| Compared to the digital signal, analog signal has many disadvantages that can not be overcome when it is used to dispose of and transfer the audio. Analog signal is not safe, hard to be controlled and easy to be interfered with. The digitization of the audio signal is the tendency. With the development of the internet, the transfer of the digital audio signal becomes more convenient. It is the direction of development of the audio technology to transfer and exchange the digital audio signal through the internet.Currently, the way of the most audio stream servers to get the audio data is through PC. Some applications get the data in other ways, and then transfer the data to the internet through the PC. In this way, the system is expensive. Further more, only one channel audio can be sampled. With the change of the people's requirement, many applications need to sample the multi channel audio signal simultaneously. So, it is significative practically to develop a multi channel field digital audio stream server system with low cost, low power and high performance.Based on referring to abundant information, a multi channel field digital audio stream server system is designed. The system introduce the technology of CPLD and IP network multicast, and implement the real time sample of four channel analog audio signal simultaneously. After the conversion from analog audio signal to digital audio signal by the sampling rate of 44.1 kHz and the sampling precision of 16 bits, the audio signal is transferred to the play terminal through the Ethernet. The digitization of the transferring makes the audio signal more controllable, safer and more anti-jamming. The system has the function of AGC (Automatic Gain Control), which can stabilize the signal on the stated level according to the change of the input audio signal, preventing the audio of the terminal from changing acutely, and protecting the terminal equipment.The first section of the paper introduces the theory relative to the digital audio stream server system, including the digitization of the audio signal and the TCP/IP protocol used in this paper.The second section introduces the design of the hardware system, including the design of the sampling audio board and network interface board.CPLD is the core of the sampling audio board, which converts the analog audio signal in every channel to digital audio signal, then stores the audio data in the data buffer. The introduction of the technology of CPLD makes the system not only flexible, extensible and easy to update and maintain, but also cabinet, steady, low power, strong in real time. The program of the CPLD is written in VHDL.The network microcontroller DS80C400 is the core of the network interface board, which implement the connection between the sampling board and the Ethernet. The network interface board sends the audio data sampled by the sampling audio board to the multicast address of the network. DS80C400 is a network microcontroller manufactured by Dallas Semiconductor with the 8051 core. It integrates the Ethernet MAC on chip. A full application accessible TCP IPv4/6 network stack is provided in the ROM.The third section of the paper introduces the design of the software system, which is implemented in assemble language. Although the software of the DS80C400 can also be written in Java or C, the software written in Java or C can not meet the need in speed because of the large numbers of the audio data of four channels and the need of the real time.In the end, the schematics and PCB are drawn according to the design of the system. The print board is debugged successfully after it is manufactured. Through tested repeatedly, the system meets the prospective function. |
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