Www.infotech.monash.edu FIT 1005 Networks & Data Communications Lecture 4 – Signal Encoding...

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FIT 1005 Networks & Data Communications

Lecture 4 – Signal Encoding Techniques

Reference: Chapter 5

Data and Computer Communications

Eighth Edition

by William Stallings

Lecture slides by Lawrie Brown

Modified Slides: http://users.monash.edu.au/~amkhan/fit1005


Type of

• Data– digital– analog

• Signal– digital– analog

• Data Encoding (Schemes)– digital to digital– digital to analog– analog to digital– analog to analog

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal

Analog Signal &

Digital Signals


Type of

• Data– digital– analog

• Signal– digital– analog

• Data Encoding (converted into) Converting source data into communication signal– digital to digital– digital to analog– analog to digital– analog to analog

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal

Analog Signal &Digital Signals


Data can be encoded in either form of signals

Analog Signals

TelephoneVoice

(sound waves)Analog Signal

Analog Data

ModemBinary Voltagepulses

Analog Signal(Carrier frequency)

Digital Data

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal

Analog Signal


Data can be encoded in either form of signals

Digital Signals

CODECAnalog data Digital Signal

Analog Data

DigitalTransmitter

Digital data Digital Signal

Digital Data

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital SignalDigital Signals


Signal Encoding Techniques (1)

• Digital signaling: For digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t).

• Analog signaling: The basis for analog signaling is a continuous constant-frequency fc signal known as the Carrier signal. E.g. AM or FM

• Baseband signal: The input signal may be analog or digital and is called the modulating signal or baseband signal.

– Baseband signals are the fundamental group of frequencies in an analog or digital waveform that may be transmitted along a channel. Examples of a digital baseband signal may be Ethernet signals operating over a LAN

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Encoding is required because

• Signals are carried by the physical medium• The performance of medium will vary depending on

the kind of signal, with varying characteristics in terms of– Attenuation– Error rate– Distance– Noise influence– etc….

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Encoding

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


signal


Modulation

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


signal


Signal Encoding Techniques (2)

• The electromagnetic signal is generated at the physical layer• The electromagnetic signal could be analog or digital signals that

must carry the data(message).

• The data must be encoded into signals:– Data Encoding is done to produce a analog or digital signal– Modulation of a analog/digital data is used to produce an analog signal

• This analog signal can then be transmitted over a network/communication link.

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Which one of the 4 combinations to choose?• Digital data/Digital signal

– Equipment for encoding is less complex– Digital transmission has less errors

• Analog data/Digital signal– Digital transmission can be done on the existing analog medium –

good return on investment (ROI)– Digital transmission has less errors

• Digital data/Analog signal– Some high data rate mediums are analog (e.g. optical fiber)– most of the unguided media are analog (e.g. Wireless)

• Analog data/Analog signal– Can be transmitted easily and cheaply; different position on the

spectrum can be shared on the same media (e.g. Frequency-division multiplexing)

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Data Encoding

Data Types

Signal Encoding

Data to Signal

EncodingSummary

Digital Data to

Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Data Encoding

• digital data to digital signal• analog data to analog signal• digital data to analog signal• analog data to digital signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Digital Data, Digital Signal

• Digital signal– discrete, discontinuous voltage pulses– each pulse is a signal element– binary data encoded into signal elements

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Properties that need to be considered while encoding

• At least 5 properties need to be considered when encoding any form of data to any signals1. Signal spectrum requirement

2. Signal synchronization capability

3. Signal error-detecting capability

4. Signal interference and noise immunity

5. Cost and complexity of the encoding/decoding equipment.

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Property 1

• Signal spectrum • digital signal can contain infinite frequencies high

bandwidth is required• Conversely lack of high frequency component means

less bandwidth required for transmission (cheaper)• No DC component - otherwise the decoding equipment

can be expensive

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



What is DC Component?

• A non-zero constant value of the Fourier series transformation of a signal is the value of the DC component

• If the signal varies between positive and negative voltages, then a non-zero difference in the area above and below the zero voltage line implies that the signal has a DC component

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



What is DC Component?

0

0

0

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


+ dc component

- dc component

0

0


Property 2

• Signal synchronization capability• Need to determine the beginning and end of each bit

position.– Not an easy task – May need a separate clock between sender and receiver to

synchronize them or– embedded

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Property 3

• Signal Error-detecting capability • can the encoding technique identify (and correct) errors

when the signal is corrupted?• Parity or CRC

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Property 4

• Signal interference and noise immunity• Certain encoded signals can be decoded correctly when

the signal is corrupted due to interferences or inducement of noise.

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Encoding

• Unfortunately we will not be able to achieve all the first four properties at the cheapest cost by a single encoding technique.

• Hence cost is an last 5th important factor that needs to be considered as well

• Each encoding technique will satisfy (either fully or partially) only a sub-set of the properties.

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


Encoding requires to distinguish between Signal element and data elementRatio r is defined as number of data elements carried by each signal elements

Signal element versus data element


Some Popular Encoding Schemes of Digital data to Digital signals

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Some Terms used w.r.t Digital data to Digital signals

• Unipolar all signal elements have either +ve or –ve voltage (same sign)

• Bipolar one logic state is represented by a +ve voltage and the other by a –ve voltage

• data rate rate of data transmission in bits per second• duration or length of a bit time taken for the transmitter

to emit a bit = 1/data rate• modulation rate rate at which the signal level changes,

measured in baud = number of signal elements per second

• mark and space another term for binary 1 and binary 0

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Encoding Schemes

Has more desirable properties

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Nonreturn to Zero-Level (NRZ-L)

• two different voltages for 0 and 1 bits

0 bit 0 / +ve voltage

1 bit -ve voltage• voltage constant during bit interval

– no mid-bit transition - i.e. no return to zero voltage– one can use absence of voltage for 0, constant positive voltage for 1 but

becomes a DC component problem– more often, negative voltage for 1 (mark) and positive voltage for 0 (space)

• NRZ-L is typically the code used to generate or interpret digital data by terminals and other devices

Presence

of -ve voltage

Zero / positive volts

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Nonreturn to Zero Invert on ones (NRZI)

• NonReturn to zero inverted on ones• constant voltage pulse for duration of bit• data encoded as presence or absence of signal transition

at beginning of bit time– transition (low to high or high to low) denotes binary 1– no transition denotes binary 0

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Nonreturn to Zero Invert on ones (NRZI)

• NonReturn to zero inverted on ones• constant voltage pulse for duration of bit• data encoded as presence or absence of signal transition

at beginning of bit time– transition (low to high or high to low) denotes binary 1– no transition denotes binary 0

• example of differential encoding since we have– data represented by changes rather than levels– more reliable detection of transition rather than level– easy to lose sense of polarity & synchronization

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



NRZ Pros & Cons

• Pros– easy to design encoder and decoder– make good use of bandwidth

• Cons– dc component– lack of synchronization capability

• used for magnetic recording• not often used for signal transmission

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Multilevel Binary Bipolar-AMI

• Use more than two levels• Bipolar-AMI

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


zero represented by no line signal one represented by positive or negative pulse

one pulses alternate in polarity no loss of sync if a long string of ones long runs of zeros still a problem no net dc component lower bandwidth easy error detection because of the alternation of the polarity Used on a T-carrier communication links


Multilevel Binary Pseudoternary

• one represented by absence of line signal• zero represented by alternating positive and negative• no advantage or disadvantage over Multilevel binary

bipolar

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Multilevel Binary Issues

• synchronization with long runs of 0’s (in the case Bipolar-AMI) or 1’s (in the case of pseudoternary)

– can insert additional bits to force transitions, ( as in ISDN )– Insertion causes wasted bit transmission leads to reduced

transmission rate• not as efficient as NRZ

– each signal element only represents one bit> receiver distinguishes between three levels: +A, 0, -A

– a 3 voltage level system could represent log23 = 1.58 bits

– requires approx. 3dB more signal power for same probability of bit error

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Manchester Encoding

• has transition in the middle of each bit period• transition serves as clock and data (property 2)• low to high represents one• high to low represents zero• An example of a bipolar encoding method• Specified for the IEEE 802 –Ethernet standards

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Differential Manchester Encoding

• Mid-bit transition can be used for clocking• transition at start of bit period representing 0• no transition at start of bit period representing 1

– this is a differential encoding scheme• used by IEEE 802.5 - token ring LANs

1

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



What does differential encoding mean?

• In differential encoding, the signal is decoded by – comparing the polarity of adjacent signal elements – rather than determining the absolute value of a signal element.

• In other words it looks for a transition rather than the absolute value.

• The above would require synchronisation with respect to the sender.

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Differential Manchester Encoding • At least 4 properties need to considered when encoding

digital data to digital signals• 1. Signal spectrum requirement - less DC component and

less frequency spectrum • 2. Signal synchronization capability – by the mid-bit

transition • 3. Signal error-detecting capability • 4. Signal interference and noise immunity –signal is

decoded by comparing the polarity of adjacent signal elements rather then the absolute value of the signal

element • 5. Cost and complexity of the encoding/decoding

equipment. - partially met

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Biphase Pros and Cons

• Con– at least one transition per bit time and possibly two– maximum modulation rate is twice NRZ– requires more bandwidth

• Pros– synchronization on mid bit transition (self clocking)– has no dc component– has error detection

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Scrambling

• Scrambling replaces long sequences of 0..0’s that would produce constant voltage by a special codes.

• these filling sequences must– produce enough transitions to provide synchronization– Should be recognized by receiver & replaced with original data– be of same length as original sequence and hence no date rate

reduction• design goals

– have no dc component– have no long sequences of zero level line signal– have no reduction in data rate– give error detection capability

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



B8ZS and HDB3

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


B8ZS (Bipolar with 8-zeros substitution)If an octet of all zeros occurs last voltage pulse is positive, Encoded as 000+–0–+

If an octet of all zeros occursLast voltage pulse is negative,Encoded as 000–+0+–

- ve0 0 0 - + 0 + -

HDB3 (High-Density Bipolar 3-zeros)If string of Four zeros sequences occurs The fourth zero(0) is replaced with a code violation(+- or -+ pulses). Successive violations are replaced with alternate polarity.


Data Encoding

• digital data to digital signal• analog data to analog signal• digital data to analog signal• analog data to digital signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Analog data, Analog Signal

• By modulating the data (baseband) with a high carrier signal fc.

• Why?– higher frequency fc can give more efficient transmission & BW

– permits frequency division multiplexing (chapter 8)

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Modulation• is a process of encoding an analog (or digital) data on to

an analog carrier signal whose frequency, say is f c Where f c is a high frequency

• Input – analog (digital) data – called baseband or modulating signal m(t)

• Output - analog signal - carried modulated signal s(t)

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


Analog Data modulating Signal m(t)

High Frequency SignalFc(t)

Amplitude Modulated SignalM(t) ʘ Fc(t)

Communication channel


Modulation Techniques

• is to vary one of the following three aspects of the

carrier signal

• Amplitude of the carrier (Amplitude modulation (AM))

• Frequency of the carrier (Frequency modulation (FM))

• Phase of the carrier (Phase modulation(PM))

• Example – AM and FM radio stations for the first two

respectively

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Analog ModulationTechniques

Amplitude Modulation

Phase Modulation

Frequency Modulation

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Data Encoding

• digital to digital

• analog to analog• digital to analog• analog to digital

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Digital Data, Analog Signal

• Use the same modulation techniques discussed for analog data, analog signals

• main use is public telephone system– has freq range of 300Hz to 3400Hz– use modem (modulator-demodulator)

• encoding techniques– Amplitude shift keying (ASK)– Frequency shift keying (FSK)– Phase shift keying (PK)

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Digital Data, Analog Signal – Modulation Techniques

Digital Data

AmplitudeShift keying

FrequencyShift keying

PhaseShift keying

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Amplitude Shift Keying• encode 0/1 by different carrier amplitudes

– usually have one amplitude zero• susceptible to sudden gain changes• inefficient• used for

– up to 1200bps on voice grade lines– very high speeds over optical fiber

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


s(t) = A * Sin (2π f t + θ)A = Amplitudef = carrier frequencyθ = Phase Angle

“0” = 0 * Sin (2π f t + 0)

“1” = A * Sin (2π f t + 0)


Binary Frequency Shift Keying

• most common is binary FSK (BFSK)• two binary values represented by two different

frequencies (near carrier)• less susceptible to error than ASK• used for

– up to 1200bps on voice grade lines– high frequency radio– even higher frequency on LANs using co-axial cable

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



“0” = A * Sin (2π f1 t + 0)

“1” = A * Sin (2π f2 t + 0)


Multiple FSK

• Each signalling element can also represent more than one bit

– then more than two frequencies are required (e.g:2 bits – 4 frequencies for 00, 01, 10 and 11)

• More bandwidth efficient• However more prone to error

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Phase Shift Keying• phase of carrier signal is shifted to represent

data• binary PSK

– two phases represent two binary digits• differential PSK

– phase shifted relative to previous transmission rather than some reference signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



“0” = A * Sin (2π f1 t + 0)

“1” = A * Sin (2π f1 t + 180)


Quadrature PSK• get more efficient use if each signal element represents

more than one bit– e.g.. shifts of /2 (90o)– each element represents two bits– split input data stream in two & modulate onto carrier & phase

shifted carrier• can use 8 phase angles & more than one amplitude

– 9600bps modem uses 12 angles, four of which have two amplitudes

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


“11” = A * Cos (2π fc t + 45)“10” = A * Cos (2π fc t + 135)“00” = A * Cos (2π fc t + 225)“01” = A * Cos (2π fc t + 315)


Quadrature Amplitude Modulation

• QAM used in asymmetric digital subscriber line (ADSL) and some wireless communication

• combination of ASK and PSK• logical extension of QPSK• send two different signals simultaneously on same carrier

frequency

– uses two copies of carrier fc, one shifted by a phase angle of 90°

– each carrier is ASK modulated– two independent signals over same medium– demodulate and combine for original binary output

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



QAM Modulator

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


send two different signals simultaneously on same carrier frequencyuses two copies of carrier fc, one shifted by a phase angle of 90°

each carrier is ASK modulatedtwo independent signals over same mediumdemodulate and combine for original binary output

ASK – signal-1

ASK – signal-2

http://www.inue.uni-stuttgart.de/german/lehre/lesungen/uet2/applet/QAM16e.html


Data Encoding

• digital data to digital signal

• analog data to analog signal

• digital data to analog signal• analog data to digital signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Analog Data, Digital Signal

• Need to convert– analog data into digital Data

> called digitization– then digital data to digital signal

• Digitization Principle: Take the amplitude of the signal at different intervals and convert the signal value into digital data.

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal




• Once the analog data is converted into digital data then we can use any of the encoding techniques that we talked earlier to convert it to digital signals

• For example we can then:– transmit using NRZ-L, NRZI or using Manchester or Differential

Manchester coding techniques– can be converted to analog signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Digitizing Analog Data

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal




• analog to digital conversion is done using a codec (coder-decoder)

• Conversion is done at least in two different ways– pulse code modulation (PCM)– delta modulation (DM)

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



An example• Let the analog data be as

shown in the left picture

• Find the amplitude of the signal at regular intervals (say at t1, t2, t3, t4, t5, t6 in the figure)

• Express each amplitude in a binary form.

• e.g. a signal voltage level, say at t2 is represented as binary code

code # [15] in base10= [1111]2

which is binary - digital data

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


t6t5t4t3t2t1


An Example (cont’d)

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


Voltage levels (M)

Binary 4-bit code

15 1111

14 1110

13 1101

12 1100

11 1011

10 1010

9 1001

8 1000

7 0111

6 0110

5 0101

4 0100

3 0011

2 0010

1 0001

0 0000


How often you need to sample the analog data?

• If you want to decode the original signal from its digital data, then the sampling theorem specifies the minimum rate at which you need to sample the analog signal– “If a signal f(t) is sampled at regular intervals of time at a rate

higher than twice the highest signal frequency, then the samples contain all the information of the original signal” (Stallings DCC8e)

– e.g.. 4000Hz voice data, requires 8000 sample per sec– Note that these are analog samples, referred to as pulse

amplitude modulation (PAM) samples– The PAM samples must be assigned to a binary code to

complete the digitization of the original analog signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



PCM Block Diagram

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Delta Modulation

• analog input is approximated by a staircase function– can move up or down one level () at each sample interval

• has binary behavior– since function only moves up or down at each sample interval– hence can encode each sample as single bit– 1 for up or 0 for down


Delta Modulation Example


Delta Modulation Operation


PCM versus Delta Modulation

• DM has simplicity compared to PCM• DM is easier to implement compared to PCM• DM has worse SNR compared to PCM• PCM has better SNR compared to DM• PCM has issue of bandwidth being used

– e.g. for good voice reproduction with PCM> want 128 levels (7 bit) & voice bandwidth of 4khz> need 8000 x 7 = 56kbps

• data compression can improve on BW issues..• PCM is the choice and still growing demand for digital

signals– Due to use of repeaters, TDM(no intermodulation noise), and efficient

switching• PCM is preferred to DM for analog signals

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal



Summary

• We looked at signal encoding techniques

– digital data to digital signal– analog data to analog signal– digital data to analog signal– analog data to digital signal

Data Types

Signal Encoding

Signal Encoding

SummaryDigital Data

to Digital Signal

Analog Data to

Analog Signal

Digital Data to

Analog Signal

Analog DataTo

Digital Signal


Www.infotech.monash.edu FIT 1005 Networks & Data Communications Lecture 4 – Signal Encoding...

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