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8/7/2019 Lecture11 & 12_Analog Data Digital Signal
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PROF. Dr. ABDUL QADEER KHAN
RAJPUT
Lecture No.11 & 12
Data Communications System & Networks
By
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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
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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
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BackgroundBackground
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Steps for converting Analog Data to Digital
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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
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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.
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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.
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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).
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PULSE AMPLITUDE MODULATIONPULSE AMPLITUDE MODULATION
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PULSE CODE MODULATIONPULSE CODE MODULATION
Figure A
Quantization
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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
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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.
Number 7 =00111 for 32 level quantization.
Number 7 =000111 for 64 level quantization.
Encoding techniques will be explained in next lecture
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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.
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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.
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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.
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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
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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)
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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.
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Teletype data
Computer data
Pulsed radar
Sonar Signals
Teletype data
Computer data
Pulsed radar
Sonar Signals
SIGNALS WHICH ARE INHERENTLY DIGITAL
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Speech
TV
Speech
TV
SIGNALS WHICH ARE ANALOG