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This tutorial is part of the NI Analog Resource Center. Each tutorial will teach you a specific topic by explaining the theory and giving practical examples. This tutorial introduces and explains the basic fundamentals of analog circuits.
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Table of Contents
Understanding Composite Video Signals
A composite video signal is a signal in which all the components required to generate a video signal are embedded in a single signal. The three main components that together form a composite signal are as follows:
- The luma signal (or luminance)-Contains the intensity (brightness or darkness) information of the video image
- The chroma signal-Contains the color information of the video image
- The synchronization signal-Controls the scanning of the signal on a display such as the TV screen
The monochrome composite signal is built of two components: luma (or luminance) and synchronization. This signal, which is usually called the Y signal, is shown in Figure 1.
The chroma signal by itself, which is usually called the C signal, is shown in Figure 2.
[+] Enlarge Image
Figure 2. Color Information Signal for a Color Bar Line (Including the Color Burst)
The composite color video signal, often called the Color Video, Blank, and Sync (CVBS) signal, is the sum of Y and C , is shown in Figure 3.
CVBS = Y + C
The two components Y and C can also be distributed separately as two independent signals. These two signals together are called either Y/C or S-video.
Parts of the Video Signal
The signal for a single horizontal video line consists of a horizontal sync signal, back porch, active pixel region, and front porch, as shown in Figure 4.
Figure 4. Parts of a Video Signal
| The horizontal sync (HSYNC) signals the beginning of each new video line. It is followed by a back porch, which is 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 the color burst, which provides information for decoding the color content of the signal. There is a good description for all the advanced set-up parameters for the video signal in the the Measurement & Automation Explorer Help. Color information can be included along with the monochrome video signal. A composite color signal consists of the standard monochrome signal (RS-170 or CCIR) with the following components added:
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Another aspect of the video signal is the vertical sync (VSYNC) pulse. This is actually a series of pulses that occur between fields to signal the monitor to perform a vertical retrace and prepare to scan the next field. 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. For simplicity, a 6-line frame is shown:
Figure 5. VSYNC pulse
It is important to realize that the horizontal size (in pixels) of an image obtained from an analog camera is determined by the rate at which the frame grabber samples each horizontal video line. That rate, in turn, is determined by the vertical line rate and the architecture of the camera. The structure of the camera's CCD array determines the size of each pixel. To avoid distorting the image, you must sample in the horizontal direction at a rate that chops the horizontal active video region into the correct number of pixels. The following is an example with numbers from the RS-170 standard:
Parameters of interest:
- # of lines/frame: 525 (this includes 485 lines for display; the rest are VSYNC lines for each of the two fields)
- line frequency: 15.734 kHz
- line duration: 63.556 microsec
- active horizontal duration: 52.66 microsec
- # active pixels/line: 640
Now, some calculations we can make:
- Pixel clock (PCLK) frequency (the frequency at which each pixel arrives at the frame grabber):
640 pixels/line / 52.66 e-6 sec/line = 12.15 e6 pixels/sec (12.15 MHz) - Total line length in pixels of active video + timing information (referred to as HCOUNT):
63.556 e-6 sec * 12.15 e6 pixels/sec = 772 pixels/line - Frame rate:
15.734 e3 lines/sec / 525 lines/frame = 30 frames/sec
Different Video Formats
The following table describes some characteristics of the standard analog video formats in common use:
NTSC: National Television Standards Committee
PAL: Phase Alteration Line
SECAM: Systeme Electronic Pour Coleur Avec Memoire
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Color Coding
For all PAL and NTSC formats, the coding is based on the Quadrature Amplitude Modulation (QAM) concept, where 2-color components are amplitude modulated in quadrature and then combined. The modulation must be decoded, so to keep track of the absolute phase needed to decode the color information, a reference signal, called the color burst, is inserted at the beginning of each line, right after the horizontal synchronization pulse (see Figures 3 and 4 above).
For the SECAM format, the 2-color components are frequency modulated using two different subcarrier frequencies and are sequentially distributed on alternated video lines. The SECAM format does not need a color burst signal.
Video Levels
The video levels define the levels and ranges for the different parts of the video signal. The unit used to define video levels is the IRE (Institute of Radio Engineers). The blanking level refers to 0 IRE and the white level refers to +100 IRE. The blanking level, which is the reference level for the video signal (usually 0 V), is different from the black level if a setup is applied to the signal as shown in Figure 6 below.
Figure 6. Video levels
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.
The following table shows the different video levels depending on the video format.
| 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 |
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.
Interlaced Scanning Concept
All composite video systems display the video image on a TV screen using an interlaced scanning technique. Figure 7 shows the interlaced scanning concept.
Figure 7. Interlaced scanning on a TV screen
The analog video signal includes synchronization pulses that control the scanning line-by-line from left to right and field-by-field from top to bottom. The pulses that control the line-by-line scanning are called the horizontal synchronization pulses (H-Sync). The pulses that control the vertical scanning are called the vertical synchronization pulses (V-Sync).
Two interlaced fields compose a complete frame. The first field, called the odd field, scans the odd lines of the video image. The second field, called the even field, scans the even lines of the video image. The process repeats for every frame.
See Also:
When Acquiring an Interlaced Image What Field Is Taken First Odd or Even?
How are Odd and Even Fields Differentiated in an Interlaced Video Signal?
Active Image
The active video image resulting from the scanning always has an aspect ratio (horizontal/vertical) of 4/3, independent of the video format. The color composite video signal shows that the scanning process requires some additional room on the left and right sides of each line, as well as on the top and bottom of the active video image region. This additional room includes the synchronization signals, color bursts, and other format-specific information, like the ITS, which are not part of the active video image. Approximately 90% of all the lines and 80% of each line can transmit the active image information. The exact values depend on the video format, as shown in the following table.
| Video Format | Lines/Frame | Active Lines | Frame Rate | Line Duration | Active Line Duration |
| NTSC | 525 | 480/486 | 29.97 frames/sec | 63.55 µs | 52.2 µs |
| PAL/SECAM | 625 | 576 | 25.00 frames/sec | 64.00 µs | 52.0 µs |
Active Lines represents the number of lines that are actually used to transmit the image information. For example, only 480 lines out of 525 lines per frame transmit the image information in NTSC. Likewise, on each line, the image information is transmitted only during the active lines sequence, which is shorter than the entire line duration. For example, of 63.55 µs, only 52.2 µs are the active line duration in NTSC. Frame rate is the scanning speed.
Gray Scale Image and Extracted Line Profile
The Complete NTSC Frame Scan image in the next section simulates the video display that would appear on a television screen if the following conditions were true:
- The television could show the entire line instead of just the active image part.
- The television was not interlacing the two fields to form a complete image frame, but instead was displaying a progressive scanning, line by line, of the entire frame.
The scanning starts (line-by-line from top to bottom) with a number of lines that represent the vertical synchronization pattern for the odd field. Immediately after the vertical synchronization pattern for the odd field, optional insertion test signals (ITS) are inserted. Finally, the actual odd field active image displays.
The process repeats for the even field, forming the complete frame.
The extracted line profile example at the bottom of Figures 8 and 9 show an actual video signal line extracted from the even field. Refer to Video Signals above for more information about video levels.
Horizontal synchronization pulses are basically simple negative pulses, which are pulses going below the level of the luminance signal. However, the vertical synchronization signals are composed of pulse trains distributed on several lines, and the pulse trains are different for odd and even fields. Figures 8 and 9 show the vertical synchronization patterns for both fields and for the three main video formats.
Figure 8. Vertical Blanking and Synchronization Signal for NTSC
Figure 9. Vertical Blanking and Synchronization Signal for PAL and SECAM
Complete NTSC Frame Scan
Figure 10 shows the result of scanning all 525 lines that compose a complete NTSC frame.
Figure 10 is a gray-scale image because it represents the intensity graph of the raw NTSC video waveform. The color information is embedded in that waveform and is not yet decoded.
You can see the color burst of the signals on the left side. The dotted pattern represents the intensity graph of the sine tone that is the color burst waveform. After decoding, the color burst would (if it were visible on the TV-monitor) appear as a solid color surface.
Relevant NI products
Customers interested in this topic were also interested in the following NI products:
- LabVIEW Graphical Programming Environment
- SignalExpress Interactive Software Environment
- Digitizers/Oscilloscopes
- Dynamic Signal Acquisition (DSA)
- Digital Multimeter (DMM)
- Data Acquisition (DAQ)
For the complete list of tutorials, return to the NI Analog Resource Center.
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