DIGITAL CCTV A Security Professional’s Guide phần 3 docx

In the Beginning 51 The cones are made up of three types: one is sensitive to red- orange light, the second to green light, and the third to blue-violet light. When a single cone is stimulated, the brain perceives the corresponding color. If our green cones are stimulated, we see green; if our red-orange cones are stimulated, we see red. If both our green and red-orange cones are simultaneously stimulated, we see yellow. The human eye cannot tell the difference between spectral yellow and some combination of red and green. Because of this physiological response, the eye can be fooled into seeing the full range of visible colors through a proportionate adjustment of just three colors: red, green, and blue. Colors are represented by bits and the more bits that are available, the more precise the color defi nition is portrayed. Digital video uses a non-linear variation of RGB called YCbCr. Cb represents luminance and Cr represents chrominance. Subtractive Color Subtractive color is the basis for printing. It is called subtractive because white, its base color, refl ects all spectral wavelengths and any color added to white absorbs or “subtracts” different wave- lengths. The longer wavelengths of the visible spectrum, which are normally perceived as red, are absorbed by cyan. Magenta Figure 3-7 Primary Colors in Subtractive Color System 52 Digital CCTV absorbs the middle wavelengths (green), and yellow absorbs the shorter wavelengths of the visible spectrum (blue-violet). Mixing cyan, magenta, and yellow together “subtracts” all wavelengths of visible light and, as a result, we see black. Printing inks comprised of the colors cyan, magenta, and yellow combine to absorb some of the colors from white light and refl ect others. Figure 3-7 illustrates how cyan, magenta, and yellow, when printed as three overlapping circles, work to produce black as well as red, green, and blue. In practice, the black produced by combining cyan, magenta, and yellow is often not black enough to provide a large contrast range, so printers often add black ink (K) to the mix, resulting in the four color printing process some- times known as CMYK. Cyan, magenta, and yellow are the primary colors in the subtractive color system. Red, green, and blue are the primary colors in the additive color system. Additive Mixing Video systems deal with subtractive color when a camera captures the light refl ected from objects the same way as our eyes. But, when a video system needs to display a color image, it has to deal with a whole new way of working with color. Images that are sources of light, such as the television screen or monitor, produce color images by a process known as additive mixing. To create a color, the wavelengths of the colors are added to each other. Before any colors have been added, there is only black, which is the absence of light. On the fl ip side, adding all three additive primary colors in equal amounts creates white. All other colors are pro- duced by mixing the three primary wavelengths of light in differ- ent combinations. When the three primary colors of light are mixed, intensities of the colored light are being added. This can be seen where the primary color illumination overlaps. Yellow is formed when red light added to green light is equal to the illumi- nation of the red and green combined. In a video signal, the color white is comprised of 30% Red, 59% Green, and 11% Blue. Since green is dominant, it is used for In the Beginning 53 the luminance or black-and-white information in the picture. Note that the common symbol for luminance is the letter Y. The lumi- nance equation is usually expressed to only 2 decimal places as Y = 0.3R + 0.59G + 0.11B. The letter R is of course representing red, B representing blue, and G representing green. Instead of sending luminance (Y) and three full color signals red, green, and blue, color difference signals are made to conserve analog bandwidth. The value of green (also Y) is subtracted from the value of Red (R-Y). The value of green is also subtracted from the value of blue (B-Y). The result is a color video signal comprised of luminance Y and two color difference signals, R-Y and B-Y. Since Y (the luminance signal) is sent whole, it can be recombined with the color difference signals R-Y and B-Y to get the original red and blue signals back for display. PICTURE QUALITY One of the most important things to a security professional is picture quality. The effort and expense of capturing video images will be of little value if, when viewed, the image is unrecognizable. The fact is that the science of removing redundant information to reduce the amount of bits that need to be transferred would not even be necessary if we lived in a world of unlimited bandwidth. For the present, at least, this is not the case. So we must learn how to use bandwidth to its fullest advantage. Choices for high quality or high image rate result in high bandwidth requirements. Choices for lower bandwidth result in reduced image quality or reduced update rate or both. You can trade off image rate for quality within the same bandwidth. THE BANDWIDTH WAGON The term bandwidth is used for both analog and digital systems and means similar things but is used in very different ways. In a digital system, bandwidth is used as an alternative term to bit rate, 54 Digital CCTV which is the number of bits per second, usually displayed as kilo- bits per second. Technically, bandwidth is the amount of electro- magnetic spectrum allocated to a telecommunications transmitter to send out information. Obviously, the larger the bandwidth, the more information a transmitter can send out. Consequently, band- width is what determines the speed and, in some cases, the clarity of the information transferred. Bandwidth is restricted by the laws of physics regardless of the media utilized. For example, there are bandwidth limitations due to the physical properties of the twisted-pair phone wires that service many homes. The band- width of the electromagnetic spectrum also has limits because there are only so many frequencies in the radio wave, microwave, and infrared spectrum. In order to make a wise decision about the Figure 3-8 Traffi c. Courtesy of WRI Features. In the Beginning 55 path we choose, we need to know how much information can move along the path and at what speeds. Available bandwidth is what determines how fast our compressed information can be transferred from one location to another. Visualize digital video as water and bandwidth as a garden hose. The more water you want to fl ow through the hose, the bigger around the hose must be. Another example can be found in comparing driving home in rush hour traffi c with transmitting video signals. If there are 500 cars, all proceeding to the same destination, how can they make the trip more expediently? A larger highway would be the obvious answer. See Figure 3-8. If those 500 cars were traveling over four lanes as opposed to two lanes, they could move with greater speed and accuracy. Now imagine those 500 cars on an eight-lane highway. The four and eight lane highways simply represent larger bandwidths. Conversely, if there is very little traffi c, a two-lane highway will be adequate. The same is true for transmitting digital data. The bandwidth requirements are dictated by the amount of data to be transferred. This page intentionally left blank Compression—The Simple Version 4 It can be diffi cult to get a simple answer to questions concerning the compression of video, especially when faced with making purchasing decisions. Manufacturers of video compression systems can choose from a variety of compression techniques, including proprietary technologies, and they each feel that certain attributes are more important than others. It can be easy to feel overloaded with information but at the same time feel like you are not getting any answers. The next few chapters will attempt to explain some of the common video compression idiosyncrasies so that better decisions can be made. 57 Compression 1. An increase in the density of something. 2. The process or result of becoming smaller or pressed together. 3. Encoding information while reducing the bandwidth or bits required. Merriam-Webster Online Dictionary 58 Digital CCTV Image compression is the same as data compression—the process of encoding information using fewer bits. Various software and hardware techniques are available to condense information by removing unnecessary data or what are commonly called redun- dancies. This reduction of information in turn reduces the trans- mission bandwidth requirements and storage requirements for audio, image, and full-motion video signals. The art or science of compression only works when both the sender and receiver of the information use the same encoding scheme. The roots of compression lie with the work of the mathemati- cian Claude Shannon, whose primary work was in the context of communication engineering. Claude Elwood Shannon is known as the founding father of the electronic communications age. Shannon investigated the mathematics of how information is sent from one location to another as well as how information is altered from one format to another. Working for Bell Telephone Labora- tories on transmitting information, he uncovered the similarity between Boolean algebra and telephone switching circuits. He theorized that the fundamental unit of information is a yes-no situ- ation in which something is or is not. Using Boolean two-value binary algebra as a code, one means “on” when the switch is closed and the power is on, and zero means “off” when the switch is open and power is off. One of the most important features of Shannon’s theory was the concept of entropy. The basic concept of entropy in informa- tion theory has to do with how much randomness is in a single or in a random event. He is also credited with the introduction of the Sampling Theory, which is concerned with representing a continuous-time signal from a (uniform) discrete set of samples. These concepts are deeply rooted in the mechanics of digital compression. COMPRESSION IN THE 1800’s The compression of data is an idea that is not necessarily new. A compression algorithm is the mathematical process for converting data into smaller packages. An early example of a compression Compression—The Simple Version 59 method is the communication system developed by Samuel Morse, known as Morse code. In 1836, Samuel Morse demonstrated the ability of a telegraph system to transmit information over wires. The idea was to use short code words for the most commonly occurring letters and longer code words for less frequent letters. This is what is known as a variable length code. Using a variable length code, information was compressed into a series of electrical signals and transmitted to remote locations. Morse code is a system of sending messages that uses short and long sounds combined in various ways to represent letters, numbers and other characters such as punctuation marks. A short sound is called a dit; a long sound, a dah. Written code uses dots and dashes to represent dits and dahs. “Morse code” World Book Online Reference Center. 2004. World Book, Inc. In the past, telegraph companies used American Morse Code to transmit telegrams by wire. An operator tapped out a message on a telegraph key, a switch that opened and closed an electric circuit. A receiving device at the other end of the circuit made clicking sounds and wrote dots and dashes on a paper tape. See Table 4-1. Today, the telegraph and American Morse Code are rarely used. Compression techniques have played an important role in the evolution of telecommunication and multimedia systems from their beginnings. As mentioned in Chapter 3, pioneers of slow scan transmission of video signals have roots in the 1950s and 60s. In the 1970s, interest in video conferencing as a business tool peaked, resulting in a stimulation of research that improved picture quality and digital coding. Early 1980s compression based on Differential Pulse Code Modulation (DPCM) was standardized under the H.120 standard. During the late 1980s, the Joint Photographic Experts Group became interested in compression of static images and they chose Discrete Cosine Transfer (DCT) as the basic unit of compression, mainly due to the possibility of progressive image transmission. 60 Digital CCTV This codec showed great improvement over H.120. The standard defi nition was completed in late 1989 and is offi cially called the H.261 standard. Compression, or the process of reducing the size of data for transmission or storage, is typically achieved by the use of encoding techniques such as these just mentioned because video sequences contain a signifi cant amount of statistical and subjective redundancy (recurring information) within frames. The ultimate goal of video compression is to reduce this information for storage and transmission by examining and discarding these redundan- cies and encoding a minimum amount of information. The perfor- mance of a video compression technique is signifi cantly infl uenced by the amount of redundancy in the image as well as on the actual compression method used for coding. CODECS One second of uncompressed NTSC video requires approximately 27 MB of disk space and must defi nitely be compressed in order to store effi ciently. Playing the video would then require decompres- sion. Codecs were devised to handle the compression of video for storage and transmission and the decompression when it is played. Table 4-1 Morse Code A N 1 . B . . O 2 . , C P 3 . . ? . . D . Q 4 . . . ( E . R 5 . . . . . ) F . S . . . 6 . . . - . . G T - 7 . . ” H . . . . U . 8 . _ . I . . V . . 9 ’ J W 0 : . . K X / ; L . Y + $ . . M Z . = . [...]... standards, including the American National Standards Institute, the International Organization for Standardization, and the International Electrotechnical Commission The definition of a standard by the American National Standards Institute (ANSI) is a set of characteristics or qualities that describes the features of a product, process or service.” ANSI is a private, non-profit organization that administers... head organization for many national standardization bodies, including: ● ● ● ● ● ● ● ● ● DIN—Deutsches Institut fuer Normung, Germany BSI—British Standards Institution, United Kingdom AFNOR—Association francaise de normalisation, France UNI—Ente Nazionale Italiano di Unificatione, Italy NNI—Nederlands Normalisatie-instituut, Netherlands SAI—Standards Australia International SANZ—Standards Association... (2) fractals, (3) discrete cosine transform (DCT), and (4) wavelets Each of these four compression techniques has advantages and disadvantages 62 Digital CCTV 1 Vector quantization is a lossy compression that looks at an array of data and generalizes what it sees Redundant data is compressed, preserving enough information to recreate the original intent 2 Fractal compression, also a lossy compression,... itself In mathematics and computer science, algorithm usually refers to a process that solves a recurrent problem An algorithm will have a well-defined set of rules and a specific stopping point A computer program can, in fact, be viewed as an elaborate algorithm The word algorithm comes from the name of the mathematician Mohammed ibn-Musa al-Khwarizmi, who was part of the royal court in Baghdad and lived... testing, and review of digital multimedia standards Having these MPEG standards as international standards insures that video encoding systems will produce files that can be opened and played with any standards-compliant decoder The major advantage of MPEG compared to other video and audio coding formats is that MPEG files are much smaller for the same quality Moving objects tend to be predictable and in... similarities within a video frame Intraframe compression exploits the redundancy within the image, known as spatial redundancy Intraframe compression techniques can be applied to individual frames of a video sequence For example, a large area of blue sky generally does not change much from pixel to pixel The same number of bits is not necessary for such an area as for an area with large amounts of detail,... international standards and assessment body for the fields of electrotechnology Established in 1988, the IEC prepares and publishes international standards for all electrical, electronic, and related technologies These serve as a basis for national standardization and as references when drafting international tenders and contracts DO YOU HAVE ALGORITHM? Before we look at individual compression standards,... similarities within sections of an image and uses a fractal algorithm to generate the sections Fractals and vector quantization require significant computing resources for compression but are quick at decompression 3 DCT samples an image, analyzes the frequency components, and discards those that do not affect the image Like DCT, discrete wavelet transform (DWT) mathematically transforms an image into... is a scene change, the new frame is tagged as the key frame and becomes the comparison image for the next frames The comparison continues until another change occurs and the cycle begins again The file size increases with every addition of a new key frame This means that the fewer changes in the camera view, the smaller the data to be stored or transferred 70 Digital CCTV There are several temporal... than the original for transmission and storage The data must be able to be returned to a good approximation of its original state There are many popular general-purpose lossless compression techniques that can be applied to any type of data We will examine a few here Please do not expect to fully understand the intricacies of these techniques from the very brief explanations and examples; rather take . techniques has advantages and disadvantages. 62 Digital CCTV 1. Vector quantization is a lossy compression that looks at an array of data and generalizes what it sees. Redundant data is compressed,. speeds. Available bandwidth is what determines how fast our compressed information can be transferred from one location to another. Visualize digital video as water and bandwidth as a garden. for both analog and digital systems and means similar things but is used in very different ways. In a digital system, bandwidth is used as an alternative term to bit rate, 54 Digital CCTV which