Video transmission
Xu Pingyi
- With the continuous development of the Internet, people want to transfer more multimedia information online. The transmission of video signals is the core of multimedia information transmission. The video signal is not problematic in computer terminals, and the key issue is the compression and transmission technology of the signal. For example, the normal television signal of the MPG-1 mode is transmitted. The speed is 120 ~ 140kbit / s; the transmission equivalent to the MPG-2 TV signal corresponding to S-VSH, the rate of 500kbit / s is required; the rate of transmission or higher quality The requirements for bandwidth are high. In addition, video transmission is high for delay and delay jitter. Therefore, video transmission will not only have a wide band, but also have better stability and scalability.
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1. Telephone line transmission video information
- Transport video signals with telephone systems, primarily applied a variety of digital subscriber line XDSL technology. It provides an uninterrupted service that directly access the Internet program. According to ITU G.902, the access network consists of a series of transport entities (such as network facilities and transmission facilities, etc.) between service node interface (SNI) and user network interface (UNI), which is the implementation of telecommunications services. Implementation systems that require transmission capabilities can be configured and managed via management interfaces (Q3). It is actually the information transfer implementation system between the local bureau and the user equipment, which can be partially or all of the traditional user local lines, including complex, cross-continuous and transmission. In principle, the number of SNI and UNIs that access to the network can be implemented, and the number is not limited. The entire communication network is divided into 3 parts: transfer network, switched network, and access network. The latest XDSL technology utilizes the double copper wire of the telephone line to achieve data transmission per second, which is much higher than the normal dial-up Internet and the ISDN adapter.
-ADSL (asymmetric user digital line) is one of XDSL technology. It transmits high quality video signals. The uplink transmission rate is lower than the downlink transmission rate. When the downlink transmission rate is affected by the transmission distance, when the distance is 2.7km, the transmission rate is 8.4 Mbit / s, and it is lowered to 1.54 Mbit / s. The uplink transmission rate is between 16 to 64kbit / s. So ADSL is more suitable for distributed services for on-demand. The key technologies of ADSL are modulation of high-speed channels. There are three types of modulation techniques: orthogonal amplitude modulation (QAM), no carrier amplitude phase modulation (CAP) and discrete multi-frequency (DMT) modulation. DMT provides a higher operating rate compared to QAM, CAP, which is a multi-carrier modulation method. It divides the two-stranded frequency band (1 MHz) of the twisted pair in the telephone network to 256 subchannels, each subchannel bandwidth of 4 kHz. It can dynamically allocate the number of bits that each channel can be carried out according to performance of each subchannel, and turn off the subchannels that cannot carry data, which makes the average transmission rate of the available frequency bands greatly improved. ADSL divides the downstream high-speed channel into four sub-carrier channels AS0-AS3, divided the duplex medium to three subchannel LS0-LS2. The choice of each channel is completed in ADSL initialization. ADSL can provide data rates in accordance with Europe, North American standards, as well as ATMs. When the system is working, the 4-channel down-directional high-speed data signal AS0-AS3 and 3-channel dual-circuit medium speed signal LS0-LS2 is synchronized, and two separate buffer-fast buffers and interleaving buffers are circulated. Code check, quantization, and forward error correction coding (FEC). The data from the interleaving buffer also needs to be interleaved. After the frequency allocation, balance, the two signals are subjected to the constellation coding and gain control, and finally the modulation of the multi-carrier changes (IDFT), and then transition to the channel, then transition to the channel. . The receiving end can restore data by reverse processing. --HDSL (high speed user digital line) is also a kind of XDSL technology. Due to the advanced modulation technology, the requirements for bandwidth are less, and the repeater is not used while transmitting, only two pairs of twists line. HDSL delivers bidirectional symmetrical signals with two pairs of twisted copper wires, and the transmission rate is 168kbit / s, providing 2048kbit / s E1 business. The HDSL unparalleled transmission distance is 3 ~ 5km, more than double the traditional PCM. It has no traditional techniques for the requirements of the twisted copper wire, so the installation is convenient. HDSL uses several coding and modulation methods such as baseband coding mode (PAM), orthogonal amplitude modulation (QAM), no carrier modulation phase adjustment (CAP), etc. to increase transmission quality, extend transmission distance. HDSL is transmitted with multi-pair parallel parallel, and the light of 2 Mbit / s data flow is uniformly transmitted on several pair of twisted copper wires, and then reduced to 2 Mbit / s.
- Currently ADSL and HDSL technology are mature and more. Other RDSL (Adaptive User Digital Line), which dynamically adjusts the access speed according to the quality and transmission distance of the transmission line; VDSL (very high speed user digital line), which is the fastest transmission rate, in a pair of pairs The rays can support the uplink transmission rate of 13 ~ 52 Mbit / s and downlink transmission rates of 1.5 ~ 2.3 Mbit / s, but the transmission distance is only 300 ~ 1 400m, which can support high definition television (HDTV) signals; SDSL (single user digital line) ), Also support T1 line and E1 line, when the transmission distance is within 304m, it can meet the requirements of equal, downlink transmission rate, on the non-load copper line line, support inexpensive 1.1Mbit / s data transmission, and very fast Reaching 2 Mbit / S; IDSL (ISDN User Digital Line) supports the data rate of bidirectional 144kbit / s on the non-load copper phone line, can implement the quality of ISDN on the distance of standard ISDN, very suitable for transmission distances more than 5km . -
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2. Transport video information with wireless technology
- The downstream speed of wireless access is higher than the upstream speed, which can reach 400kbit / s because the user's uplink uses analog modem, and is coupled to the local ISP through standard SLIP / PPP. The actual speed is determined according to the modem rate.
- Another wireless technology is a local domain multi-point distribution service (LDMS), which is a two-way digital broadcast system that is mainly transmitted from the ground equipment. LMDS uses G band, due to high frequency, the transponder of the network station and the network center transmits data. The role of this communication center is to process all path selection, line exchange, and some problems on the Internet. Although LMDS is not a fully swap network, it can establish a virtual point-to-point connection. As other high speed access programs, LMDS is also an asymmetric system. It divides 1GHz bandwidth for 850 MHz for downlink communication, and its remaining 150MHz is used for uplink communication. According to the design requirements, LMDS specifies many users to different channels, and the bandwidth of these channels is approximately between 20 and 40 MHz.
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3. Other available video transmission technology
- Now, protocols and standards related to transfer video data are in improved. For example, ATM can integrate video, data, and audio, and its cell has a length of 53bit, which contains 5BIT's header, for error control, address information, and priority control, etc., the remaining 48bit is used to carry data. Since ATM is a connection-oriented communication method, just like a call, the ATM transmits the contents of the first register to all switches connected to the channel, which is responsible for transmitting each cell to the next node. The data stream itself does not have to consider the problem of path selection, which makes ATM strong scalability. Further, the connection cost of the ATM is determined by the amount of data transmitted, and there is no relationship with the transfer distance. The ATM network can provide with a rate of 25 mbit / s ~ 1 gbit / s. In addition, the ATM switching device can dynamically adjust the data transmission rate between the various devices in a transparent manner, so it is very suitable for point-to-point pair transmission.
- In addition to ATM, there are some other communication protocols available for video networks. Distributed Queuing Double Bus Technology (DQDB) can provide transmission rate of 34 to 140 mbit / s in duplex mode, which uses two bus to transfer data to all connection points, and the terminal node of the bus can generate an empty data package. This package is transmitted to other nodes to query information that needs to be transmitted. DQDB is a non-connected network. The communication rate between each endpoint is different. The length of each cell is also 53bit, where 5bit is address information, which is the same as the length and format of the ATM, so they are each other. The structure is compatible. The dual bus structure increases repetition, so that the stability of DQDB can be greatly improved. In this way, once a channel is completely unreasonable, the system can be reset within several seconds. - Broadband ISDN (B-ISDN) can also be used for video transmission, which can transmit video and other data at a rate higher than 150 Mbit / s. The main technique of B-ISDN is the fiber plus time division multiplex (TDM). In the United States, it is based on (SONET), which is based on synchronous digital grading technology in Europe.
