and differential versions• Absolute NRZ: NRZ-L (L=Level)(like ntl)

– 0 = Positive voltage– 1 = Negative voltage

Amplitude

Time

0 1 0 1 1 1 0 0


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• Differential NRZ: NRZ-I (I=Inverted)– A bit is represented by the transition of the

voltage level, not the voltage level itself!– 0 = No inversion at beginning of bit interval– 1 = Inversion at beginning of bit interval

Amplitude

Time

0 1 0 1 1 1 0 0


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• Evaluation– No DC component– Simple– Few high frequency components– Synchronization

• No synchronization at large (consider a string of the same bit)

• NRZ-I provides synchronization for every 1 encountered can handle strings of 1s(superior to NRZ-L)


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Return to Zero (RZ)(Return to Zero (RZ)(bipolar formbipolar form))

• Targets to solve the synchronization problem• A scheme that handles both strings of both 1s

and 0s• Voltage level change for every bit value

three levels: +,-, 0– 0 = Transition from negative to zero– 1 = Transition from positive to zero

Amplitude

Time

0 1 0 1 1 1 0 0


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Return to Zero (RZ)Return to Zero (RZ)

• Variations used also for magnetic recording (no synchronization capability)

• Evaluation– Solves synchronization problem– Two signal changes / bit

More transitions Occupies more bandwidth


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BiphaseBiphase

• Signal changes in the middle of the bit interval, but does not return to zero

• Signal change bit representation synchronization

• Manchester:– 0 = Transition from positive to negative– 1 = Transition from negative to positive

Amplitude

Time

0 1 0 1 1 1 0 0


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BiphaseBiphase• Differential Manchester:

– 0 = Transition at the beginning of bit period– 1 = No transition at the beginning of bit period

• Evaluation:– Not as simple– Higher frequency components (as RZ)– Synchronization capability– No DC component

Amplitude

Time

0 1 0 1 1 1 0 0


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BipolarBipolar

• Like in RZ, three voltage levels are used

• Zero voltage level used for binary 0

• Categories:– Alternate Mark Inversion (AMI)– Bipolar 8-Zero Substitution (B8ZS)

North America– High Density Bipolar 3 (HDB3)

Europe and Japan


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Alternate Mark Inversion (AMI)Alternate Mark Inversion (AMI)

• Uses three voltage levels– 0 = Zero volts– 1 = Non-zero voltage, opposite in polarity to

the last logical 1

• Evaluation– No DC component– Synchronized only for 1s, not 0s– Error detection

Amplitude

Time

0 1 0 1 1 1 0 0


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Bipolar 8-Zero Substitution (B8ZS)Bipolar 8-Zero Substitution (B8ZS)

• Adds synchronization for long strings of 0s• North American system• Same working principle as AMI except for eight

consecutive 0s

• Evaluation– Adds synchronization without changing the DC balance– Error detection possible

Amplitude

Time

0 0 0 0 0 0 0 01 0 1

Violation Violation

10000000001 +000+-0-+01 in general 00000000000V(-V)0(-V)V


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High Density Bipolar 3 (HDB3)High Density Bipolar 3 (HDB3)

• Goal like B8ZS to improve Sync of AMI• Just like AMI except 4 0’s are replaced by code• For 0000 use 000V or B00V

– Where B and V are + or –– And V is AMI violation, B is Balance Bit

• Use 000V if EVEN number of + and – pulses so far

• Use B00V if ODD, and B is opposite last pulse


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High Density Bipolar 3 (HDB3)High Density Bipolar 3 (HDB3)• Same goal as B8ZS• Based on AMI• Replaces every four consecutive 0s based on

– Number of pulses since last substitution– Polarity of last logical 1

Last 1 polarity

# of 1s + -

Even(revised) 0000000+ 0000000-

Odd(revised) 0000-00- 0000+00+


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High Density Bipolar 3 (HDB3)High Density Bipolar 3 (HDB3)

• Example: (revised 1-13-06)

– Number of 1s since last substitution is even, last 1 negative (before this string)

– Encode 100000000001

Amplitude

Time

0 0 0 0 0 0 0 01 0 0 1

Data Conversion Methods

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