| With the development of information technology and the popularity of network, the need for supplying high quality video/audio multimedia services grows rapidly. Videoconference is a multimedia communication tech that can achieve the real-time and interactive communication with people who are in deferent places. Apply of Videoconference tech can save customer's time and improve the work efficiency etc. Videoconference tech has a wide applicable scope, and it has a good prospect. During the evolution of videoconference tech, two important trends are: the protocol used is transitioned from H.320 to H.323 and the method of codec is changed from hardware codec to software codec.To acquire an in-depth knowledge of videoconference tech and establish a solid basis for latter correlation research, according to the trend of videoconference development, the author of this article selects H.261 protocol, G.723.1 protocol and RTP protocol for in-depth research and discussion, and then based on these protocols this paper illuminates the realization of videoconference terminal system by software implementation. Videoconference terminal system is mainly composed of four classes: main form class, H261 class, G7231 class and RTP class. H261 and G7231 classes are responsible to video/audio's encoding and decoding. Main form class sends codec request to H261 and G7231 classes and get result from these classes. Through the communication between main form class and RTP class, main form class gets information of videoconference server and information of other terminals. On the basis of implementation of videoconference terminal system, this article puts forward a new method, namely decode-in-two-phases, to improve the speed of bit stream decoding. The basic concept of this method can be used in all decoding systems for decoding vlc table.Decode-in-two-phases method is a tradeoff of the difficulty and the efficiency on the basis of decode-in-one-phase method. The main concept of the decode-in-one-phase method is according to the max length of the code in vlc table designs a structure array, using the bit value of code as the address of the array element that is corresponding to the code and using the run and level represented by the code as the corresponding array element's members. When got x bits from bit stream, it uses the bit value of x bits as the address to get the run and level from structure array.There are two conditions to use decode-in-one-phase method. One is the restriction for the max length of code. It is suitable for vlc tables in which the max length of code is smaller than 11 with a view to difficulty of designing the structure array and the needed space for the structure array. Another is the use rate of the code in vlc table. This rate referes to the number of run and level closes to the number of structure array's element. In the Table 5 (VLC Table for TCOEFF) of H.261 protocol, the max length of code is 14 and the use rate of the code is very low. So decode-in-one-phase method is not suitable for Table 5 of H.261 protocol.Decode-in-two-phases method is a method via executing decode-in-one- phase two times to break the limitation of the decode-in-one-phase method. The working process of decode-in-two-phases method is: it first gets x bit from bit stream, then uses the bit value of x bit as address to get the corresponding array element (for example: sa). According to the value of flag (a member of sa) we can know the first time decode is successful or not. We can also get the value of r(a member of sa) which represents the difference of x and the real length of code . If value of flag is 1, that means the first time decode is successful, then the pointer pointing to the bit stream header move back r bit, then getting the run and level from sa for afterward handling. If value of flag is zero, that means the length of code is x plus r, so we get r bit again from bit stream for decode again. The run and level can be gotten from another structure array. Moreover, the result of x adding r is seen a... |
PDF Full Text Request