The network camera has the networking function, and maximizes the function of the analog camera, which is undoubtedly a new star in the field of surveillance. Network cameras generally use the finished CCD camera as the front end of the video capture. The CCD camera accounts for a large part of the hardware cost, and the CCD camera outputs an analog signal. A video A / D converter must be added to the system. The development of CMOS cameras for network cameras is of great significance to reduce design cost and complexity.
1 Introduction to the chip used in the image acquisition system
1.1 Image sensor OV7720
The OV7720 is a highly integrated CMOS Camera Chip sensor that provides all the functions of a VGA image processor on a single chip. A unique feature of the OV7720 is that it has a large main beam angle, which can significantly reduce the height of the module, and the height is the key factor that allows the camera to be assembled into the current ultra-thin laptop; the second highlight of the OV7720 is the ability to work at 60f / s VGA mode, or working in 120 f / s QVGA mode to achieve the best performance of the device. The OV7720 integrates complete control of the image sensor, including exposure control, gamma correction, white balance, color saturation, and color tone control. All image processing functions can be implemented internally through its serial camera control bus (SCCB) interface. Control the programming of the register set. The OV7720 uses proprietary sensor technology to improve image quality, and produces clear, pure, and completely stable color images by reducing or eliminating undesirable factors such as fixed pattern noise, smearing, and floating. The main technical parameters are shown in Table 1. .
1.2 Camera control chip OV529
OV529 is a camera control chip that can convert signals from an image sensor (such as OV7720) into data in a compressed image format. Figure 1 shows the logical relationship between the OV529 and the sensor, host and E2PROM connection. The chip has the following technical features: it can be used in low-cost, low-power, high-resolution single-chip camera controllers in mobile PDA systems and notebook computers; no additional DRAM is required; there is an 8-bit parallel camera chip data interface; built-in Sampling circuit, clamping circuit and window circuit; can be used for VGA, CIF, SIF, QCIF, 160 & TImes; 128, 160 & TImes; 120, 128 & TImes; 128, 80 & TImes; 64 and more image resolution; built-in color conversion circuit can be used for 4 b / 8 b grayscale, 16 b YUV, 12 b / 16 b / 24b RGB image preview; with serial camera control bus, built-in microcontroller, optional serial E2PROM memory, general-purpose I / O pins and 2 host interfaces (RS 232, 5wire), etc.
2 Overall system design
2.1 System composition
The overall design of the system is divided into hardware design and software design. The hardware design module mainly includes a microcontroller module, a network interface module and a camera control module. The microcontroller module is mainly composed of the main control chip, DATAFLASH and SDRAM. The main control chip is the core of the entire system and is responsible for the scheduling of the entire system. Embedded Linux kernel and its file system, application software and system configuration file are solidified in DATAFASH. SDRAM is used as memory for system operation. The network interface module mainly cooperates with the main control chip to transmit the MPEG-4 code stream. The control module of the camera is mainly composed of OV529 and OV7720 chips, and the sensor video stream is transmitted to OV529 through SCCB (Serial Camera Control Bus); OV529 converts the video stream into MEPG-4 code stream by controlling OV7720, and completes the control of the camera command Function. When the system starts, the microcontroller transfers the Linux kernel to SDRAM through SPI, and the system starts from SDRAM.
The software design part mainly includes three parts: embedded Linux transplantation, CGI control program and MEPG-4 decoding program. The embedded Linux system is stored in DAT-AFLASH controlled by AT91SAM7X256, which is responsible for the scheduling of the entire system software. The CGI control program is mainly responsible for the control of the camera and the related settings of the MPEG-4 video stream. The MEPG-4 decoding program is mainly responsible for decoding the MPEG-4 data stream obtained through the network.
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