1、视频会议基础知识.ppt

* CIF best obtainable resolution is roughly half of that of the NTSC broadcast standard. This resolution is suitable for group conferences, where each participant takes up only small fraction of the viewing area. The lower quality, QCIF, is limited to only half the resolution of CIF in each dimension. This resolution is suitable for single-person conferencing, where only the head and shoulders of one person are shown. * The TV broadcast of 30 frames per second (fps) produces a perfectly smooth picture. This is important for entertainment but not for videoconferencing. The picture rate, though theoretically allowed to be at the NTSC standard, 30 fps, will normally be lower. This feature provides a wide range of freedom in reducing the transmission requirements. * An added feature is motion compensation, which uses frame-to-frame comparison of blocks that are not only in the same but also in adjacent locations. This way it can be determined in which direction a part of the picture has moved between frames. Motion compensation can significantly improve the quality of the picture. H.261 requires implementation of motion compensation in the decoder section of the codec, but it is optional in the encoder. * At 384 Kbps or above, it is probably feasible to assign 48 Kbps or 64 Kbps to audio, resulting in high quality without imposing undue constraint on video. * The wider the audio bandwidth, the more intelligible and natural speech becomes. The 3-Khz bandwidth for G.711 and G.728 corresponds to toll quality, similar to that experienced in normal analog telephone service, and provides good intelligibility. The 7-Khz bandwidth of G.722 is higher quality and will provide more natural sounding speech but not as good as FM radio or CD quality. * An important feature of the digital audio system is its inherent transmission delay. The different processing to which video and audio signals are subjected basically produces different delays. * * * For the audio track the data rate