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0 PrefaceCharacter superposition has been widely used in closed-circuit television, image systems, car video systems, highway toll systems, video surveillance and other systems. TW2834 is a 4-channel video controller from TeehWell. It is mainly used in video surveillance systems. The OSD function of the chip can easily and quickly superimpose Chinese character information on video.
1 TW2834 chip performance characteristics
The TW2834 has four high performance NTSC/PAL video decoders, a two-color display controller and dual video encoders. It also integrates four built-in analog anti-aliasing filters, four 10-bit analog-to-digital converters and proprietary digital gain, clamp controllers, and high-performance Y/C splitters for cross-noise. The TW2834 features high video free zoom and four built-in detectors for motion detection and video loss detection. The TW2834 has flexible video display control that provides the flexibility to place input video anywhere on the display. In addition, TW2834 also has excellent image overlay function, such as superimposing Chinese characters, character information, mouse pointer graphics, windows, etc. on the video.
2 TW2834 Chinese character superposition principle
The character overlay function of TW2834 is mainly composed of font RAM, display RAN and overlay control module. Figure 1 is a schematic diagram showing the structure of the display RAM.
In addition, the font RAM can store the dot matrix information data of the character, and the display RAM stores the number, position and attribute of the displayed character. The size of the displayed characters can be defined by the number of pixels in the horizontal and vertical directions. TW2834 can support 16 pages × 2 (display and record mode each 16 pages) font groups. Each font group can support up to 128 characters. When the font dot matrix becomes larger, the number of characters stored in the font group is reduced accordingly.
Font RAM can be understood as a repository for displaying fonts, and display RAM can be understood as a display. When superimposing character information, the display font should first be uploaded to the font RAM, and the area to be superimposed should be pointed to the font RAM area, so that the superimposed characters on the video can be realized. The structure diagram of its font RAM is shown in Figure 2.
3 Display the writing of superimposed characters in RAM
In the display RAM, the display attributes of each display character include blending, blinking, color, type, character number, etc., which can be set by 2 bytes of 12-bit data, each superimposed character line has a font page, a font number , horizontal and vertical sizes and attributes. The color, position, size, etc. of the superimposed characters can be determined by the display attributes of the characters and the attributes of the character lines. The display properties of characters and the display properties of character lines can be achieved by setting the registers of the TW2834.
There are three different modes for setting the character information of the display RAM, and each mode can transfer data through the serial interface or the parallel interface. The setting mode of the display RAM can be set by the mode control register. The setting mode has three modes: mode 0, mode 1 and mode 2. The specific setting mode is shown in Fig. 3.
4 TW2834 based video character overlay
4.1 Hardware Circuit Design
The hardware block diagram of video character superposition using TW2834 is shown in Figure 4. The design of this circuit needs to pay attention to the following aspects:
(1) In this circuit design, the choice of processor should be based on the requirements of the application to select the appropriate microprocessor. The basis for selecting a microprocessor is the processing speed of the microprocessor and the size of the memory. The peripheral module functions must meet the design requirements. This design uses the SST89E516RD2 microcontroller, which has IAP (Programmable in Application) function with 56K bytes of program memory space;
(2) The selection of the RAM memory chip is to select a memory of a suitable size according to the requirements of the system. In the video superimposed Chinese character application, usually only one piece of RAM is needed for storing data such as dot matrix information of superimposed characters, and the size of the RAM directly affects the number of pages in which the font RAM stores characters. If you want to superimpose the information in the recording mode with TW2834, you need to add another RAM chip.
(3) The function of the font memory is to store the dot matrix information of Chinese characters. When the processor receives the superimposition command from the host computer PC, it will extract the dot matrix information of the superimposed Chinese characters from the font memory and write it to the font RAM, and set the display character attribute of the display RAM to realize the superposition of the Chinese characters on the video.
4.2 Writing of Chinese character lattice information
Use TW2834 to realize the superposition of Chinese character information on the video. The important part is to write the dot matrix information of Chinese characters into the font RAM. When the writing of the font RAM is completed, the display character attribute of the display RAM is set. The superposition of Chinese character information can be realized on the video. The flow chart for writing Chinese character dot matrix information into the font RAM is shown in FIG. 5.
5 Conclusion
This paper introduces the method of superimposing Chinese character information on video by using TW2834 chip. This method has important guiding significance for realizing the superposition of Chinese character information in video surveillance system using TW2834 chip. Since the use of TW2834 to superimpose Chinese character information in the video surveillance system has the advantages of simple design, high reliability, good practicability, etc., it is undoubtedly a better choice to realize the superposition of Chinese character information on the video monitoring system by using TW2834.
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