Analysis of common video signal transmission characteristics and conversion depth

1. Component Video

The optical system of the camera splits the light beam of the scene into three basic colors: red, green and blue. Photosensitive equipment then converts the three monochrome images into separate electrical signals. In order to identify the left edge and top of the image, synchronization information is added to the electrical signal. The synchronization information between the display terminal and the camera can be attached to the green channel, sometimes to all three channels, or even transmitted as one or two independent channels. The following are several common synchronization signal addition modes and representation methods :

-RGsB: The synchronization signal is attached to the green channel and transmitted by three 75Ω coaxial cables.

-RsGsBs: The synchronization signal is attached to three channels of red, green and blue, and three 75Ω coaxial cables are transmitted.

-RGBS: The synchronization signal is transmitted as an independent channel with four 75Ω coaxial cables.

-RGBHV: Synchronization signal is transmitted as two independent channels of line and field, and five 75Ω coaxial cables are transmitted.

RGB component video can produce high-quality images from the camera to the display terminal, but the transmission of such a signal requires at least three independent channels to be processed separately, so that the signal has the same gain, DC offset, time delay and frequency response. The transmission of component video The characteristics are as follows:

-Transmission medium: 3-5 shielded coaxial cables

-Transmission impedance: 75- Common connectors: 3-5 & TImes; BNC connector

-Wiring standard: red = red primary color (R) signal line, green = green primary color (G) signal line, blue = blue primary color (B) signal line, black = line synchronization (H) signal line, yellow = field synchronization ( V) Signal line, public ground = shielded network cable (see drawing VP-03)

2. Composite video (Composite-Video)

Due to the unequal gain of each channel of the component video signal or the error of the DC offset, the color displayed by the terminal will change slightly. At the same time, due to the length error of multiple transmission cables or the use of different transmission paths, this will cause the timing deviation of the color signal, resulting in blurred image edges and even multiple separated images in severe cases.

Inserting the NTSC or PAL codec makes the video signal easy to handle and transmits along a single line. This is composite video. The composite video format is a compromise solution for long-distance transmission. The chroma and brightness share the frequency bandwidth of 4.2MHz (NTSC) or 5.0-5.5MHz (PAL), and there is relatively large crosstalk between each other, so it is still necessary to consider the frequency response And timing issues, the use of multi-level codecs should be avoided. The transmission characteristics of composite video are as follows:

-Transmission medium: single shielded coaxial cable

-Transmission impedance: 75- Common connectors: BNC connector, Lotus (RCA) connector

-Wiring standard: pin = coaxial signal line, shell common ground = shielded network cable (see drawing VP-01)

3. Color difference signals (Y, RY, BY)

When processing video signals and transmitting images, the RGB component video method is not the method with the highest bandwidth utilization because the three component signals all require the same bandwidth.

Human vision is more sensitive to changes in brightness details than changes in color, so we can use the entire bandwidth for brightness information and the remaining available bandwidth for color difference information to improve the bandwidth utilization of the signal.

Processing the video signal components into luminance and color difference signals can reduce the amount of information that should be transmitted. A full-bandwidth brightness channel (Y) is used to represent the brightness details of the video signal. The bandwidth of the two color-difference channels (RY and BY) is limited to approximately half of the brightness bandwidth, which still provides sufficient color information. Using this method, the conversion between RGB and Y, RY, BY can be achieved through a simple linear matrix. The bandwidth of the color-difference channel is implemented after the linear matrix. When the color-difference signal is restored to the RGB component video display, the brightness details are restored at the full bandwidth, and the color details are limited to an acceptable range.

Color difference signals also have many different formats and have different applications. In the commonly used composite PAL, SECAM and NTSC systems, the coding coefficients are different, see the following table:

4. Digital Video (SDI)

There are many different formats of digital video, and they are used in different areas. This refers to "serial digital video" (Signal-Digital Interface), generally abbreviated as SDI interface.

After gamma correction, the RGB signal is transformed into a luminance component Y and two chromaticities Pb and Pr in a linear matrix. Since the human visual perception is more sensitive to changes in brightness details than changes in color, the brightness signal Y passes through the transmission system with a higher bandwidth (SDTV of 5.5 MHz). After the low-pass filtering of the luminance signal, the sampling frequency is 13.5MHz, and a 10-bit 13.5MB / s code stream is generated in the A / D converter; after the same process, the two chrominance signals are in the A / D converter Two 10-bit 6.75MB / s streams were generated, and the three video channels were multiplexed to form a 27MB / s 10-bit parallel data stream (Y, Cb, Cr).

The 27MB / s 10-bit parallel data stream is sent to the shift register (serializer), adding clock and scrambling, and forming a 270Mb / s serial data stream (SDI) according to the TV specifications.

5. Video format conversion

The different formats of the video determine the signal's performance in various aspects such as brightness, chroma, contrast, sharpness, sharpness, and highest resolution. From the above analysis of various video formats, we can know that the level of high-definition quality of video can be roughly sorted as right (from high to low):

Among them, currently the highest-level DVI digital video signal is elected, but there are shortcomings that can only be transmitted over a short distance (effective distance is about 5 meters), SDI digital video has the advantages of editing and longer distance transmission, RGBHV and VGA are actually a unified grade There are two kinds of signals because of the different components of the signal. S-Video has a significant improvement in brightness utilization compared to Video (abbreviation of composite video), and effectively eliminates color creep. The RF format is the lowest Level signal is only used in the scope of monitoring and public TV.

Engineering applications often face the conversion process of many signal formats. What rules should be followed for the conversion of these different formats? What effect will it have in the end? Generally considered:

The conversion from low-level format to high-level format has a relatively obvious quality improvement, such as the early octave scanner or quadruple octave scanner, as well as the currently popular intelligent video regulators, all of which are Video-RGBHV (composite video-component video ) Conversion process, there is a significant improvement in improving the quality of the signal. Because these products use multi-bit digital technology to ensure that the signal quality (sharpness, brightness, signal-to-noise ratio) can be highly restored.

DVI digital video is usually converted to SDI or RGBHV, the clarity of the original signal is lost after the conversion, but the DVI signal is transmitted over a long distance; the actual effect of converting the VGA signal to RGBHV has not been improved, because the two are at the same level, But it solves the problem of synchronous universal matching of VGA signals, and can transmit longer distances.

The conversion process from high-level format to low-level format (such as VGA to Video) will cause serious losses regardless of any aspect of the original signal, including brightness, chroma, color, contrast, sharpness, sharpness, and highest resolution. This conversion does not make any sense, but it has a certain use value in the early days, such as: converting the computer's VGA signal into Video for tape recording, TV and TV wall display, or for "capture" transmission in video conferences.

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