《多媒体技术原理-视频中的基本概念课件.ppt》由会员分享，可在线阅读，更多相关《多媒体技术原理-视频中的基本概念课件.ppt（53页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、Chapter 5Fundamental Concepts in Video5.1 Types of Video Signals5.2 Analog Video5.3 Digital Video5.4 Further Exploration5.1 Types of Video SignalsComponent video（分量视频）Composite video（复合视频）S-Video（超视频）5.1 Types of Video Signals Component video Higher-end video systems make use of three separate video
2、 signals for the red,green,and blue image planes Each color channel is sent as a separate video signal.Most computer systems use Component Video,with separate signals for R,G,and B signals.For any color separation scheme,Component Video gives the best color reproduction since there is no“crosstalk”b
3、etween the three channels.This is not the case for S-Video or Composite Video,discussed next.Component video,however,requires more bandwidth and good synchronization of the three components.5.1 Types of Video Signals Component videoComposite Video One Signal Composite video color(“chrominance”)and i
4、ntensity(“luminance”)signals are mixed into a single carrier wave.Chrominance is a composition of two color components(I and Q,or U and V).In NTSC TV,e.g.,I and Q are combined into a chroma signal,and a color subcarrier is then employed to put the chroma signal at the high-frequency end of the signa
5、l shared with the luminance signal.The chrominance and luminance components can be separated at the receiver end and then the two color components can be further recovered.Composite Video One Signal Composite video When connecting to TVs or VCRs,Composite Video uses only one wire and video color sig
6、nals are mixed,not sent separately.The audio and sync signals are additions to this one signal.Since color and intensity are wrapped into the same signal,some interference between the luminance and chrominance signals is inevitable.Composite Video One Signal Composite videoS-Video Two Signals S-Vide
7、o as a compromise,(separated video,or Super-video,e.g.,in S-VHS)uses two wires,one for luminance and another for a composite chrominance signal.As a result,there is less crosstalk between the color information and the crucial gray-scale information.S-Video Two Signals S-Video The reason for placing
8、luminance into its own part of the signal is that black-and-white information is most crucial for visual perception.In fact,humans are able to differentiate spatial resolution in grayscale images with a much higher acuity than for the color part of color images.As a result,we can send less accurate
9、color information than must be sent for intensity information we can only see fairly large blobs of color,so it makes sense to send less color detail.S-Video Two Signals S-Video small round connector with two separate video signals,one carrying brightness(luminance),the other carrying color(chroma).
10、Also referred to as Y/C video电视扫描和同步 扫描分类 非隔行扫描（逐行扫描）(progressive scanning)计算机显示器 隔行扫描(interlaced scanning)电视5.2 Analog Video In TV,and in some monitors and multimedia standards as well,another system,called“interlaced”scanning is used:The odd-numbered lines are traced first,and then the even-number
11、ed lines are traced.This results in“odd”and“even”fields two fields make up one frame.In fact,the odd lines(starting from 1)end up at the middle of a line at the end of the odd field,and the even scan starts at a half-way point.5.2 Analog Video Figure 5.1 shows the scheme used.First the solid(odd)lin
12、es are traced,P to Q,then R to S,etc.,ending at T;then the even field starts at U and ends at V.The jump from Q to R,etc.in Figure 5.1 is called the horizontal retrace,during which the electronic beam in the CRT is blanked.The jump from T to U or V to P is called the vertical retrace.Fig.5.1:Interla
13、ced raster scan5.2 Analog Video5.2 Analog Video Because of interlacing,the odd and even lines are displaced in time from each other generally not noticeable except when very fast action is taking place on screen,when blurring may occur.For example,in the video in Fig.5.2,the moving helicopter is blu
14、rred more than is the still background.5.2 Analog VideoFig.5.2:Interlaced scan produces two fields for each frame.(a)The video frame,(b)Field 1,(c)Field 2,(d)Difference of Fields(a)(b)(c)(d)5.2 Analog Video Since it is sometimes necessary to change the frame rate,resize,or even produce stills from a
15、n interlaced source video,various schemes are used to“de-interlace”it.The simplest de-interlacing method consists of discarding one field and duplicating the scan lines of the other field.The information in one field is lost completely using this simple technique.Other more complicated methods that
16、retain information from both fields are also possible.TV Standards NTSC美国、加拿大等大部分西半球国家，以及日本、韩国、菲律宾 PAL 德国、英国等一些西欧国家，及中国、朝鲜等国家 SECAM法国、及东欧国家World TV StandardsNTSC NTSCP AL P ALSECAM SECAMP AL/SECAM P AL/SECAMUnknown UnknownNTSC Video NTSC(National Television System Committee)TV standard uses the fami
17、liar 4:3 aspect ratio(i.e.,the ratio of picture width to its height)and uses 525 scan lines per frame at 30 frames per second(fps).It is mostly used in North America and Japan.NTSC Video Features of NTSC NTSC follows the interlaced scanning system,and each frame is divided into two fields,with 262.5
18、 lines/field.Thus the horizontal sweep frequency is 52529.97 15,734 lines/sec,so that each line is swept out in 1/15.734 103 sec 63.6sec.Since the horizontal retrace takes 10.9 sec,this leaves 52.7 sec for the active line signal during which image data is displayed(see Fig.5.3).NTSC Video Fig.5.4 sh
19、ows the effect of“vertical retrace&sync”and“horizontal retrace&sync”on the NTSC video raster.Fig.5.4:Video raster,including retrace and sync dataNTSC Video Vertical retrace takes place during 20 lines reserved for control information at the beginning of each field.Hence,the number of active video li
20、nes per frame is only 485.Similarly,almost 1/6 of the raster at the left side is blanked for horizontal retrace and sync.The non-blanking pixels are called active pixels.Since the horizontal retrace takes 10.9 sec,this leaves 52.7 sec for the active line signal during which image data is displayed(s
21、ee Fig.5.3).It is known that pixels often fall in-between the scan lines.Therefore,even with non-interlaced scan,NTSC TV is only capable of showing about 340(visually distinct)lines,i.e.,about 70%of the 485 specified active lines.With interlaced scan,this could be as low as 50%.NTSC Video NTSC video
22、 is an analog signal with no fixed horizontal resolution.Therefore one must decide how many times to sample the signal for display:each sample corresponds to one pixel output.A“pixel clock”is used to divide each horizontal line of video into samples.The higher the frequency of the pixel clock,the mo
23、re samples per line there are.Different video formats provide different numbers of samples per line,as listed in the Table 5.1.Format Samples per lineVHS 240S-VHS 400-425Betamax 500Standard 8 m 300Hi-8 mm 425Table 5.1:Samples per line for various video formatsColor Model and Modulation of NTSC NTSC
24、uses the YIQ color model,and the technique of quadrature modulation is employed to combine(the spectrally overlapped part of)I(in-phase)and Q(quadrature)signals into a single chroma signal C:C=I cos(Fsct)+Qsin(Fsct)(5.1)This modulated chroma signal is also known as the color subcarrier,whose magnitu
25、de is,and phase is tan1(Q/I).The frequency of C is Fsc 3.58 MHz.The NTSC composite signal is a further composition of the luminance signal Y and the chroma signal as defined below:composite=Y+C=Y+Icos(Fsct)+Qsin(Fsct)(5.2)Color Model and Modulation of NTSC Fig.5.5:NTSC assigns a bandwidth of 4.2 MHz
26、 to Y,and only 1.6 MHz to I and 0.6 MHz to Q due to human insensitivity to color details(high frequency color changes).Fig.5.5:Interleaving Y and C signals in the NTSC spectrum.Decoding NTSC Signals The first step in decoding the composite signal at the receiver side is the separation of Y and C.Aft
27、er the separation of Y using a low-pass filter,the chroma signal C can be demodulated to extract the components I and Q separately.To extract I:Multiply the signal C by 2 cos(Fsct),i.e.,Decoding NTSC Signals Apply a low-pass filter to obtain I and discard the two higher frequency(2Fsc)terms.Similarl
28、y,Q can be extracted by first multiplying C by 2sin(Fsct)and then low-pass filtering.Decoding NTSC Signals The NTSC bandwidth of 6 MHz is tight.Its audio subcarrier frequency is 4.5 MHz.The Picture carrier is at 1.25 MHz which places the center of the audio band at 1.25+4.5=5.75 MHz in the channel(F
29、ig.5.5).But notice that the color is placed at 1.25+3.58=4.83 MHz.So the audio is a bit too close to the color subcarrier it was a cause for potential interference between the audio and color signals.It was largely due to this reason that the NTSC color TV actually slowed down its frame rate to 301,
30、000/1,001 29.97 fps.As a result,the adopted NTSC color subcarrier frequency is slightly lowered tofsc=30 1,000/1,001 525 227.5 3.579545 MHz,where 227.5 is the number of color samples per scan line in NTSC broadcast TV.PAL Video PAL(Phase Alternating Line)uses 625 scan lines per frame,at 25 frames/se
31、cond,with a 4:3 aspect ratio and interlaced fields.PAL uses the YUV color model.It uses an 8 MHz channel and allocates a bandwidth of 5.5 MHz to Y,and 1.8 MHz each to U and V.The color subcarrier frequency is fsc 4.43 MHz.In order to improve picture quality,chroma signals have alternate signs(e.g.,+
32、U and-U)in successive scan lines,hence the name“Phase Alternating Line”.This facilitates the use of a(line rate)comb filter at the receiver the signals in consecutive lines are averaged so as to cancel the chroma signals(that always carry opposite signs)for separating Y and C and obtaining high qual
33、ity Y signals.PAL is widely used in Western Europe,China,India,and many other parts of the world.SECAM Video SECAM stands for Systme Electronique Couleur Avec Mmoire the third major broadcast TV standard.SECAM also uses 625 scan lines per frame,at 25 frames per second,with a 4:3 aspect ratio and int
34、erlaced fields.SECAM and PAL are very similar.They differ slightly in their color coding scheme:In SECAM,U and V signals are modulated using separate color subcarriers at 4.25 MHz and 4.41 MHz respectively.They are sent in alternate lines,i.e.,only one of the U or V signals will be sent on each scan
35、 line.Comparison of the major TV StandardsTable 5.2:Comparison of Analog Broadcast TV SystemsTV SystemFrame Rate(fps)No.of Scan LinesTotal Channel Width(MHz)Bandwidth Allocation(MHz)Y I or U Q or VNTSC 29.97 525 6.0 4.2 1.6 0.6PAL 25 625 8.0 5.5 1.8 1.8SECAM 25 625 8.0 6.0 2.0 2.05.3 Digital Video T
36、he advantages of digital representation for video Video can be stored on digital devices or in memory,ready to be processed(noise removal,cut and paste,etc.),and integrated to various multimedia applications;Direct access is possible,which makes nonlinear video editing achievable as a simple,rather
37、than a complex,task;Repeated recording does not degrade image quality;Ease of encryption and better tolerance to channel noise.Chroma Subsampling Since humans see color with much less spatial resolution than they see black and white,it makes sense to“decimate”the chrominance signal.Interesting(but n
38、ot necessarily informative!)names have arisen to label the different schemes used.Chroma SubsamplingSubsamplingSubsampling 对亮度信号和色差信号采用相同的采样频率进行采样 对亮度信号和色差信号采用不同的采样频率进行采样 图像子采样的概念 对色差信号使用的采样频率比对亮度信号使用的采样频率低的采样方法Chroma SubsamplingSubsamplingSubsampling 在数字图像压缩技术中得到广泛应用 最简便的图像压缩技术恐怕就要算图像子采样。基本依据是人的视觉系
39、统所具有的两个特性 人眼对色度信号的敏感程度比对亮度信号的敏感程度低，利用这个特性可把颜色信号去掉一些而使人不易察觉 人眼对图像细节的分辨能力有一定的限度，利用这个特性可把图像中的高频信号去掉而使人不易察觉 Chroma SubsamplingSubsamplingSubsampling 4:4:4 这种采样格式不是子采样格式，它是指在每条扫描线上每4 个连续的采样点取4 个亮度Y 样本、4 个红色差Cr 样本和4 个蓝色差Cb 样本，每个像素用3 个样本表示 4:2:2 在每条扫描线上，每4 个连续的采样点取4 个亮度Y样本、2 个红色差Cr 样本和2 个蓝色差Cb 样本，平均每个像素用2
40、个样本表示 4:1:1 在每条扫描线上，每4 个连续的采样点取4 个亮度Y样本、1 个红色差Cr 样本和1 个蓝色差Cb 样本，平均每个像素用1.5 个样本表示 4:2:0 在水平和垂直方向上，每2 个连续采样点上取2 个亮度Y 样本、1 个红色差Cr 样本和1 个蓝色差Cb 样本，每个像素用1.5 个样本表示Chroma Subsampling To begin with,numbers are given stating how many pixel values,per four original pixels,are actually sent:The chroma subsampli
41、ng scheme“4:4:4”indicates that no chroma subsampling is usedeach pixels Y,Cb and Cr values are transmitted,4 for each of Y,Cb,Cr.The scheme“4:2:2”indicates horizontal subsampling of the Cb,Cr signals by a factor of 2.of four pixels horizontally labelled as 0 to 3,all four Ys are sent,and every two C
42、bs and two Crs are sent,as(Cb0,Y0)(Cr0,Y1)(Cb2,Y2)(Cr2,Y3)(Cb4,Y4),and so on(or averaging is used)Chroma Subsampling The scheme“4:1:1”subsamples horizontally by a factor of 4.The scheme“4:2:0”subsamples in both the horizontal and vertical dimensions by a factor of 2.Theoretically,an average chroma p
43、ixel is positioned between the rows and columns as shown Fig.5.6.Scheme 4:2:0 along with other schemes is commonly used in JPEG and MPEG(see later chapters in Part 2).Chroma SubsamplingFig.5.6:Chroma subsamplingCCIR Standards for Digital Video CCIR is the Consultative Committee for International Rad
44、io,and one of the most important standards it has produced is CCIR-601,for component digital video.This standard has since become standard ITU-R-601,an international standard for professional video applications adopted by certain digital video formats including the popular DV video.The CCIR 601 stan
45、dard uses an interlaced scan,so each field has only half as much vertical resolution(e.g.,240 lines in NTSC).Table 5.3 shows some of the digital video specifications,all with an aspect ratio of 4:3.CCIR Standards for Digital Video CIF stands for Common Intermediate Format specified by the CCITT.The
46、idea of CIF is to specify a format for lower bitrate.CIF is about the same as VHS quality.It uses a progressive(non-interlaced)scan.All the CIF/QCIF resolutions are evenly divisible by 8,and all except 88 are divisible by 16;QCIF stands for“Quarter-CIF”.it provides convenience for block-based video
47、coding in H.261 and H.263 which will be discussed later in Chapter 10.CCIR Standards for Digital Video Note,CIF is a compromise of NTSC and PAL in that it adopts the NTSC frame rate and half of the number of active lines as in PAL.Table 5.3:Digital video specificationsCCIR 601525/60NTSCCCIR 601625/5
48、0PAL/SECAMCIF QCIFLuminance resolution 720 x 480 720 x 576 352 x 288 176 x 144Chrominance resolution 360 x 480 360 x 576 176 x 144 88 x 72Color Subsampling 4:2:2 4:2:2 4:2:0 4:2:0Fields/sec 60 50 30 30Interlaced Yes Yes No NoHDTV(High Definition TV)The main thrust of HDTV(High Definition TV)is not t
49、o increase the“definition”in each unit area,but rather to increase the visual field especially in its width.The first generation of HDTV was based on an analog technology developed by Sony and NHK in Japan in the late 1970s.MUSE(MUltiple sub-Nyquist Sampling Encoding)was an improved NHK HDTV with hy
50、brid analog/digital technologies that was put in use in the 1990s.It has 1,125 scan lines,interlaced(60 fields per second),and 16:9 aspect ratio.Since uncompressed HDTV will easily demand more than 20 MHz bandwidth,which will not fit in the current 6 MHz or 8 MHz channels,various compression techniq