As the key technology of 3G mobile communication, wireless video communication has been widely concerned. But it is for long-distance multi-user service, development is difficult, need many auxiliary technical support. However, the short-range wireless video communication can be realized by relatively mature technology. It can be applied to the parts of video monitoring that are not easy to connect and the short-range inspection. It is different from the video transmission in 3G technology, and has the characteristics of easy development and short transmission distance. In order to achieve wireless video transmission, it is necessary to apply video compression coding with large compression ratio to meet the stringent requirements of wireless transmission rate. MPEG-4 coding is the second generation video coding scheme based on image content, and the coding scheme of object composition is also combined in the standard, which divides the image into different video objects (VO) according to the content of the image. In the process of coding, foreground and background images adopt different coding strategies: the coding compression of foreground image preserves the details and smoothness of compressed image as much as possible; For the background video object, the coding strategy of large compression ratio is adopted, and other backgrounds are used to compose a new background at the decoder. Therefore, it can realize video coding with large compression ratio, which not only solves the block effect, but also solves the problem of bandwidth limitation in wireless transmission. Therefore, MPEG-4 is regarded as the main video coding standard in wireless communication system.
Circuit system principle:Using dedicated MPEG-4 encoder can get good performance price ratio, so ml86410 chip is a better solution for MPEG-4 video compression; At the same time, nRF24L01 and nrf24lu1 chips are used to realize wireless transmission. They not only have wireless air rate up to 2 Mbit · s-1, but also the latter has Wireless SoC chip with enhanced 8051 core and USB2.0 protocol, which solves the connection problem with PC better; The controller uses the widely used low-power FPGA to coordinate and control the wireless transmission of video stream, as well as the control and mode configuration of OV7620 and ml86410. The accurate logic of FPGA ensures the reliability of transmission.
NRF24L01 and its connecting circuit
NRF24L01 of Nordic semiconductor company is used for wireless connection of lower computer. It is a new type of single chip RF transceiver, working in 2.4 ~ 2.5 GHz ISM band, using GFSK modulation, hardware integration of OSI link layer, the highest air rate of 2Mbit · s-1, using SPI interface to connect the controller. Built in functional modules such as frequency synthesizer, power amplifier, crystal oscillator and modulator, and integrated with enhanced ShockBurst technology, its output power and communication channels can be configured by program, with 125 channels. NRF24L01 has low power consumption, and its working current is only 9 Ma when transmitting at – 6 DBM; When receiving, the working current is 12.3 ma. SPI interface is used to connect with FPGA controller, which is responsible for receiving the configuration information of each device when power on, and wireless sending of video data stream after configuration. Among them, CE is the transmit and receive switching pin, CSN is the chip selection, IRQ is the interrupt signal output pin, and the connection circuit diagram is shown in Figure 4.
Nrf24lu1 and its application circuit
Nrf24lu1 + is a wireless transceiver chip introduced by Nordic, which combines high-performance RF transceiver with single-chip usbdongle function. It can realize the conversion of wireless data to USB data, so as to realize the connection with PC. Nrf24lu1 + includes an enhanced 8051 MCU core, wireless transceiver module and device controller conforming to the full speed USB2.0 standard. Nrf2alu1 + significantly enhances the performance of anti wideband interference and intermodulation distortion (IMD). Nrf24lu1 + chip needs only low-cost 16 MHz crystal oscillator, decoupling circuit, matching network and antenna. The voltage range of USB power supply is 4.0 ~ 5.25 v. Nrf24lu1 + is a monolithic structure with an overall dimension of 5 mm × 5 mm。 Its maximum speed is 2 Mbit · s-1, compatible with all nrf24 series chips; Enhanced shockedbus technology can be used to automatically package / unpack and transmit data packets; Using multiceiver technology, six wireless devices can be supported at the same time. The frequency band, output energy and other RF parameters can be programmed and adjusted through RF registers. The circuit diagram is shown in Figure 5.
PA and LNA
Only NRF chip can be used for short distance wireless communication. In order to increase the distance of wireless video transmission, PA needs to be added. In order to increase the data communication distance, it is necessary to increase the receiving sensitivity of the chip input, so it is necessary to select the appropriate LNA and filter. Max2240 and max2644 of Maxim company are selected in the design, and the connection circuit is shown in Figure 6.
Max224o is designed for 2.4-2.5 GHz band applications and meets the requirements of Bluetooth, HomeRF, 802.11 and other FSK modulation systems Max2644 is a high third order intermodulation point low noise amplifier designed for WLAN, Bluetooth and other devices working in 2.4 GHz band. In the amplifying circuit, the microwave switch adopts the low-cost SPDT microwave switch produced by Hittite company, model hmc545. The switch is specially designed for 3G and ISM band equipment, and its insertion loss is only 0.25 dB. It adopts sot package, small size, and can be controlled by CMOS or TTL level. VDD using NRF chip_ PA pin controls the switch between wireless transmission and reception.
FPGA with reliable logic function is used to coordinate the conversion and transmission of MPEG-4 video stream to wireless data stream, which can ensure the performance of wireless video transmission; At the same time, the terminal uses SOC chip with wireless function and USB2.0 protocol to connect with PC, which not only ensures the reliability of wireless transmission, but also makes the circuit miniaturization and easy to use. The test shows that when the wireless rate is lower than 2 Mbit · s-1, the reliable transmission of wireless video can be realized.