Video signal measurement and generation

A composite video signal is one in which all the components needed to create a video signal are combined in the same signal. The three main components that make up a composite signal are as follows:

● Luminance signal – contains information about the intensity (brightness or darkness) of the video image

● Color signal - contains the color information of the video image

● Synchronous signal - controls the scanning of signals on displays such as TV screens

A monochrome composite signal is made up of two components: luminance and sync. Figure 1 shows this signal (often called the Y signal).

Figure 1: Monochrome composite video signal (brightness transitions from white to black)

The color signal is generally referred to as the C signal, and is shown in FIG. 2 .

Figure 2: Color information signal of color bars (including color bursts)

The composite color video signal is usually called the color video, blanking and synchronization (CVBS) signal, which is the sum of Y and C, as shown in Figure 3.

CVBS = Y + C

Figure 3: Color composite video signal with color bars

The two components, Y and C, can be transmitted separately as two independent signals. Together, these two signals are called Y / C or S-video.

2. Video signal composition

A single horizontal video line signal consists of a horizontal sync signal, a back porch, an active pixel field, and a front porch, as shown in Figure 4.

Figure 4: Video signal composition

The horizontal sync (HSYNC) signal indicates the start of each new video line. It is followed by the back porch, which is used as a reference level to remove the DC component from a floating (AC-coupled) video signal. This is accomplished by a clamping interval for monochrome signals, which occurs during the back porch. For composite color signals, clamping occurs during the horizontal sync pulse, and since most of the back porch is used for the color burst, it provides information to decode the color content of the signal. All the setup parameters for the video signal are clearly described in the MAX Help.

Color information can be contained in a monochrome video signal. A composite color signal contains the standard monochrome signal (RS-170 or CCIR) with the following components added:

● Color Burst: Located on the trailing porch, this is a high frequency field that provides a phase and amplitude reference for subsequent color information.

● Color signal: This is the actual color information. It consists of two quadrant components modulated onto a carrier at the color burst frequency. The phase and amplitude of these components determine the color content of each pixel.

Another aspect of the video signal is the vertical sync (VSYNC) pulse. This is actually a sequence of pulses that occurs between fields and is used to tell the display to complete vertical retracking and prepare to scan the next field. There are several lines in each field that do not contain active video information. Some contain only the HSYNC pulse, while others contain equalization and a sequence of VSYNC pulses. These pulses were defined in the early days of broadcast television and have been part of the standard ever since, although later hardware technology has been able to avoid the use of some of the additional pulses. The composite RS-170 cross signal, including the vertical sync pulse, is shown in Figure 5, and for simplicity, a 6-line frame is shown below:

Figure 5: VSYNC pulse

It is important to understand that the vertical size (in pixels) of an image from an analog camera is determined by the rate at which the frame grabber samples the horizontal video lines. This rate is determined by the vertical line rate and the camera architecture. The structure of the camera's CCD array determines the size of each pixel. To avoid image distortion, you must sample the horizontal direction at a rate that divides the horizontal active video field into the correct number of pixels. Here is an example from the RS-170 standard: [page]

Parameters of interest:

● Lines/frame: 525 (including 485 lines for display; the rest are VSYNC lines between every two fields)

● Line frequency: 15.734 kHz

● Line duration: 63.556 microseconds

● Activity level duration: 52.66 microseconds

● Active pixels/lines: 640

Now, we can do some calculations:

● Pixel clock frequency (how often each pixel reaches the frame grabber): 640 pixels/line / 52.66 e-6 seconds/line = 12.15 e6 pixels/line (12.15 MHz)

● Pixel line length of active video + timing information (called HCOUNT): 63.556e-6 seconds * 12.15e6 pixels/second = 772 pixels/line

● Frame rate: 15.734 e3 lines/second / 525 lines/frame = 30 frames/second

3. Different video formats

The following table describes some characteristics of common standard analog video formats:

NTSC: National Television Standards Committee

PAL: Phase Alternation Line

SECAM: Systeme Electronic Pour Coleur Avec Memoire

4. Color coding

For all PAL and NTSC formats, the encoding is based on the concept of quadrature amplitude modulation (QAM), where the two color components are combined after quadrant amplitude modulation. The modulation must be decoded, so tracking the absolute phase requires decoding the color information. A reference signal called a color burst is inserted at the beginning of each line, after the horizontal sync pulse (see Figures 3 and 4 above).

For all SECAM formats, the two color components are frequency modulated using two different subcarrier frequencies and then sequentially stepped on different video lines. The SECAM format does not require a color burst signal.

5. Video signal level

Video signal levels define the level and range of different parts of a video signal. The organization used to define video signal levels is the IRE (Institute of Radio Engineers). The blanking level corresponds to 0 IRE and the white level corresponds to +100 IRE. The blanking level is the reference level of the video signal (usually 0 V), as shown in Figure 6 below. If certain settings are made to the signal, the blanking level and white level are different.

Figure 6: Video signal levels

For NTSC, a 7.5 IRE setting is usually used, raising the black level to +7.5 IRE. For PAL and SECAM, the black level is the same as the blanking level, both at 0 IRE.

The following table shows the different video signal levels depending on the video format.

Analog composite video signals are defined as voltage sources with an output impedance of 75 Ω. When loaded with a 75 Ω impedance, the white level sync is typically 1 V peak-to-peak. Therefore, the unloaded signal is nominally 2 V peak-to-peak. [page]

6. Interlaced Scanning Concept

All composite video systems use interlaced scanning to display video images on a television screen. Figure 7 shows the interlaced scanning concept.

Figure 7: Interlaced scanning on a TV screen

The analog video signal contains pulses that control scanning from left to right line by line and from top to bottom field by field. The pulse that controls the line scanning is called the horizontal synchronization pulse (H-Sync). The pulse that controls the vertical scanning is called the vertical synchronization pulse (V-Sync).

Two interleaved fields make up a complete frame. The first field is called the odd field and scans the odd lines of the video image. The second field is called the even field and scans the even lines of the video image. The entire process is repeated for each frame.

7. Moving images

The scanned active video image always has a 4/3 aspect ratio (horizontal/vertical), which is independent of the video format. Color composite video signals mean that the scanning process requires some additional space on the left and right sides of each line, as well as at the top and bottom of the active video image field. This extra space contains sync signals, color bursts, and other format-specific information such as ITS, which is not part of the active video image. Approximately 90% of all lines and 80% of each line are capable of conveying active image information. The exact values ​​depend on the video format, as shown in the table below.

Active lines represent the number of lines that are actually used to transmit the image and information. For example, in NTSC, only 480 lines out of 525 lines in each frame are used to transmit image information. Likewise, in each line, image information is only transmitted during the active line sequence, which is shorter than the duration of the entire line. For example, in NTSC, only 52.2 μs out of 63.55 μs is the active line duration. Frame rate is the scanning speed.

8. Grayscale image and extracted line spectrum profile

The full NTSC frame scan images in the next section simulate how video might appear on a television screen, assuming the following conditions are met:

● The TV is able to display the entire line, not just the active image portion.

● Instead of interlacing two fields to get a complete image frame, the TV scans the entire frame line by line.

The scan starts with the lines representing the vertical sync pattern of the even field (line by line from top to bottom). An optional test signal (ITS) is inserted after the vertical sync pattern of the even field. Finally, the actual odd field active image is displayed.

This process is repeated for even fields to form a complete frame.

Description: Most lines begin with a horizontal sync pulse followed by a color burst pattern signal. The active picture (or ITS) that follows shows intensity variations, where higher signal levels represent higher brightness.

The extracted spectral profiles at the bottom of Figures 8 and 9 show the active video signal lines extracted from the even fields. For more information on video levels, see the previous section on video signals.

Horizontal sync pulses are generally simple negative pulses that are below the luminance signal level. However, the vertical sync signal consists of a sequence of pulses that are stepped across multiple lines and are different for odd and even fields. Figures 8 and 9 show the vertical sync patterns for two fields and three major video formats. [page]

Figure 8: Vertical blanking and sync signals for NTSC

Figure 9: Vertical blanking and sync signals for PAL and SECAM

9. Full NTSC frame scan

Figure 10 shows the results of scanning the 525 lines that make up a complete NTSC frame.

Figure 10: Complete NTSC frame scan

Figure 10 is a grayscale image because it represents the intensity map of the original NTSC video waveform. Color information is embedded in this waveform but has not yet been encoded.

You can see the color burst of the signal on the left. The dot pattern represents the intensity graph of the sinusoidal beats that make up the color burst waveform. After decoding, the color burst looks like a monochrome surface (if visible on a TV monitor).