模擬視頻信號解析與數字視頻信號的對應關係1：模擬視頻信號的波形解析(Z)
1. 模擬視頻信號的波形解析
主要包括composite、component、VGA等模擬視頻信號的波形解析；
2. 數字視頻的數據格式
主要包括BT601、BT656和BT1120等並行數據格式的描述和說明；
3. 模擬視頻信號與數字視頻信號的對應關係
着重說明視頻信號ADC前後模擬視頻信號與數字視頻信號的對應關係。
2. 模擬視頻信號的波形解析
2.1 Composite Signal 複合視頻信號
2.1.1 複合視頻信號的概念
在一個信號中包含了亮度信號、色度信號與同步信號（包括場同步、行同步信號及行場消隱信號） ，稱爲複合視頻信號。
又稱爲CVBS，表示Color，Video，Blanking，Sync，或者composite video baseband signal。
複合視頻信號把亮度、色度與同步信號複合在一個信號通道上傳輸，也就是在傳輸前需要把色度信號與亮度信號“合成”在一個信號裏，在傳輸後再將色度信號與亮度信號“分離”開來，送到顯示電路處理。
在色度信號與亮度信號的“合成”與“分離”過程中，因爲亮度信號與色度信號之間的相互干擾以及複合視頻信號本身帶寬的限制等，影響了圖像的質量。複合視頻信號，沒有象射頻電視廣播信號那樣經過調製、音/視頻混合/分離、放大、檢波、解調等過程，傳輸的圖像質量，相對射頻電視廣播信號要好一些，但相對其它視頻信號，傳輸的圖像質量是比較差的，水平分辨率一般可達 350-450 線。
由於行頻、場頻與色度信號的編碼方式不同，複合視頻信號又有 PAL、NTSC、SECAM 制式之分。
注：PAL、NTSC、SECAM制式的描述和區別見專題“NTSC、PAL、SECAM 三種制式的比較”。簡單的區別見表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 |
在複合視頻信號的波形中，亮度與同步信號加在一起，稱爲亮度信號Y(Luminance，Luma)。色調與色飽和度通過一定的轉換，轉換成色差信號，然後調製在色副載波上，已調色差信號即爲色度信號C (Chrominance，Chroma)。色度信號的相位代表色相，即顏色，其幅度代表色飽和度。
2.1.2 複合視頻信號波形解析
2.1.2.1 行信號解析
reference from:
1. http://www.digitalcreationlabs.com/docs/AN10_digital_video_overview.pdf
2. http://zone.ni.com/devzone/cda/tut/p/id/4750
圖1. Monochrome Composite Video Signal (Luma Steps from White to Black)
圖2. Color Composite Video Signal for a Color Bar Line
圖3 典型NTSC/PAL Composite信號波形(1行信號)
典型的NTSC/PAL Composite信號波形如圖1所示。
完整的Composite信號由下列幾個部分組成：
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組成Horizontal Blanking，所以有下列關係成立：
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.
可參考圖4 Composite Signal IRE Level and timing specification。
圖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)
圖5. 場信號組成
A series of pulses that occurs between fields to signal the monitor to perform a vertical retrace and prepare to scan the next field.
圖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.
圖6. Vertical Interval構成
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.
圖7. Complete NTSC Frame Scan
2.2 S-Video Signal
Reference from: http://zh.wikipedia.org/zh-cn/S-Video
S-端子（英語：S-Video），或稱“獨立視訊端子” ，而當中的S是“Separate”的簡稱。也稱爲Y/C（或誤稱爲S-VHS或“超級端子（Super Video）”）。S-端子的光亮度（Y; greyscale，灰階）訊號和調製色度（C; colour，色彩）訊號由獨立電線或電線組傳送，不像複合視頻訊號（composite video）是將所有訊號打包成一個整體進行傳送。
在合成視頻，光亮度的訊號經由低通濾波器排除高頻的色度訊號，因高頻率的色度訊號及光亮度訊號一部分是重疊的。而S-端子把兩種訊號分開，這種就不用把經由低通濾波器取出光亮度的訊號。這樣可以給予光亮度的訊號有更大的帶寬，也解決了訊號重疊的問題。因此，受干擾的點陣訊號被排除。這表示S-端子能從完整原先的影像訊號轉送比合成訊號更多的訊息，因此與合成影像相比，S-端子更有效使圖像在低失真的情況下，原畫再生。
但是，影像訊號分離爲亮度與色度兩部分，因此S-端子有時也被視爲是一種合成影像訊號，但就品質上而言，S-Video是色差訊號中最差的一種，遠不如其他更爲複雜的色差影像訊號（如RGB或YPbPr）， 但較之另外一種模擬信號複合視訊銳利，干擾較少。S-Video與這些更高階色差影像的差別在於，S-Video將色度的訊號合爲一條訊號進行傳送，因此 色度的訊號必須先經過編碼，而且NTSC、PAL或SECAM等影像訊號透過S-Video進行傳送時皆有不同的編碼方式。所以爲了使訊號間達到完全相容 性，必須兼顧S-Video接頭與色度編碼方式兩者的相容性。
2.3 Component Video
2.4 VGA
兩種同步方式：separate HS/VS，SOG。
2.5 模擬視頻信號彙總
圖8. 模擬視頻信號格式和信號電平彙總