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MPEG Picture Order

It must be noted that in MPEG the order of the picture in the coded stream is the order in which the decoder process them. The reconstructed frames are not necessarily in the correct form of display. The following example in table 4.7 shows such a case at the encoder output, in the coded bitstream and at the decoder input, and at the decoder output.


 
Table 4.7: MPEG Source, Encoder and Decoder Frame Sequence
At the encoder input                    
12 3 4 5 6 78 9 10 11 12 13
IB B P B B PB B I B B P
At the encoder output                    
14 2 3 7 5 610 8 9 13 11 12
IP B B P B BI B B P B B
At the decoder output                    
12 3 4 5 6 78 9 10 11 12 13

Since the order of pictures at the decoder is not always in the display order, this leads to potential for delays in the encoder/decoder loop This is also true of H.261 - at its highest compression ratio, it may incur as much as 0.5 seconds delay - not very pleasant for interactive use!


next up previous contents
Next: Scaleable Extensions Up: MPEG Previous: MPEG Multiplexing
Jon CROWCROFT
1998-12-03