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)
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
|
Video Format |
NTSC |
PAL |
HDTV/SDTV |
VGA |
XGA |
|
Description |
Format for North America and Japan |
Format for Most of Europe andSouth America |
High Definition/ Standard Definition Digital Television Format |
Video Graphics Array (PC) |
Extended Graphics Array (PC) |
|
Vertical Resolution Format (visible lines per frame) |
Approx 480 (525 total lines) |
Approx 575 (625 total lines) |
1080 or 720 or 480; 18 different formats |
480 |
768 |
|
Horizontal Resolution Format (visible pixels per line) |
Determined bybandwidth, ranges from 320 to 650 |
Determined by bandwidth, ranges from 320 to 720 |
1920 or 704 or 640; 18 different formats |
640 |
1024 |
|
Horizontal Rate (kHz) |
15.734 |
15.625 |
33.75-45 |
31.5 |
60 |
|
Vertical Frame Rate (Hz) |
29.97 |
25 |
30-60 |
60-80 |
60-80 |
|
Highest Frequency (MHz) |
4.2 |
5.5 |
25 |
15.3 |
40.7 |
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. http://www.digitalcreationlabs.com/docs/AN10_digital_video_overview.pdf
2. http://zone.ni.com/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 region
The 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 provides a phase and amplitude reference for the subsequent color information.
3. Active video region
4. Front porch region
Frontporch 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-p
The 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.
|
Video Format |
Sync Level |
Blanking Level |
Black Level |
White Level |
Peak Level |
Burst Amplitude |
|
NTSC |
–40 IRE |
0 IRE |
+7.5 IRE |
+100 IRE |
+120 IRE |
20.0 IRE |
|
PAL |
–43 IRE |
0 IRE |
0 IRE |
+100 IRE |
+133 IRE |
21.5 IRE |
|
SECAM |
–43 IRE |
0 IRE |
0 IRE |
+100 IRE |
+130 IRE |
N/A |
表2. Video Levels by Format
6. The analog composite video signal is defined as a voltage source with an output impedance of 75 Ω. The sync-to-white level is normally 1 Vpk-pk when loaded with a 75 Ω resistance. Therefore, the unloaded signal is nominally 2 Vpk-pk.
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 signal
There 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 Scan
2.2 S-Video Signal
Reference from: http://zh.wikipedia.org/zh-cn/S-Video
S-端子（英语：S-Video），或称“独立视讯端子” ，而当中的S是“Separate”的简称。也称为Y/C（或误称为S-VHS或“超级端子（Super Video）”）。S-端子的光亮度（Y; greyscale，灰阶）讯号和调制色度（C; colour，色彩）讯号由独立电线或电线组传送，不像复合视频讯号（composite video）是将所有讯号打包成一个整体进行传送。
在合成视频，光亮度的讯号经由低通滤波器排除高频的色度讯号，因高频率的色度讯号及光亮度讯号一部分是重叠的。而S-端子把两种讯号分开，这种就不用把经由低通滤波器取出光亮度的讯号。这样可以给予光亮度的讯号有更大的带宽，也解决了讯号重叠的问题。因此，受干扰的点阵讯号被排除。这表示S-端子能从完整原先的影像讯号转送比合成讯号更多的讯息，因此与合成影像相比，S-端子更有效使图像在低失真的情况下，原画再生。
但是，影像讯号分离为亮度与色度两部分，因此S-端子有时也被视为是一种合成影像讯号，但就品质上而言，S-Video是色差讯号中最差的一种，远不如其他更为复杂的色差影像讯号（如RGB或YPbPr）， 但较之另外一种模拟信号复合视讯锐利，干扰较少。S-Video与这些更高阶色差影像的差别在于，S-Video将色度的讯号合为一条讯号进行传送，因此 色度的讯号必须先经过编码，而且NTSC、PAL或SECAM等影像讯号透过S-Video进行传送时皆有不同的编码方式。所以为了使讯号间达到完全相容 性，必须兼顾S-Video接头与色度编码方式两者的相容性。
2.3 Component Video
2.4 VGA
两种同步方式：separate HS/VS，SOG。
2.5 模拟视频信号汇总
图8. 模拟视频信号格式和信号电平汇总
