Unit 1

Lecture 5

Digital Signal Encoding A Digital To Digital Data Transmission Method

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Coding• In communications & information processing the code is system of

rules to convert information - such as a letter, word, sound, image, or gesture—into another form. Sometimes in shortened or secret, form or representation for communication through a channel or storage in a medium.

• A first example is the invention of language, which enabled a person, through speech, to communicate what he or she saw, heard, felt, or thought to others.

• Speech limits the range of communication to the distance a voice can carry, and limits the audience to those present when the speech is uttered.

• The invention of writing, which converted spoken language into visual symbols, extended the range of communication across space and time.

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Encoding • Encoding is the technical term used for coding

in data communication.• The process of encoding converts information

from a source into symbols for communication or storage. Decoding is the reverse process, converting code symbols back into a form that the recipient understands.

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Morse Encoding Morse code is a method of transmitting text

information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment.

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Different Conversion/Transmission Schemes

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Before we discuss various line coding schemes, let us first have an idea of different data conversion schemes.

• Digital to digital conversion• Line Coding• Block Coding• Scrambling

• Analog to Digital Conversion• PAM• PCM

– Nyquist Theorem• Digital To Analog Conversion

• ASK, FSK, PSK & QAM– Constellation

• Analog to Analog Conversion• AM, FM &PM 6

Different Techniques Used in Data Transmission/Conversion

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DIGITAL-TO-DIGITAL CONVERSIONDIGITAL-TO-DIGITAL CONVERSION

• We know that data can be either digital or We know that data can be either digital or analog.analog.

• We know that signals that represent data can We know that signals that represent data can also be digital or analog. also be digital or analog.

• The digital data can be converted & represented The digital data can be converted & represented into digital signals. This conversion can be done into digital signals. This conversion can be done by three techniques: by three techniques: 1.1. line coding line coding 2.2. block coding & block coding & 3.3. ScramblingScrambling

Line coding is always needed; block coding and Line coding is always needed; block coding and scrambling may or may not be neededscrambling may or may not be needed..

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Line CodingLine Coding

• Line coding is the process of converting digital data to digital signals. We assume that data, in the form of text, numbers, graphical images, audio, or video, are stored in computer memory as sequences of bits.

• Line coding converts a sequence of bits to a digital signal. At the sender, digital data are encoded into a digital signal; at the receiver, the digital data are recreated by decoding the digital signal. Figure shows the process.

• Line code is also called as digital baseband modulation or baseband transmission.

Line coding and decoding

Codec

Codec • A codec is a device or computer program capable of encoding or

decoding a digital data stream or signal. Codec is a short word of coder-decoder or, less commonly, compressor-decompressor.

• A codec encodes a data stream or signal for transmission, storage or encryption, or decodes it for playback or editing. Codecs are used in videoconferencing, streaming media and video editing applications.

• A video camera's analog-to-digital converter (ADC) converts its analog signals into digital signals, which are then passed through a video compressor for digital transmission or storage. A receiving device then runs the signal through a video decompressor, then a digital-to-analog converter (DAC) for analog display.

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Some of the important characteristics of line coding are:

1.Signal element vs data element2.Data rate vs Signal rate3.Bandwidth 4.Baseline Wandering5.DC Component6.Self Synchronization7.Built in Error Detection8.Immunity to Noise & Interference9.Complexity

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Some Common Characteristics of line coding

1. Data Element Vs Signal Element• Data is the smallest unit which can be sent & it is bit• Signal is the smallest time wise unit of digital signal• A signal only carries data element• A single signal can carry one or multiple data

element as well• We are also defining a ratio r which is no of data

element carried by each signal for the purpose of understanding relation between data element & signal element. So, r = data element/signal element.

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Figure: Signal elements versus data elements

2. Data Rate vs Signal RateThe data rate defines the number of data elements (bits) sent in 1s. The unit is bits per second (bps). The data rate is sometimes called the bit rate also.The signal rate is the number of signal elements sent in 1s. The unit is the baud. The signal rate is sometimes called the pulse rate, the modulation rate, or the baud rate.

S = N/rSavg = c x N/r

c is a case factor which is normally 1, it can have other valueS = Signal rate in baudN = data rate in bps

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A signal is carrying data in which one data element is encoded as one signal element (r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1?

Example 1

SolutionWe assume that the average value of c is 1/2. The baud rate is then

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3. Bandwidth• The actual bandwidth of a digital/data signal is

infinite• The effective bandwidth of data signal is found to

be finite• The minimum bandwidth is calculated as

Bmin = c x N x (1/r)

• The maximum data rate of a channel is calculated by Nyquist formula as below:

Nmax = 2 x B x log2 L B = Bandwidth &

L = Levels of signals16

A signal has two data levels with a pulse duration of 1 ms. Calculate the pulse rate and bit rate.

Pulse Rate = 1/ 10Pulse Rate = 1/ 10-3-3 = 1000 pulses/s = 1000 pulses/s

Bit Rate = Bit Rate =

= 1000 x log= 1000 x log22 2 2

= 1000 bps= 1000 bps17

Example 2

Pulse Rate x logPulse Rate x log22 L L •If signal level is 2 then bit rate is same as pulse rate•If signal is 4 then bit rate is 2 times to pulse rate•If signal is 8 then bit rate is 3 times to pulse rate

A signal has four data levels with a pulse duration of 1 ms. Calculate the pulse rate and bit.

Pulse Rate = = 1000 pulses/sPulse Rate = = 1000 pulses/s

Bit Rate = PulseRate x logBit Rate = PulseRate x log22 L = 1000 x log L = 1000 x log22 4 = 2000 bps 4 = 2000 bps

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Example 3

• Switch to lecture of channel capacity from here.

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The maximum data rate of a channel is Nmax = 2 × B × log2 L (defined by the Nyquist formula). Does this agree with the previous formula for Nmax?

SolutionA signal with L levels actually can carry log2 L bits per level. If each level corresponds to one signal element and we assume the average case (c = 1/2), then we have

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Example 3

4. Baseline wandering In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called as baseline. The incoming signal power is evaluated against this baseline to determine the value of the data element. A long string of 0s or 1s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly. A good line coding scheme needs to prevent baseline wandering.

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5. DC ComponentWhen the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies (results of Fourier analysis). These frequencies around zero are called DC (direct-current) components which present problems for a system that cannot pass low frequencies or a system that uses electrical coupling (via a transformer). For example, a telephone line cannot pass frequencies below 200 Hz. Also a long-distance link may use one or more transformers to isolate different parts of the line electrically. For these systems, we need a scheme with no DC component.

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Figure DC component

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6. Self Synchronization• To correctly interpret the signals received from the sender, the

receiver’s bit intervals must correspond exactly to the sender’s bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might misinterpret the signals. Figure shows a situation in which the receiver has a shorter bit duration. The sender send 1011001, while the receiver receives 110111000011.

• A self-synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. If the receiver’s clock is out of synchronization, these points can reset the clock.

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• In telecommunications, a self-synchronizing code, or comma-free code, is a line code that can be easily synchronized. Such line codes have the property that the code which is made of a part of the code word, or two overlapping code words is not a valid code. An example, take the code words 11 and 00, and the code 11 00 00 11 00. The spaces have been added to show the different words, and are not really in the code.

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6. Self Synchronization

• Let's now assume that four letters (two code words) are read. The code 1000 is not a valid code, because 10 is not one of the two code words defined. Similarly, 0001. Even though 00 is a valid word, 01 is not. The only valid way to read two valid words from the example given is by starting at the very beginning, or just after one of the spaces, which have been inserted for clarity only.

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In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps?

SolutionAt 1 kbps, the receiver receives 1001 bps instead of 1000 bps.

At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps.

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Example 4

7. Built in Error Detection – It is desirable to have a built-in-error-detecting capability in the generated code to detect some of or all the errors that occurred during transmission.

8. Immunity to Noise & Interference – Another desirable code characteristic is code that is immune to noise and other interferences.

9. Complexity – A complex scheme is more costly to implement than a simple one. For example, a scheme that uses four signal levels is more difficult to interpret than one that uses only two levels.

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