Wireless video communications has been widely concerned as the third generation of mobile communication key technologies. But it is a long-distance multi-user service, and the development is difficult, and numerous auxiliary technical support is required. However, near-distance wireless video communications can be implemented with a more mature technology, which can be applied to portions of the non-linked part of video surveillance and close patrol, which is different from video transmission in 3G technology, with easy development, transmission distance, etc. Features. To implement wireless video transmission, the video compression coding to which large compression ratio is applied satisfies the harsking of the wireless transmission rate, and the MPEG-4 encoding is the second generation video coding scheme based on the image content, and the encoding scheme synthesized by the object is also combined. The standard is divided into different video objects (VO) based on the content of the image. During the encoding process, the foreground and background object uses different encoding strategies: the outrageous image compression is as possible to retain the details of the compressed object; the background video object, the high compression ratio encoding strategy is used, while Decoding a new background with other backgrounds. Therefore, it can achieve a large compression ratio video code, not only solve the block effect, but also solve the bandwidth limit problem at the time of wireless transmission, so MPEG-4 is used as the main video coding standard in the wireless communication system.
Circuit System Principle: With a dedicated MPEG-4 encoder, you can get good cost performance, so the ML86410 chip is a better MPEG-4 video compression solution; the NRF24L01 and NRF24LU1 chips are used to achieve wireless transmission, which not only has up to 2 Mbit · S-1 wireless aerial rate, and the latter has a wireless SOC chip with enhanced 8051 kernel and USB2.0 protocol, better solves the connection problem with the PC; the controller uses a widely used low power FPGA To coordinate the wireless transmission of the control video stream and control and mode configuration for OV7620 and ML86410, FPGA is accurately logically guaranteed the reliability of the transmission.
NRF24L01 and its connection circuit
The lower computer's wireless connection uses Nordic Semiconductor's NRF24L01. It is a new single-piece RF transceiver, working in 2.4 ~ 2.5 GHz ISM band, using GFSK modulation, hardware integrated OSI link layer, the highest air rate reaches 2Mbit · s -1, connect the controller using the SPI interface. Functional modules such as built-in frequency synthesizer, power amplifier, crystal oscillator, and modulator combine enhanced Shockburst technology, and its output power and communication channel can be configured by program, with 125 channels. The NRF24L01 power consumption is low, and the operating current is only 9 mA when the power is from -6 DBM; when receiving, the operating current is 12.3 mA. The SPI interface is connected to the controller FPGA, which is responsible for receiving configuration information of each device when booting. Wireless transmission of the video data stream after completion of the configuration. Among them, CE is transmitting and receiving switching pins, and CSN is a chip select, IRQ is an interrupt signal output pin, and the connection circuit diagram is shown in FIG.
NRF24LU1 and its application circuit
NRF24LU1 + is a wireless transmission and reception chip that Nordic introduced with a high-performance RF transceiver with a single-chip USBDongle function, which enables wireless data to USB data forms, which enables connectivity to the PC. NRF24LU1 + contains 1 enhanced 8051MCU kernel, wireless transceiver module, which meets the full-speed USB 2.0 standard device controller. NRF2alu1 + significantly enhances anti-broadband interference and intermodulation distortion (IMD) performance. The external components required by the NRF24LU1 + chip are just a low-cost 16 MHz crystal, decoupling circuit, matching network, and antenna. USB operating power supply voltage range 4.0 ~ 5.25 V.NRF24LU1 + is a single piece of structure, shape size 5 mm × 5 mm. Its highest rate 2 Mbit · S-1, compatible with all NRF24 series chips; use enhanced ShockedBust technology to implement packets Automatic packaging / unpacking and transmission processing; use multicEiver technology to support 6 wireless devices, frequency bands, output energy, and other radio frequency parameters can be programmed by RF registers, and the circuit diagram is shown in Figure 5.
PA and LNA
Single only NRF chip can only be close to wireless communication, to increase the distance of wireless video transmission, it is necessary to add PA. From increasing data communication distance, it is also necessary to increase the reception sensitivity of the chip input, and select the appropriate LNA and the filter. . Designed to use MaxIM's MAX2240 and MAX2644, the connection circuit is shown in Figure 6.
The MAX224O is designed for 2.4 ~ 2.5 GHz bands, complying with Bluetooth, Homerf, 802.11 standards and other FSK modulation systems. The MAX2644 is a low noise amplifier designed for equipment designed for WLAN, Bluetooth, etc. in 2.4 GHz bands. In the magnifying circuit, the microwave switch uses the low-cost SPDT microwave switch produced by Hittite. The switch is in particular 3G and ISM band. The insertion loss is only 0.25 dB, using SOT package, small size, available CMOS Or TTL level control. Control wireless transmission and reception switching with VDD_PA pins of NRF chips.
The FPGA coordination MPEG-4 video streaming to the wireless data stream is used to ensure the performance of wireless video transmission; at the same time, the terminal uses wireless functions and USB2.0 protocol to implement and PC. Connection, not only guarantees the reliability of wireless transmission, but also makes circuit miniaturization and convenient use. Test indicates that the wireless video can be achieved with reliable transmission of wireless video when the wireless rate is less than 2 Mbit · S-1.
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