通信与信息工程专业英语教程 (14)[19页]

To make the compressed bit stream resilient to transmission errors, one must add redundancy to the stream, so that it is possible to detect and correct errors. Typically, this is done at the channel by using FEC, which operates over coded bit streams generated by a source coder.
Lesson 8 Video Communications
8.1 Overview of Video Communication 8.2 Typical Video Applications 8.3 Communication Networks
A video communication system typically involves five steps
At the receiver end, the received packets are FEC decoded and unpacked, and the resulting bit stream is then input to the video decoder to reconstruct the original video. In practice, many applications embed packetization and channel encoding in the source coder as an adaptation layer to the network.
Networks: PSTN, ISDN, B-ISDN, Internet
Wireless networks, Broadcast channels
code encode encoding encoder packet packetization unpacked packetizer multiplex multiplexing demultiplexing compress redundancy error-free predictive delay jitter real-time buffer synchronization one-way two-way broadcast multicast FEC VLC HDTV MPEG ADSL ATM
Error control: Compressed video streams are vulnerable
Typical applications: Real-time delivery Interactive two-way visual communications
One-way video streaming, One-way video downloading
LAN MAN WAN TCP/IP
A video communication system typically involves five steps, as shown in Figure 8-1. The video is first compressed by a video encoder to reduce the data rate, and the compressed bit stream is then segmented into fixed or variable-length packets and multiplexed with other data types, such as audio. The packets might be sent directly over the network, if the network guarantees bit-error-free transmission. Otherwise, they usually undergo a channel encoding stage, typically using forward error correction (FEC) and interleaving, to protect them from transmission errors.
Error control in video communications is very challenging
Typical video applications
CommunicationLeabharlann Baidunetworks
Five steps: Video source encoding
Multiplexing, packetizing, channel encoding Network Demultiplexing, depacketizing, channel decoding Video source decoding
Error control in video communications is very challenging for several reasons. First, compressed video streams are extremely vulnerable to transmission errors because of the use of temporal predictive coding and variable-length coding (VLC) by the source coder. Due to the use of temporal prediction, a single erroneously recovered sample can lead to errors in the following frames, as illustrated in Figure 8-2.