CVBS全电视信号的一些基本知识（转）2011-03-20 10:32在我们的电视天线信号线里就只有两跟线，中间有一根很粗的线，外围包着一层的线，这是为了防止外界信号的干扰。
在这两根线中一个是地线，一根是全电视信号线，外围的是地线。
做视频处理很难免要接触电视信号，了解全电视信号的原理。
当我们把电视的信号线接到示波器上看其波形时会发现其波形很乱，但总是有一些规律可循：每隔一段特别乱的波形之后有一个很小的低电平。
在这其中，中间那些特别乱的波形其实就是有效像素电平的高低信号，那些很小的电平信号就是一些同步信号。
我主要用的是PAL制式的CVBS信号。
1、关于像素时钟：大约在13.5MHz，由采样定理得出的采样信号为27MHz，像素时钟就是来同步像素有效信号的，每一个像素时钟来一个像素值；2、关于行同步信号：顾名思义就是同步行扫描的信号，每行来一次，低电平有效（对于正电视信号而言），每来一次行同步信号就意味着本行扫描结束，新的一行就要开始了；3、关于场同步信号：顾名思义就是同步场扫描的信号，每场来一次，低电平有效，每来一次就意味着本场扫描结束新的一场就要开始；4、关于场、帧的概念：从屏幕上头扫到下头叫做一场，但是并不等同于一帧，一帧图像是指能够组成完整画面的图像数据，在隔行扫描中一帧包括两场：奇场和偶场；5、关于CVBS波形电平的解析：（假设为正电视信号）设最低电平为0,最高电平为1,在两者之间有一合理的分界值x，认为x到1之间的为像素值，将这个区间划分为256份（假设精度为8位），每一个值对应一个灰度值，其中x代表黑色，1代表白色，中间为各级灰度。
(一个电平就可以表示一个256之内的数字，模拟电平)x以下的电平不是有效地像素值也可以说是黑色，那些同步信号就融合在其中，包括行同步信号和场同步信号，场同步信号比行同步信号要宽很多，具体的都有自己的时间长短定义，这样才能保持发送和接收段信号的一致性，才能够恢复原来的图像；6、关于奇偶场的概念；就是一帧分两场扫描，先扫描奇场再扫描偶场，两场组成一帧。
7、关于场消隐和行消隐：跟在场同步和行同步之后，当一行扫到屏幕的最右头或者一场扫描到屏幕的最底端时，必须返回进行下一行或者下一场的扫描，但是又不能让人眼看出来，因此就诞生的场消隐和行消隐信号，在此期间回扫器件，虽然也是在扫描但是看不出来就像隐藏的一样。
模拟视频信号解析与数字视频信号的对应关系1：模拟视频信号的波形解析(Z)2011-05-11 15:51:49| 分类：视频技术|字号订阅Original URL:/User/yieress/Article/43197_1.htm1. 主要内容1. 模拟视频信号的波形解析主要包括composite、component、VGA等模拟视频信号的波形解析；2. 数字视频的数据格式主要包括BT601、BT656和BT1120等并行数据格式的描述和说明；3. 模拟视频信号与数字视频信号的对应关系着重说明视频信号ADC前后模拟视频信号与数字视频信号的对应关系。
2. 模拟视频信号的波形解析2.1 Composite Signal 复合视频信号2.1.1 复合视频信号的概念在一个信号中包含了亮度信号、色度信号与同步信号（包括场同步、行同步信号及行场消隐信号），称为复合视频信号。
又称为CVBS，表示Color，Video，Blanking，Sync，或者composite video baseband signal。
复合视频信号把亮度、色度与同步信号复合在一个信号通道上传输，也就是在传输前需要把色度信号与亮度信号“合成”在一个信号里，在传输后再将色度信号与亮度信号“分离”开来，送到显示电路处理。
在色度信号与亮度信号的“合成”与“分离”过程中，因为亮度信号与色度信号之间的相互干扰以及复合视频信号本身带宽的限制等，影响了图像的质量。
复合视频信号，没有象射频电视广播信号那样经过调制、音/视频混合/分离、放大、检波、解调等过程，传输的图像质量，相对射频电视广播信号要好一些，但相对其它视频信号，传输的图像质量是比较差的，水平分辨率一般可达 350-450 线。
由于行频、场频与色度信号的编码方式不同，复合视频信号又有 PAL、NTSC、SECAM 制式之分。
注：PAL、NTSC、SECAM制式的描述和区别见专题“NTSC、PAL、SECAM 三种制式的比较”。
简单的区别见表1。
表1. Typical Frequencies for Common TV and Computer Video Formats在复合视频信号的波形中，亮度与同步信号加在一起，称为亮度信号Y(Luminance，Luma)。
色调与色饱和度通过一定的转换，转换成色差信号，然后调制在色副载波上，已调色差信号即为色度信号C (Chrominance，Chroma)。
色度信号的相位代表色相，即颜色，其幅度代表色饱和度。
2.1.2 复合视频信号波形解析2.1.2.1 行信号解析reference from:1. /docs/AN10_digital_video_overview.pdf2. /devzone/cda/tut/p/id/4750图1. Monochrome Composite Video Signal (Luma Steps from White to Black)图2. Color Composite Video Signal for a Color Bar Line图3 典型NTSC/PAL Composite信号波形(1行信号)典型的NTSC/PAL Composite信号波形如图1所示。
完整的Composite信号由下列几个部分组成：1. Sync pulse region （Horizontal Sync）Horizontal sync is the -40 IRE pulse occurring at the beginning of each line. This pulse signals the picture monitor to go back to the left side of the screen and trace another horizontal line of picture information.2. Back porch regionThe portion of the video signal which lies between the trailing edge of the horizontal sync pulse and the start of the active picture time. Burst is located on back porch.Used as a reference level to remove any DC components from the floating (AC coupled) video signal. This is accomplished during the clamping interval for monochrome signals, and takes place on the back porch. For composite color signals, the clamping occurs during the horizontal sync pulse because most of the back porch is used for color burst, which provides information for decoding the color content of the signal.Ps：color burst, located on the back porch, is a high-frequency region, which providesa phase and amplitude reference for the subsequent color information.3. Active video region4. Front porch regionFrontporch is defined to start right after the last active video data to right before the leading edge of sync pulse.注：1、2、4组成Horizontal Blanking，所以有下列关系成立：Totol Video of one Row = Active Video + Horizontal Blanking= Active Video + Back porch + Sync Pulse Region + Front porch 注：1. Horizontal blanking is the entire time between the end of the active picture time of one line and the beginning of active picture time of the next line. It extends from the start of front porch to the end of back porch.可参考图4 Composite Signal IRE Level and timing specification。
图4 Composite Signal IRE Level and timing specification注：1. IRE is an arbitrary unit where 140 IRE = 1Vp-pThe 0 IRE point is at blanking level, with sync tip at -40 IRE and white extending to+100 IRE. IRE stands for Institute of Radio Engineers, the organization which defined the unit.2. Breezeway: The area of a composite video signal defined as the time between the rising edge of the sync pulse and the start of the color burst.3. The color burst, also commonly called the "color subcarrier," is 8 to 10 cycles of the color reference frequency. It is positioned between the rising edge of sync and the start of active video for a composite video signal.Another description: A small reference packet of the subcarrier sinewave, typically 8 or 9 cycles, which is sent on every line of video. Since the carrier is suppressed, this phase and frequency reference is required for synchronous demodulation of the color information in the receiver.4. SETUP, In NTSC systems, video black is typically 7.5 IRE above the blanking level. This 7.5 IRE level is refer red to as the black setup level, or simply as setup.5. For NTSC, a setup of 7.5 IRE is usually applied, moving the black level to +7.5 IRE. For PAL and SECAM, the black level is aligned with the blanking level at 0 IRE.2.1.2.2 Vertical Blanking Interval (VBI)图5. 场信号组成A series of pulses that occurs between fields to signal the monitor to perform a vertical retrace and prepare to scan the next field.图6. A composite RS-170 interlaced signalThere are several lines between each field that contain no active video information. Some contain only HSYNC pulses, while several others contain a series of equalizing and VSYNC pulses. These pulses were defined in the early days of broadcast television and have been part of the standard ever since, although newer hardware technology has eliminated the need for some of the extra pulses. A composite RS-170 interlaced signal is shown in Figure 5, including the vertical sync pulses.Vertical Blanking Interval includes pre- and post-equalizing pulse and vertical sync pulses, as well as several lines of blanked video. These are full lines of video on which there is no active picture.图6. Vertical Interval构成Note:1. Depending on whether it is the odd or even field, there will be 6 post-equalizing pulses, but either 5 or 6 half lines. In the even field, there are only 5 half lines. The first half-line of inactive video is called line 9. In the odd field, there are 6 post-equalizing pulses and 6 half lines, so that the first full line of inactive video is called line 10.图7. Complete NTSC Frame Scan2.2 S-Video SignalReference from: /zh-cn/S-VideoS-端子（英语：S-Video），或称“独立视讯端子”，而当中的S是“Separate”的简称。
