Circuit diagram of VGA video interface to PAL signal
Many aircrafts in China have used PAL monitors similar to color televisions. These devices can be used as display terminals for airborne computers when the resolution requirements for displaying graphics are not very demanding. However, the display interface of a general computer does not directly support the PAL system, and it is necessary to properly convert between the VGA interface and the PAL system. The AD725 of the American Analog Devices Company has this function. Figure 1 is the specific conversion circuit. AD725 supports PAL system and NTSC system. PAL is a line-by-line inverted quadrature balanced amplitude modulation system, which is the standard adopted by China's television; NTSC is the standard of the National Television System Committee and is adopted by the United States. The VGA interface mainly includes 2 synchronization signals and 3 color signals, namely horizontal synchronization (line synchronization) signal HSync, vertical synchronization (frame synchronization) signal VSync, and R (red), G (green), B (blue) three colors Intensity signal. Among them, HSync and VSync are TTL logic levels, RGB is an analog signal. The RGB output is connected to the corresponding input of the AD725 through AC coupling, and a resistor of 75 should be connected to the analog signal near the AD725. The maximum peak-to-peak voltage is 714mV. In order to enhance noise suppression, a larger input resistance and capacitance are required, and each input is connected with a 0.1 F series capacitor. The HSync and VSync logic level inputs are used to generate integrated synchronization within the AD725. If the integrated synchronization signal is used directly, the integrated synchronization signal is connected to the HSync input terminal, and the VSync terminal is connected to the high level H (+ 2V).
PAL / NTSC mode conversion is determined by the pin STND. When input STND = H, the chip works in NTSC mode; when STND = L, the chip works in PAL mode. The working mode in the figure is set by H1, H2 line adjustment, both should be H or L at the same time. When working in PAL system, the crystal frequency of G1 should be 17.734475MHz; when working in NTSC mode, G1 should be 14.318180MHz. The video output in Figure 1 provides two methods: one is that all TVs have R / F input, and the corresponding signal is the integrated video (COMP) output of pin 10; the other uses brightness and chrominance to form the image control, In the figure, pin 11 (LIMA) is the brightness output, and pin 9 (CRMA) is the chroma output.
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