Analog Video 101

This paper is part of the National Instruments Signal Generator Tutorial and High-Speed Digitizers Tutorial series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining the theory and giving practical examples. This tutorial covers the fundamentals of video signal measurement and generation.

For additional signal generator concepts, refer to the Signal Generator Fundamentals main page.
For additional high-speed digitizer concepts, refer to the High-Speed Digitizers Fundamentals main page.

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.

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.

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. The NI Measurement & Automation Explorer Help includes a description of all the advanced setup parameters for the video signal. You can incorporate color information 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: Color burst: Located on the back porch, this is a high-frequency region, which provides a phase and amplitude reference for the subsequent color information. Chroma signal: This is the actual color information. It consists of two quadrature components modulated onto a carrier at the color burst frequency. The phase and amplitude of these components determine the color content at each pixel.

Another aspect of the video signal is the vertical sync (VSYNC) pulse. This is actually a series of pulses that occurs 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 six-line frame is shown:

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 µs
• active horizontal duration: 52.66 µs
• # active pixels/line: 640

You can make these calculations:

• 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

Table 1 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

Format Where It Is Used Mode Signal Name Frame Rate, Scanning Speed (frame/sec) Vertical Line Resolution Line Rate (lines/sec) Image Size (WxH) pixels NTSC North and Central America, Japan Mono RS-170 30 525 15,750 640x480 Color NTSC Color 29.97 525 15,734 PAL Europe (except France), Australia, parts of Africa and South America Mono CCIR 25 405 10,125 768x576 Color PAL Color 25 625 15,625 SECAM France, Eastern Europe, Russia, parts of the Middle East and Africa Mono 25 819 20,475 N/A Color 25 625 15,625

Table 1. Characteristics of Standard Analog Video Formats

Color Coding

For all PAL and NTSC formats, the coding is based on the quadrature amplitude modulation (QAM) concept, where two-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, you insert a reference signal, called the color burst, at the beginning of each line, right after the horizontal synchronization pulse (see figures 3 and 4).

For the SECAM format, the two-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.

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.

Table 3 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

Table 2. Video Levels by Format

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 V_pk-pk when loaded with a 75 Ω resistance. Therefore, the unloaded signal is nominally 2 V_pk-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.

 

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 no matter 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 percent of all the lines and 80 percent of each line can transmit the active image information. The exact values depend on the video format, as shown in Table 3.

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

Table 3. Active Video Image Values

"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 appears on a television screen if the following conditions are true:

• The television shows the entire line instead of just the active image part.
• The television is not interlacing the two fields to form a complete image frame. Instead, it is displaying a progressive scan, 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.

 

Note: Most lines start with a horizontal synchronization pulse followed by the color burst pattern. Then the active image (or the ITS) displays as an intensity change, where a higher signal level corresponds to brighter intensity.

The extracted line profile example at the bottom of figures 8 and 9 shows an actual video signal line extracted from the even field. Refer to "Parts of the Video Signal" above for more information about video levels.

Horizontal synchronization pulses are basically simple negative pulses, which are pulses that dip 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 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, if it were visible on the TV monitor, would appear as a solid color surface.

Relevant NI Products

NI Analog Video Analyzer datasheet and pricing

NI Analog Video Generator datasheet and pricing

NI Digital Video Analyzer datasheet and pricing

Reader Comments | Submit a comment »

thanks,it was very useful for me
- arvind, tvtn. arvind.jarwal@rediffmail.com - Mar 9, 2008

Its good
The information provided is really nice and useful for beginners
- Sep 12, 2007

Very nice with the update, very useful information
- simon nielsen - Aug 24, 2007

Its very helpful for beginners to understand about Video Signals
- Jul 30, 2007

Resourceful
Very Resourceful, Better Images would be great, but this still gets the point across of how Video works. Maybe how it is developed with NI on a certain generator would be good too!
- Aric Salois, MSCL-Johnson Space Center. Aric.Salois1@jsc.nasa.gov - Dec 19, 2006

Excellent job

- Dec 16, 2006

Excellent reference
Excellent document - well written.
- James - Nov 29, 2006

More info on color modulation
Very nice document. Adding some more details and graphs on the color modulation would make it even better.
- Rolf Bankersen, Le Batel. NI@le-batel.tmfweb.nl - Dec 30, 2005