Based On The Low-voltage Distribution Network Transmission, Public Address System (pas)

Public Address System(abbreviated as PAS) is a necessary equipment for public places such as intelligent building, airport, railway station and integrated building and has a wide range of applications. However, current PAS has the disadvantages such as requiring a large amount of wiring, inconvenient system maintenance and inflexible area-dividing mode. The objective of this paper is to utilize low-voltage power line as the transmission medium of PAS and develop a PAS based on transmission over low-voltage power distribution networks, in order to resolve the defects existing in conventional PAS.The paper made a brief introduction to present application status of PAS and brought up the idea of researching on PAS based on transmission over low-voltage power distribution networks. New PAS possesses advantages such as avoiding wiring, convenient maintenance, flexible area-dividing mode and readily expanding. Hindrance to Research is that the using of power line as a communication medium has technical difficulties.System scheme of Speech coding plus Spread Spectrum Communication was presented based on a full analysis of noise characteristic, attenuation characteristic and impedance characteristic of low-voltage power line. Spread Spectrum Carrier(abbreviated as SSC) technology is adopted to overcome problems existing in signal transmission over power line. High quality, low rate MBE compression algorithm was used to complete speech encoding and decoding. Relative products were selected to carry out system's hardware and software design. The successfully developed experimental system was tested to evaluate the data transmission ability and speech quality of the system. Analysis of testing results showes that two factors mainly influence data transmission ability: transmitting signal's amplitude and carrier frequency. Good speech quality is found to have high clarity and naturalness. Experimental results have demonstrated that the scheme is reasonable and can be used for public places addressing. The system requires two improvements: (1) the ability to counter the severe communication condition of low-voltage power line and (2) the ability to transmit music signal.The success of system study has made it possible to launch researches onVIfollowing topic: audio/video system based on the transmission over low-voltage power distribution networks, for example, internal telephone system or picture intercom system.