Exfo EXpert IPTV Test Tools (FTB-1 / 2 Pro) Manual de usuario descargar pdf (Pagina 48)

For inter-frame or motion-based coding (P and B frames), motion vectors are determined for

each block and encoded. As for intra-frame coding, errors can render a whole slice or frame

unusable. In simple inter-frame coding systems, the loss of one I or P frame can make all

subsequent frames unusable until the next I frame is received—resulting in a significant

period of degraded, frozen, or blank video.

Note: the H.264

(MPEG-4 AVC) codec

standard introduces

two new frame types,

Figure 2-2. Error

propagation through

frames in a typical

GOP

“Switching I” (SI) and

“Switching P” (SP), which are designed to enable the decoder to more easily switch between

video streams with different bitrates. VQmon/HD reports SI and SP frame metrics when

H.264 is used.

Figure 2-2 shows the impact of encoding errors on various frame types (I, B, and P) in a typical

Group of Pictures (GOP).

In most cases, the standards for video coding provide considerable flexibility to both the

encoder and decoder, allowing a range of cost/performance tradeoffs to be made. This can

make it difficult to precisely assess the impact of network impairments without knowledge of

the exact implementation.

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Exfo EXpert IPTV Test Tools (FTB-1 / 2 Pro) Manual de usuario Pagina 48