Research On Codec Technology Of Free-space Optical Voice Communication System

Optical communication can be divided into wired and wireless communication, and both of which have advantages and disadvantages. Free space optical communication, a branch of wireless optical communication, combines the advantages of wired and wireless optical communication. It not only has a high transmission rate, wide bandwidth and good confidentiality, but also not need to pre-laid communication links and flexible installation. Thus is received widespread attention and research. This article is based on the free space optical communication and voice codec technology theory, using programmable logic device, laser, acousto-optic modulator and photomultiplier tube to build the wireless optical voice communication system.This paper analyzes the free-space optical communications theory, the digitization process of voice signal and the classification of voice codec technology, then selects the A-law and ADPCM codec technology as the research targets. After the determining of codec technology, this paper protocols the overall framework of communication system. The working principles of the key devices were introduced to make comparative analysis and selection. The final plan is to choose a semiconductor with 637 nm wavelength as the laser light source, PMT as the photo detector, acousto-optic modulator as the external modulator to build the voice communication system.In order to ensure the normal operation of the communication system, the correlation signal transmitter and receiver of the voice codec technology are programmed and simulation to analyze whether the function of each sub-module is normal. The key components of the system, such as laser and acousto-optic modulator, are tested simultaneously. The results show that the laser can accept external modulation which is less than 5MHz and internal modulation which is 30 MHz at most. At the same time, this paper tests the acousto-optic modulator, using different modulation types, amplitude and frequency. The results indicate that the modulator needs TTL square wave input signal which is greater than 2.4V to achieve digital modulation.Based on the above design and selection, firstly, this article builds and tests the simple A-law codec technology; secondly, designs and verifies the ADPCM codec technology on the basis of A-law. Then builds a platform for communication system successfully and conducts a number of experiments which artificially receives the decoded signal indoors. After several tests, the MOS value is about 4.2. It can convey the information without distortion, and realize the normal voice communication basically.Finally, a summary has been made, and then analyzes the experimental results and further improvements in the future.