lecture11 & 12_analog data digital signal

8/7/2019 Lecture11 & 12_Analog Data Digital Signal

1/29

PROF. Dr. ABDUL QADEER KHAN

RAJPUT

Lecture No.11 & 12

Data Communications System & Networks

By


2/29


3/29


4/29

Digital baseband and digital carrier transmission systems have many advantages over their

analog counterparts. Some of these advantages are:

1. Digital transmission systems are more immune to noise (due to the

availability of regenerative repeaters, which can be used instead of analog amplifiers at

intermediate points throughout the transmission channel).

2. Digital transmission systems allow us to use multiplexing at both the

basebandand carrier levels (e.g., TDMA, CDMA and OFDMA),

which means we can easily carry multiple conversations on a single

physical medium (channel).

3. The possibility of using channel coding techniques (i.e., error

correcting codes) in digital communications improves the signal-to-

noise ratio (SNR) at the receiver.

ANALOG DATA-DIGITAL SIGNALANALOG DATA-DIGITAL SIGNAL


5/29

4. The ability to use spread spectrum techniques in digital systems help

overcome jamming and interference and allows us to hide the

transmitted signal within noise if necessary.

5. Using computers to perform signal processing (DSP) on digital

signals eliminates the need to build expensive and bulky discrete-

component devices.

Actually, due to these important advantages many communication systems are transitioning

these days from analog to digital communications. Within the next decade most of

communication systems will become digital, with analog communication playing a minor role.

ANALOG DATA-DIGITAL SIGNALANALOG DATA-DIGITAL SIGNAL


6/29

BackgroundBackground


7/29

Steps for converting Analog Data to Digital


8/29

INTRODUCTION

The process of transmitting analog data onto digital links can be regarded as

digitization i.e. analog data is converted into digital (using ADC) and then transmitted.

At the receiver side it is converted back into analog (using DAC).

There are two popular techniques for it.

PCM (Pulse Code Modulation)

Delta Modulation


9/29

PULSE CODE MODULATIONPULSE CODE MODULATION

PCM involves following steps:

Sampling (Through PAM)

Quantization

Encoding

The Pulse Amplitude modulation serves the basis for

Pulse Code Modulation.


10/29

I. Sampling:

Sampling is the process in which only a relatively-small set of values, called discrete samples

{mn}, are taken to represent the signal m(t) instead of the (time-continuous) infinite set of

values included in the analog signal (see the following figure).

In uniform sampling, the time interval between successive samples is set to a constantvalue

equal to T, called the sampling time. In this case, the sampling frequency is fs = 1/T.


11/29

As specific examples, telephone conversations are sampled at 8 kHz (twice the 4 kHz

bandwidth of the human voice signal), while compact disc (CD) audio is sampled at

44.1 kHz (twice the 22.05 kHz bandwidth of music signals1).


12/29


13/29


14/29


15/29


16/29

PULSE AMPLITUDE MODULATIONPULSE AMPLITUDE MODULATION


17/29

PULSE CODE MODULATIONPULSE CODE MODULATION

Figure A

Quantization


18/29


19/29


20/29

In voice telephony, for example, the number of quantization levels is chosen to be L = 2^8 = 256,while for compact disc (CD) audio, the number of quantization levels is L = 2^16 = 65,536

possible values per sample. Of course, a bigger value of L means a smaller interval v, and hence

a smaller round off error caused by the quantization process (called quantization error).

The number of quantization levels L is an important parameter in digital systems because it

decides (see next step) how many bits will be used to represent the value of each sample. For

example, ifL = 256, the value of each sample can be in one of 256possibilities, which means

that each sample must be mapped (encoded) into 8 bits.

This is because 8 binary bits can be in 2^8 = 256 possible states (00000000, 00000001,

00000010, 00000011, . . ., 11111111). ForL = 65,536, we need 16 bits to encode each sample

value.

Quantization


21/29

ENCODINGThe quantized level are converted into binary code using some suitable encoding

scheme (to be discussed later).

For a 16 level quantization (24), 4 bits are required.

For example:

Number 7 =0111 for 16 level quantization.



Encoding techniques will be explained in next lecture


22/29

CompandingCompanding is a process, which consists of compressing the signal at the

transmitter and expanding the signal at the receiver. In this process it in possibleto pre-distort the signal before it in modulated and un-distort it after

demodulation. This process is used instead of tampered quantizing, in which the

difference b/w adjoining levels are made small for small signals and gradually

large for large signals. The quantizing noise in disturbed so as to effect small

signals some what less and large signal somewhat more with companding, same

results are obtained but much more easily. In companding, the signal to be

transmitted in passed through an amplifier, whichhas correctly adjusted non-

linear transfer characteristic, favoring small amplitude signals. These are then

artificially large when they are quantized, and so the effect of quantizing noise

upon them is reduced. The correct amplitude relations are restored by the

expander in the receiver.


23/29

Delta ModulationWith delta modulation, an analog input is approximated by a staircase function that moves up

or down by one quantization level () at each sampling interval (Ts). An example is shown in fig

in next slide. The important characteristics of this staircase function are that its behavior is

binary. At each sampling time, the function moves up or down a constant amount . Thus the

output of the delta modulation process can be can be represented as a single binary digit for

each sample. In essence a bit stream is produced by approximating the derivative of an analog

signal rather than its amplitude. A 1 is generated if the staircase function is to go up during the

next interval; a 0 is generated otherwise .

The transition (up or down) that occurs at each sampling interval is chosen so that the staircase

function tracks the original analog waveform as closely as possible. Thus the staircase is always

changed in the direction of the input signal. The output of the DM process is therefore a binary

sequence that can then be used at the receiver to reconstruct the staircase function.


24/29

Figure : Delta Modulation.

The staircase function can then be smoothed by some type of integrator process or by

passing it through a low pass filter to produce an analog approximation of the analog

input signal.


25/29

There are two important parameter in a DM scheme

1.The size of the step assigned to each binary digit, .

2.The sampling rate.

must be chosen to produce a balance b/w two types of errors or noise. When

the analog waveform is varying slowly, there will be quantizing noise. This noise

increases as is increased. On the other hand when the analog waveform is changing

more rapidly then the staircase can follow, there is slope overload noise. This noise

increases as is decreased.

It should be clear that accuracy of the scheme could be improved by increasing the

sampling rate. However this increase the data rate of the output signal. The principal

advantage of DM over PCM is the simplicity of its implementation. In general, PCM

exhibits better SNR characteristics at the same data rate.

Delta Modulation


26/29

OTHER PULSE MODULATIONSOTHER PULSE MODULATIONS

The previous discussed types were of digital nature,

there are other pulse modulations that are analog in

nature. They are:

PAM (Pulse Amplitude Modulation)

PWM (Pulse Width Modulation)

PPM (Pulse Position Modulation)


27/29

OTHER PULSE MODULATIONS (CONT.)PAM: The Amplitude of the pulse is varied according to the signal at discrete time intervals.

PWM: The width of the pulse is varied according to the signal at discrete time intervals.

PPM: The Position the pulse is varied according to the signal at discrete time intervals.


28/29

Teletype data

Computer data

Pulsed radar

Sonar Signals

Teletype data

Computer data

Pulsed radar

Sonar Signals

SIGNALS WHICH ARE INHERENTLY DIGITAL


29/29

Speech

TV

Speech

TV

SIGNALS WHICH ARE ANALOG

lecture11 & 12_analog data digital signal

Documents