VGA/XGA TV Box Design Based on CS7123 Chip
The CS7123 chip is a high-speed/high-precision video DAC chip independently designed by Shenzhen Xinhai Technology Co., Ltd. It includes three 10-channel current steering (Current Steering) DACs with a maximum sampling speed of 240MHz. The block diagram of the CS7123 is shown in Figure 1. It consists of three high-speed, 10-bit input video DA converters, a standard TTL input, and a complementary output high-impedance analog output current source. It has three independent 10-bit input ports that can operate from a single supply of 5V or a single supply of 3.3V.
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The CS7123 also has two additional video control signals, the SYNC sync signal and the BLANK black level control signal. The CS7123 also has a standby mode. The CS7123 is fabricated in a 5V CMOS process, and the monolithic CMOS structure ensures low power performance. The main function of CS7123 is to convert three RGB signals into analog RGB signals and output them to the display. In addition, it can also be used as a three-way separate DAC.
Figure 1: Block diagram of the CS7123 structure.
The CS7123 is compatible with a variety of high-precision display systems, including RS-343A and RS-170, and can be widely used in HDTV, digital video systems (1600×1200@100Hz), high-resolution color pictures, digital RF modulation, image processing. , medical B-ultrasound equipment, video signal reproduction, etc.
The TV function of the computer is one of the mainstream consumption functions in the personal computer entertainment market. There are currently three kinds of hardware used to receive TV programs through computers or computer monitors. Among them, the most popular features are VGA (for CRT monitors) and XGA (for CRT and LCD monitors) TV boxes. They are easy to install, have clear TV signals, and can be used only with a simple connection to a computer monitor. The usage is basically the same as that of a TV. Figure 2 shows the basic schematic of a VGA/XGA TV box.
In Figure 2, the signal from the tuner is video-decoded to the video processing chip. The video processing chip outputs three RGB digital signals to the video DAC, and then outputs three analog RGB signals to the display. Among them, the speed of the video DAC directly determines the resolution and refresh rate of the output image, and its accuracy determines the quality of the output image. The relationship between the speed and resolution of the DAC can be obtained by the following relationship: clock frequency = point rate = (horizontal resolution) × (vertical resolution) × (refresh rate) / (retracement coefficient); horizontal resolution = line Number of pixels; vertical resolution = number of pixels in the frame.
The refresh rate is the speed required for screen refresh, typically 60 Hz for progressive scan systems and 30 Hz for interlaced scans; the retrace factor must take into account that a particular portion of the total time of each frame is blank. Typical values ​​are, for example, 0.8.
Therefore, if there is a 1024 x 1024@60 Hz image system, the flyback factor is 0.8. Then: clock frequency = point rate = 1024 × 1024 × 60 / 0.8 = 76.8 Hz. The clock frequency of the required DAC is 78.6MHz. The CS7123 has a maximum clock frequency of 240MHz, so it can support resolutions up to 1600×1200@100Hz.
Figure 2: Basic schematic of the VGA/XGA TV box.
Figure 3 shows a typical circuit connection diagram for the CS7123.
When designing the PCB, it should be noted that the CS7123 should be as close as possible to the connector, which reduces spike noise and reflection due to impedance mismatch. In addition, a ground plane should be laid on the signal line to increase the high-frequency Power Supply voltage rejection ratio.
For optimum performance, a source termination resistor is required for each analog output. The magnitude of this resistor is related to the application environment. When the load is a double 75 ohm load, the resistance of this source matching resistor is also 75 ohms. Also note that this resistor should be as close as possible to the chip to reduce reflections.
Summary of this article
At present, the application of digital video is moving towards a large screen and high definition, and the TV box is also developing in this direction. At present, the general VGA TV box supports 800×600@75Hz resolution, while the XGA TV box supports 1024×768@75Hz resolution. The CS7123 has high accuracy and speed, and can support resolutions up to 1600×1200@100Hz. It is suitable for high-definition VGA/XGA TV boxes.
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