CHAPTER : 04 Encoding & Modulation

Engr Rehan Ali Shah

How information is transferred depends on its original format and on format used by communication hardware.

If you want to send a Love letter by smoke signal you must know which smoke patterns match which words in your message before you actually build your fire.

Words are information and puffs of smoke are representation of that information.

A simple signal by itself doesn’t carry information any more than a straight line conveys words.

The signal must be manipulated so that it contains identifiable changes, recognizable to sender and receiver as representing the information.

Data stored in a computer in the form of 0,1’s to be carried from one place to another, this is called Digital-to-digital conversion.

Some time we want to convert analog signal(like telephone) into digital signal, also called digitization an analog signal.

Digital to Analog signal conversion or modulating a digital signal.

Often an analog signal is sent over long distance using analog media. This is known as analog to analog conversion or modulating an analog signal.

Digital to dig: or conversion is the representation of digital information by digital signal.

For example you can transfer your data to the printer, both the original data and transmitted data are digital.

In this type of encoding, the binary 1s and 0s generated by a computer are translated into sequence of voltage pluses that can be propagate over a wire.

The digital to digital encoding hardware and resulting digital signal.

Of the many mechanisms for digital to digital encoding

Unipolar is very simple and primitive. Digital transmission system work by sending voltage pulses along the medium link.

Unipolar encoding is so named b/c it use only one polarity this polarity is assign to one of the two binary states.

Polar encoding use the two level of voltage, one is +ve and –ve. By using both level.

In most polar encoding methods the average voltage level on the line is reduced that’s why the DC component problem is alleviated.

In nrz encoding, the level of signal is always either positive or negative. This nrz also divided into two categories or two method.

1-NRZ-L. 2-NRZ_I. NRZ_L In nrz-l encoding the level of signal depends

on type of bit it represent. A +ve voltage mean o and –ve 1,the level of signal depends upon state of bit.

In nrz-I, an inversion of voltage level represent a 1 bit. It is the transmission b/w positive and a negative voltage that represent a 1 bit. A 0 bit is represented by no change.

Nrz-I is superior to nrz-l due to the synchronization provided by the signal change each a time a 1 bit is encountered.

Figure shows the nrz-l and nrz-I representation of the series of bit.

To assure synchronization, there must be signal change for each bit. The receiver can use these changes to buildup, update and synchronize its clock.

One solution is return to zero, in RZ the signal change not b/w bits but during each bit .like nrz-l,

But unlike nrz-l ,halfway through each bit interval, the signal return to zero. A 1 bit represent by positive –to-zero and 0 negative-to-zero.

The main disadvantage of RZ encoding is that it requires two signal changes to encode one bit.

Probably the best existing solution to the problem of synchronization is biphase encoding. There are two types of biphase encoding in use on networks today.

1- Manchester. 2- differential Manchester. Manchester:- Manchester encoding uses the inversion at

the middle of each bit interval for both synchronization and bit representation.

A negative to positive transition represent binary 1and a +ve to –ve transition represents binary 0. by using a single transition for a dual purpose.

differential Manchester:- in differential Manchester, the inversion at the

middle of the bit interval is used for synchronization.

the presence or absence of an additional transition at the beginning of the internal is used to identify the bit. A transition means binary o and no tran: binary 1.

Bipolar encoding, like RZ uses three voltage level : positive, negative and zero.hower ,the zero level in bipolar encoding is used to represent binary 0.

The 1s are represented by alternating positive and negative voltage. If the first 1 bit is +ve amplitude the second is represented by –ve.

Three types of bipolar encoding are in popular by the data communications.

Bipolar alternate mark inversion is the simplest type of bipolar encoding.

In the name AMI, the word mark comes from telegraphy and mean 1. so AMI means alternate 1 inversion. Zero voltage represent binary o.

Binary 1s are represented by alternating positive and negative voltages.

A variation of bipolar AMI is called pseudoternary.

We some times need to digitize an analog signal for example, to send human voice over a long distance, we need to digitize, the digital signal are less prone to noise.

This require a reduction of the potentially infinite number of values in an analog message so that they can be represented as a digital steam with a minimum loss of information.

In analog to digital conversion, we r representing the information contained in a continuous wave form as a series of digital pulses(1s or 0s).

The first step in analog to digital conversion is called PAM.

This technique takes an analog signal, samples it and generates a series of pulses based on the result of the sampling.

The method of sampling used in PAM is more useful to other area of engineering then it is to Data communication.

PAM is the foundation of an important analog-to-digital conversion method called PCM.

PAM uses the technique called sample and hold at a given moment.

The reason PAM is not useful to data comm:

PCM modifies the pulses created by PAM to create a completely digital signal. To do so, the PCM first quantizes the PAM pulses.

Quantization is a method of assigning integral values in a specific range to sampled instances.

The binary digits are then transformed into a digital signal using one of the digital to digital encoding techniques.

PCM is actually made up of four separate processes .

PCM is the sampling method used to digitize voice in T-line transmission in the North American Telecommunication system.

So the question is that, how many samples are sufficient , actually, it require remarkably little information for the receiving device to reconstruct an analog signal.

Nyquist Theorem, to ensure the accurate reproduction of an original signal using PAM, the sampling rate must be at least twice of the original signal.

Digital to Analog conversion or D-to-A Modulation is the process of changing one of the characteristics of an analog signal based on the information in digital signal.

When you transmit data from one computer to another across a public access phone line.

The digital data must be modulated on an analog signal that has been manipulated to look like two distinct values corresponding to binary 1 and o.

Before we discuss specific methods of D-to-A two basic issues must be define:

1- Bit rate. 2- Baud rate. The fewer signal unit required, the more

efficient the system and the less bandwidth required to transmit more bits, so we r more concerned to the baud rate.

The baud rate determines the bandwidth required to send the signal.

In analog transmission, the sending device produce a high frequency signal that acts as a basis for information signal.

The base signal is called the carrier signal. Digital information is then modulated on

carrier signal by modifying one or more of its characteristics .

Modulation or shifting keys and modulation signal.

In Amplitude shift keying , the strength of carrier signal varied to represent binary 1 or 0. both frequency and phase remain constant while the Amplitude changes.

The peak amplitude of the signal during each bit duration is constant and its value depends on the bit 0 or 1.

Unfortunately, ASK transmission is highly susceptible to noise interference.

A popular ASK technique is called OOK. In OOK one of the bit value represented by no voltage .

This advantage is a reduction in the amount of energy required to transmit information.

What is the bandwidth u known very well. We get a spectrum of many simple

frequencies.

In FSK , the frequency of the carrier signal is varied to represent binary 1 or 0.

The frequency of the signal during each bit duration is constant and its value depends on the bit (0,1) both peak amplitude and phase remain constant .see the fig:

FSK avoids most of the noise problem of ASK.

It can ignore voltage spikes

Although FSK shifts b/w two carrier frequencies. It is easier to analyze as two coexisting frequencies.

the bandwidth required for FSK transmission is equal to the baud rate of the signal plus the frequency shift. BW=(fc1-fc0)+N .

The process of modulation produces a composite signal.

In phase shift keying , the phase of the carrier is varied to represent binary 1 or 0. both peak amplitude and frequency remain constant as the phase change.

If we start with a phase of 0 degree to represent binary 0, then we can change the phase to 180 degree to send binary 1.

The phase of the signal during each bit duration is constant and its value depends on bit (0,1).

The method is often called 2-PSK or BPSK, b/c two different phases (0 and 180) are used.

PSK is not susceptible to the noise degradation that effects ASK. Nor the bandwidth limitation of FSK.

This means that smaller variation in the signal can be detected reliably by the receiver ,instead of utilizing only two variation of a signal , each represent 1 bit ,we can use four variation and let each phase shift represent two bits.