Design and Implementation of VGA/TV Converter Based on VX1937

Many aircraft in the United States use PAL displays similar to color TVs. If the display resolution is not demanding, these existing devices can be used as display terminals for onboard computers. However, the display interface of general computers does not directly support the PAL format, and appropriate conversion between the VGA interface and the PAL format is required. In order to achieve the conversion of the display controller VGA signal to the analog TV video signal and be compatible with the display mode of the analog video of the multi-function display, the author designed a VGA/TV video converter based on VXl937. This video converter converts the general VGA signal into a broadcast-quality PAL video signal. It does not require software support, is reliable in operation, and is easy to operate. The chromaticity, contrast, etc. can be adjusted by pressing buttons.

1 VXl937 Introduction

Figure 1 is a block diagram of the internal structure of VX1937. The figure shows that the VX1937 video converter is used to process the analog R, G, B signals of standard VGA and convert them into video signals of NTSC or PAL format. VX1937 can convert 640×480 (85 Hz), 800×600 (75 Hz), 1 024×768 (60 Hz) true color progressive images into video signals of NTSC or PAL format. All video signals are processed digitally. It integrates 4 10-bit D/A converters. The 4 output pin combinations of VX1937 can output composite video and S-video (brightness and chrominance separation) signals at the same time. The output of composite video, S-video (brightness and chrominance separation), RGB or YUV can be selected through the hardware switch. VX1937 has a built-in OSD (On Screen Display) function, which is directly controlled by hardware. The sharpness, brightness, contrast, saturation and chrominance can be adjusted by buttons.

2 Converter Design and Working Principle

The VGA/TV video converter is mainly composed of a power supply circuit, a video conversion circuit and a video conversion control circuit. Its hardware system block diagram is shown in Figure 2. The VGA signal output by the onboard display is input into the VX1937. The VX1937 performs A/D sampling on a frame of VGA progressive scan video signal according to the refresh rate of the input VGA signal, and after buffering, clamping, and caching in the external SDRAM, after odd and even field reorganization and data interpolation processing within the field, it is converted into an analog video signal through D/A at the output end.

2.1 Video front-end processing

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The VGA signal input can be A/D converted at a sampling rate of 27 MHz for each channel of R, G, and B signals through three 8-bit A/D converters. HSYNC and VSYNC are buffered by Schmitt triggers. The conversion reference voltage of the A/D converter can be set by yourself. The designed RGB signal level range is 0-0.85 V, and the given external reference voltage VK cannot be lower than the maximum value of the RGB signal level. Digital video processing is performed using YUV color components. Therefore, the RGB-YUV matrix in VXl937 is used to convert the input RGB signal into a YUV component signal in 4:2:2 format. Among them, the output length of each YUV component is 8 bits.

2.2 Digital Video Coding

The core of VX1937 is the encoder part, which is mainly used to read 4:2:2 format YUV digital video data from external storage devices and then encode it. Since the input signal is divided into two signal components, brightness and chrominance, and the chrominance signal is modulated by a composite digital subcarrier, the brightness and chrominance signals can be interpolated to twice the pixel rate respectively, and converted into analog composite video signals through two 10-bit D/A converters, and then output through the 10-bit converter of the third channel. It is necessary to pay attention to adjusting the VX1937 format control pin, selecting different clock parameters, and setting the output of the video converter to the PAL standard TV signal output compatible with the onboard multi-function display.

2.3 Hardware control part

After the VX1937 is powered on, there will be a default display mode. When the OSD menu button is pressed, the screen shown in Figure 3 will appear. This means that the OSD hardware control function has been enabled, and the red font indicates which item is selected (item 2 in Figure 3). The horizontal and vertical offset, brightness, contrast, color, saturation, sharpness and other functions can be adjusted by pressing the button. Other functions such as picture freeze, picture zoom in, picture zoom out, TV mode, output mode and power saving mode can be controlled by specific pins.

3 System PCB Design

During the design process, considering the adaptability of the equipment to the outside, the following measures are mainly taken: 1) The casing is made of metal aluminum, and the metal part of the casing is well grounded to prevent electromagnetic interference. 2) In order to ensure that the terminals of the ground wires of each component can be connected for a long time and reliably, tinned copper wiring lugs are used. At the same time, the wiring lugs are fixed to the base with screws to ensure long-term good grounding, and the entire component is treated with moisture-proof. 3) The key panel is in good contact with the casing through copper wires to prevent external electromagnetic waves from interfering with the normal operation of the equipment. 4) High and low frequency filter capacitors are connected in parallel at the power output end to avoid the influence of high and low frequency noise of the power supply on components. In PCB design, it is also necessary to pay attention to: the digital ground is separated from the analog ground, and an EMI filter device should be used to reduce the power supply noise of VDDA; the overall circuit pin length from LPF to VXl937 should be less than 0.5 inches; VDDA and VSSA terminals should be connected to 0.1 μF bypass capacitors to reduce noise.

4 Conclusion

The VGA/TV converter is used to convert the video signal output by the microcomputer in the progressive scanning mode, and can obtain the PAL standard TV video signal of the broadcast TV level. In the experiment, a good PAL standard TV video signal was obtained, which has a high application value.