SOURCE CODING PROF. A.M.ALLAM LEC1: Introduction 9/9/2017 Digital Communication System Source of Information User of Information Source Encoder Channel Encoder Modulator Source Decoder Channel Decoder De-Modulator Channel Flash Back on Digital Communication System Communication systems are designed to transmit the information generated by a source to some destination Analog/Digital Convert to digital form (ASCII) It enables the following: -Amount of information from a given source -Minimum storage and bandwidth needed to transfer data from a given source -Limit on the transmission rate of information for reliable comm. over a noisy channel -Data compression Channels can only transport physical signals, e.g., electrical signals. Therefore, digital signals must be converted to appropriate formats The channel introduce errors. Channel coding is used for controlling errors in data transmission over unreliable (noisy channel) communication channels 1
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SOURCE CODING PROF. A.M.ALLAM
9/9/2017 1
LEC1: Introduction
9/9/2017
Digital Communication System
Source of
Information
User of
Information
Source
Encoder
Channel
Encoder
Modulator
Source
Decoder
Channel
Decoder
De-Modulator
Channel
Flash Back on Digital Communication System
Communication systems are designed
to transmit the information generated
by a source to some destination
Analog/Digital
Convert to digital form (ASCII)
It enables the following:
-Amount of information from a
given source
-Minimum storage and bandwidth
needed to transfer data from a
given source
-Limit on the transmission rate of
information for reliable comm.
over a noisy channel
-Data compression
Channels can only transport physical signals, e.g., electrical signals. Therefore, digital signals
must be converted to appropriate formats
The channel introduce errors. Channel coding is used for controlling errors in data
transmission over unreliable (noisy channel) communication channels 1
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LEC1: Introduction
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Digital Communication System
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Signal
Source Filter Sampler Quantizer
Mapper
Filter: pass the required signal and remove the undesired signal
Sampler: sample and hold; generates discrete signal continuous value; i.e.,
infinite number of levels
Quantizer: generates discrete signal discrete values; i.e., finite (limited) number of levels
(could be uniform or nonuniform) Mapper: sample and hold; generates discrete signal continuous
value; i.e., infinite number of levels
t
x(t) Analogue
t
x(t) Digital; binary sequence
At the end one gets a digital values s[n] which are in practice numbers that
are stored in a computer to be further processed, that is why we do ADC
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Digital Communication System
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Signal
Source Filter Sampler Quantizer
Mapper Source
Encoder
Source Coding and Compression:
-It is created by identifying and using structures that exist in the data, -Is the art or science of representing information in a compact form
Characters in a text file
Numbers that are samples of speech or image waveforms
Sequences of numbers that are generated by other processes
Exs:
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Digital Communication System
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Signal
Source Filter Sampler Quantizer
Mapper Source
Encoder
-Why we are in need to data compression?
These number of bits or bytes required to represent a multimedia data can be huge, Exs:
To digitally represent 1 second of video without compression (using the CCIR 601 format),
we need more than 20 megabytes, or 160 megabits
To represent 2 minutes of uncompressed CD quality music (44.100 samples per second, 16
bits per sample) requires more than 84 million bits. Downloading music from a website at
these rates would take a long time
-All compression methods are computer programs:
Encoder: Mapping of s[n] into a bit stream b
Decoder: Mapping of the bit stream b into the discrete decoded signal s’[n]
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LEC1: Introduction
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Digital Communication System
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Signal
Source Filter Sampler Quantizer
Mapper Source
Encoder
Channel
Encoder
Parity check (Odd or Even)
-Adding some redundancy in the binary information sequence to overcome the problems of
interference and noise
Tx: 0 → 000 or 1 → 111
Assume Tx: 0 → 000
Rx: 0 0 0 → correct 0
0 0 1 → maybe 0
1 0 0 → maybe 0
1 0 1 → maybe 1
1 1 1 → correct 1
Code Ratio R= 1/3
Channel Coding n > k
k
X
Parity check: (odd or even)
i/p 2bits → o/p 3bits
0 0 → 000
0 1 → 011
1 0 → 101
1 1 → 110
(number of 1’s should be even)
Code Ratio R = 2/3
Code Repetition
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LEC1: Introduction
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Digital Communication System
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Signal
Source Filter Sampler Quantizer
Mapper Source
Encoder
Channel
Encoder
Digital
Modulator
Digital Modulator:
-It maps the binary information sequences into signal waveforms
0 bit
1 bit
b bits , b=2M
So(t)
S1(t)
Si(t), i=1,2,3,…,M
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Digital Communication System
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The discrete signal is temporally sampled and its amplitude is represented
using k = 3 bits/sample and 8 different levels
temporal signalsone dimensional are typically Speech and audio signals-
spatial temporal signalsthree dimensional are Videos-
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Digital Communication System
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-Pictures are two dimensional spatial signals
-8 x 8, 16 x 16, 32 x 32, and 128 x 128 samples (from left to right)
-Each sample is represented with 8 bits
-Each square represents average of luminance values it covers
Ex: Sampling of picture with different spatial sampling rates
-1, 2, 4, 8 bits/sample
-The spatial sampling rate is fixed 128x128
Ex: Quantization of picture with different bits/sample
SOURCE CODING PROF. A.M.ALLAM
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LEC1: Introduction
9/9/2017
Source coding and Data Compression
Source coding or compression is required for efficient transmission or storage, leading to one or
both of the following benefits:
• Transmit more data given throughput (channel capacity or storage space)
• Use less throughput given data
Original
Signal Encoder
Compression
Algorithm
Compression Algorithms
sc Compressed
Signal
Original
Signal Decoder
Reconstruction
Algorithm sc Compressed
Signal Fewer bits
Original data can be recovered from the compressed data exactly or not depends on the compression technique