COMPRESSION PRINCIPLES

Introduction



Everybody is familiar with the word "Compression" in different ways. Compression process has different meaning in chemical, mechanical, civil and metallurgical engineering etc. But moment we thing about compression in electronic engineering one word which strikes our mind is "Digital". These days it is practically a cliche to claim that all electronic communication is engaged in a digital revolution. Before we discuss the impact of compression on digital revolution, let us analyse digital revolution.



1. Digital Revolution



All sensory signals including audio and video signals are analog at the point of origination or at the point of perception. Although analog system has been in use for decades but following defects in it are unavoidable.



*

Difficulty of maintaining throughout the broadcast chain a true analog of the original signal.
*

Distortions in phase and amplitude occur at all points in the system causing deterioration of the technical quality of the picture.
*

Excessive attention is required in designing circuits so that these distortions are limited to manageable proportions thus increasing the cost of the system.



In case of digital the signal in first converted into a sequence of 0s and 1s (called bits) which may be transmitted or stored. The receiver then reconverts or decodes the bits into a replica of the original signal.



The main advantage of digital representation of information is the robustness of the bit stream. It can be stored and recovered, transmitted and received, processed and manipulated, all virtually without error. The only requirement is that a zero bit be distinguishable from a one bit, a task that has become quite easy since the invention of integrated circuit and high speed computational technology.



Digital revolution has brought a range of benefits but there are two main reasons for its unstoppable adoption.



*

The possibility to replicate in a more economic and compact way the different system components that technologies specific of a field had made possible.
*

The net saving in bandwidth usage compared to the corresponding analogue solution.



Some examples of digital revolutions offered by music record, speech telephony and satellite broadcasting are as follows :



*

The vinyl disc, phonograph cylinder etc. has existed for over 100 years but now CD has entered in a big way.
*

Analogue speech has existed for over 100 years but now PCM is used in the network by billions of people unbeknownst to them.
*

Analogue satellite television has been in operation for 2 decades but now digital television is being watched by millions of people.



From the above it may be concluded that inexpensive integrated circuits, high speed communication networks, rapid access, dense storage media and computing architectures that can easily handle video-rate data are now rapidly making the analog standard obsolete.



Digital audio and video signals integrated with computers, telecommunication networks, and consumer products have fueled the digital revolution. At the heart of this revolution is the digital compression of audio and video signal. This digital revolution is probably not possible without the support of compression. The compression for digital revolution is like the heart for human body. As proper functioning of heart is essential for human body to survive, similarly proper compression is necessary to make digital revolution a complete success.



Digital Revolution without compression - Is it feasible ?



The first step in digital revolution is digitization " which means sampling plus quantization. Let us examine the status of video signal after simple digitization. Video signal is converted into the digital format in accordance with the international standard CCIR Rec. 601. The three source signals, namely the luminance signal Y and the two colour difference signals (chrominance) Cr and Cb are all converted to digital format using 8 bit linear PCM coding and three separate analog to digital converters (ADC). The Y signal is sampled at the rate of 13.5 million samples per second whereas Cr and Cb are sampled at 6.75 million samples/second. The output from the three ADCs are multiplexed together to form a single stream having a bit rate of 216 million bits per second.



High rate bit stream can be transmitted/stored by a modulation scheme called QPSK (Quadrature Phase Shift Keying). From the incoming bit stream two bits are taken at a time to constitute, what is known as a symbol. In QPSK, the digital modulation is carried out by varying the phase of the RF carrier in accordance with the value of an incoming symbol. As QPSK symbol is composed of two bits it can have four possible values (00, 01, 10, 11). Accordingly the transmitted RF carrier takes one of four phase values (45o, 135o, 225o, 315o). The RF double sided bandwidth required for a digitally modulated signal is equal to the input symbol rate. Continuing with our above example, the 216 mbps stream gives a symbol rate of 108 million symbols per second which will require a 108 MHz RF channel for transmission.DVCPRO



But for analog TV transmission present bandwidth allocations are only 7 MHz (VHF) and 8 MHz (UHF). Therefore simple digitization of video signal will result in enormous bandwidth expansion. In other words it may be said that digital revolution with simple digitization is just not feasible.



So how digital revolution can be achieved ?



Audio and video signals have large amount of redundancy. It means that after simple digitization audio and video signal will have very large quantity of bits which may be removed without sacrificing the quality. This is achieved by a process known as "Compression". Following paragraphs will explain this wonderful process.





2. The orange juice analogy - A simple way of understanDVCPROding Video Compression Concept



The concept of complete video compression process can be easily understood with the help of an orange juice analogy described below :



Assume that you are owner of a very big orange orchard. The price of orange in your area has crashed due to excellent crop of oranges. You may incur a huge loss if you sell oranges at your place. To make profit you decide to convert oranges into orange pulp and export to other countries. The figure 1 describes the complete process up to the consumption of orange juice by consumer. This process is very much similar to the video signal compression process where oranges are analog to video signal. The comparison is described below :

DVCPRO



1.

Psycho-visual redundancy : In this type of redundancy the information that appears during very small duration that our eye cannot perceive is not transmitted. In other words limitations in human visual system and human audio system are exploited.







1.

information that appears during very small duration that our eye cannot perceive is not transmitted. In other words limitations in human visual system and human audio system are exploited.







Fig. 1 Orange Juice Analogy of Video Compression












ORANGES





VIDEO SIGNAL

1.


Washing of oranges to remove dust etc. otherwise it may spoil the orange juice.


1.


Filtering of video signal to remove noise etc. otherwise it may corrupt the video signal.

2.


Removing peel, skin, seed from oranges


2.


Removing unwanted information from the video signal.

3.


By crushing process converting orange into orange juice


3.


By digitization process converting analog video into digital video.

4.


Water content in the orange juice is redundant. An expensive plant is used to remove the redundant water content from the orange juice without destroying its quality


4.


A significant amount of information content in the video signal is redundant. An expensive encoder is used to remove the various types of redundant information from the digitized video signal without destroying its quality.

5.


The output of the plant is pulp of orange juice whose physical quantity is much less than the orange juice quantity which is fed to the plant.


5.


The output of the encoder is compressed digital video whose number of bits are much less than the bits of digitized video fed to the encoder.

6.


The orange pulp is packed in small tins. Necessary information such as name of company, details of ingredients, weight, directions for handling and converting pulp into almost like fresh orange juice.


6.


DVCPROThe bits of compressed digital video are converted into small packets. Header of each packet contains necessary information such as name, number of bits, and direction for decoding to convert it into almost like original video signal.

7.


These small sealed tins are packed into big cartons for storage/ transportation. These tins in the carton are transported by some carriers.


7.


Packets of bits are multiplexed to form group of pictures (GOP). These are now ready for storage in hard disc or transmission by modulating on carrier.

8.


Consumer follows the instructions written on each tin to prepare almost like fresh orange juice from the pulp. For this he is required to add correct quantity of water and ice etc.


8.


The decoder on receiving the packets converts the compressed bits into almost like original video signal as per the information given in the header.

9.


The process of making orange juice from the pulp is very simple as compared to the process of converting orange juice into pulp.


9.


The decoding process of converting compressed bits into video signal is much simpler than encoding process. TheDVCPROrefore decoders are cheaper and less complicated than encoder.



10.


Manufacturer do not specify how exactly pulp is prepared. It is their trade secret. They only specify its ingredients and instruction about how to prepare orange juice from the pulp.


10.


Encoder is like a black box. The manufacturer do not specify, how coding is done. It is their trade secret. only bit stream and decoding format is described.

11.


The orange pulp must be prepared as per the standard specifications like ISI etc.


11.


The compression process must follow the international standards like JPEG, MPEG, H261 etc.

DVCPRODVCPRO

Overview of the complete video compression process has been explained above with the help of the orange juice analogy. Now let us try to analyse the compression in more technical manner.





All sensory signals including audio and video signals are analog at the point of origination or at the point of perception. Although analog system has been in use for decades but following defects in it are unavoidable.



*

Difficulty of maintaining throughout the broadcast chain a true analog of the original signal.
*

Distortions in phase and amplitude occur at all points in the system causing deterioration of the technical quality of the picture.
*

Excessive attention is required in designing circuits so that these distortions are limited to manageable proportions thus increasing the cost of the system.



In case of digital the signal in first converted into a sequence of 0s and 1s (called bits) which may be transmitted or stored. The receiver then reconverts or decodes the bits into a replica of the original signal.



The main advantage of digital representation of information is the robustness of the bit stream. It can be stored and recovered, transmitted and received, processed and manipulated, all virtually without error. The only requirement is that a zero bit be distinguishable from a one bit, a task that has become quite easy since the invention of integrated circuit and high speed computational technology.



Digital revolution has brought a range of benefits but there are two mDVCPROain reasons for its unstoppable adoption.



*

The possibility to replicate in a more economic and compact way the different system components that technologies specific of a field had made possible.
*

The net saving in bandwidth usage compared to the corresponding analogue solution.



Some examples of digital revolutions offered by music record, speech telephony and satellite broadcasting are as follows :



*

The vinyl disc, phonograph cylinder etc. has existed for over 100 years but now CD has entered in a big way.
*

Analogue speech has existed for over 100 years but now PCM is used in the network by billions of people unbeknownst to them.
*

Analogue satellite television has been in operation for 2 decades but now digital television is being watched by millions of people.



From the above it may be concluded that inexpensive integrated circuits, high speed communication networks, rapid access, dense storage media and computing architectures that can easily handle video-rate data are now rapidly making the analog standard obsolete.



Digital audio and video signals integrated with computers, telecommunication networks, and consumer products have fueled the digital revolution. At the heart of this revolution is the digital compression of audio and video signal. This digital revolution is probably not possible without the support of compression. The compression for digital revolution is like the heart for human body. As proper functioning of heart is essential for human body to survive, similarly proper compression is necessary to make digital revolution a complete success.



Digital Revolution without compression - Is it feasible ?



The first step in digital revolution is digitization " which means sampling plus quantization. Let us examine the status of video signal after simple digitization. Video signal is converted into the digital format in accordance with the international standard CCIR Rec. 601. The three source signals, namely the luminance signal Y and the two colour difference signals (chrominance) Cr and Cb are all converted to digital format using 8 bit linear PCM coding and three separate analog to digital converters (ADC). The Y signal is sampled at the rate of 13.5 million samples per second whereas Cr and Cb are sampled at 6.75 million samples/second. The output from the three ADCs are multiplexed together to form a single stream having a bit rate of 216 million bits per second.