Portable on-site law enforcement recorder based on SPCA1527A

0 Introduction
In modern society, the creation of urban civilization and the development of a harmonious society cannot be separated from the inspection work of law enforcement agencies such as the public security, industry and commerce, and urban management on various social departments. However, disputes and conflicts in law enforcement often occur, which not only affects the image of law enforcement departments, but also is not conducive to creating a harmonious law enforcement environment. The on-site law enforcement recorder designed in this paper is a professional recording device dedicated to law enforcement. It can record the on-site law enforcement of law enforcement personnel in real time, comprehensively and accurately, thereby effectively avoiding malicious complaints, reducing frivolous legal proceedings, and providing effective evidence for the criminal responsibility of violent anti-law behavior. It is suitable for various law enforcement departments such as public security, traffic police, urban management, justice, procuratorate, fire protection, industry and commerce, electricity, water, tobacco, emergency rescue and disaster relief. The
on-site law enforcement recorder designed in this paper adopts an adjustable shoulder-mounted integrated portable structure, with an integrated design of the host and camera. It can be worn on the shoulder or chest, and the camera can be rotated and adjusted 180 degrees. After completing the system settings in advance, the on-site law enforcement situation can be recorded with one-button operation, which does not increase the burden on law enforcement personnel and does not affect their other work. This recorder is suitable for professional law enforcement and can also be promoted to civilian occasions as an audio and video recording device.

1 System Overall Design
The system uses Lingjia SPCA1527A main control chip, which is a professional video and camera chip that can provide high-quality audio and video recording and PMP multimedia playback. Its resolution, frame rate and other parameters are better than other non-professional chips. The main control frequency reaches 800MHz and is embedded with Linux2.6.31-6 operating system. The system runs fast and has reliable performance, meeting the basic requirements of digital video, digital photography, digital recording and other functions of on-site law enforcement recorders.

The industrial-grade OV7725 closed integrated camera module is used, which can be suitable for various high temperature, low temperature, dry, humid and other external environments; the internal dual-channel CMOS image sensor filter is used to meet the needs of clear shooting during the day and at night.
The dynamic random access memory (SDRAM) uses Samsung's 64MB K4S641632K chip, which can be synchronized with the CPU clock, can store dynamic data, and randomly read the specified address data.

2 Main functions and features of the system
2.1 Video acquisition
The system can record audio and video synchronously, or record or record separately. There are multiple modes for video recording, and different resolutions and frame rates can be selected according to needs. The optional modes are: 1) HD 720p mode, 1280 720, 30 frames/second; 2) VGA mode, 640 480, 60 frames/second; 3) D1 mode 720 480, 30 frames/second. The video adopts standard H. 264 encoding, video file output formats include AVI, MOV, ASF, MP4, WMV, etc. The default format is AVI, which can be directly output to any NTSC, PAL display device, and is equipped with a dedicated USB cable to store the video on the PC. The video acquisition process can be carried out in segmented recording, timed recording, loop recording, etc. When taking pictures alone, the photo resolution can reach 5 million pixels. Infrared LEDs can be used to shoot when the low illumination is less than 0.11x.
2.2 Audio acquisition
The audio signal is input by a microphone, and audio acquisition is achieved through an external amplifier and a mono PMW digital-to-analog converter. Adaptive differential pulse code modulation (ADPCM) is used for audio encoding and decoding, and the sampling frequencies are: 8kHz, 11kHz, 16kHz, 22kHz, 32kHz, 44kHz, and 48kHz. The system hardware has an AV/IN port, through which the input signal of any NTSC, PAL audio and video device (CMOS/CCD camera, DVD, TV, etc.) can be connected and recorded, and the sound can also be output to the above devices.
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3 Hardware Design
3.1 Implementation of Audio and Video Recording Function
Lingjia SPCA1527 is a low-cost system-on-chip with a complete digital camera system. In terms of video, it embeds Lingyang's high-quality image processing tools, has high-speed JPEG decoding and a wide range of peripheral interfaces, and realizes low-cost DSC (Dynamic Stability Control). Lingjia SPCA1527 supports many popular CCDs and supports RGB and YCbCr formats in color processing.
This system uses CMOS image sensor OV7725 for video acquisition, which can directly convert optical signals into analog current signals. The current signals are amplified and converted to analog to digital to achieve image acquisition, storage, transmission, processing and reproduction. OV7725 has the advantages of small size, low power consumption, and resistance to shock and vibration. Its overall performance is stable, high sensitivity, low noise, large dynamic range and fast response. It has self-scanning function, small image distortion and no afterimage. OV7725 supports multiple output modes, mainly RGB565/555/444, GRB422 and YCbCr422 formats. The digital video signal collected by OV7725 is transmitted to the SPCA1527 main control chip. The wiring diagram of OV7725 and the main control chip is shown in Figure 2 and Figure 3.

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3.2 Memory
The memory chip K4S641632K is directly connected to SPCA1527A, and the circuit schematics are shown in Figure 2 and Figure 4 respectively.

4 Software Design
The software is based on Linux2.6.31-6 embedded system and implemented in C++ language. The program flow chart is shown in Figure 5.
The program flow is as follows: Turn on the system handheld device, the system first performs self-test and then initializes. The setting time of this process is 2s. If the system buttons are not operated, that is, the video/recording button or the camera button is not pressed, the system automatically enters the main menu. The operator can select functions through the main menu, such as video playback, photo browsing, file deletion, video or photo mode selection, function setting, etc. After turning on the machine, you can also enter the photo mode directly by pressing the corresponding function key, such as the camera button. The photo file is directly stored in the SD or MMC card. When the card is full, the system automatically stops shooting.

5 Conclusion
The on-site law enforcement recorder designed in this paper can meet the basic needs of law enforcement agencies for audio and video recording during on-site law enforcement, and can effectively cooperate with relevant departments in law enforcement and reduce law enforcement disputes. In order to make the system more suitable for practical use, the following functions are planned to be improved in the next development process: adding Bluetooth module to realize wireless intercom function; adding wireless transceiver module to realize wireless Internet access, emergency call, remote monitoring and other functions. This article has a certain reference value for the development of other civilian audio and video recorders, such as vehicle-mounted audio and video recorders.