關於MPEG2中的影象序列和影象組頭GOP
影象序列
影象序列是由影象組構成的，是隨機存取段落。
sequence_header_code – The sequence_header_code is the bit string ‘000001B3’ in hexadecimal
sequence_end_code – The sequence_end_code is the bit string ‘000001B7’ in hexadecimal.
MPEG-2為了實現隨機訪問，在碼流中會插入repeat_sequence_header,它可以插在I、P幀或GOP前面；假如是場編碼，只能插在第一場前面。
影象組頭
影象組（GOP）是為方便隨機存取而加的，影象組是隨機存取視訊單位
M=1 N=X：IPPP...
M=2 N=X：IBPBP...
M=3 N=X：IBBPBBP...
MPEG-2標準中的GOP語法結構如下：
group_start_code – The group_start_code is the bit string ‘000001B8’ in hexadecimal.
closed_gop – This is a one-bit flag which indicates the nature of the predictions used in the first consecutive B-pictures (if any) immediately following the first coded I-frame following the group of picture header.closed_gop is set to ‘1’ to indicate that these B-pictures have been encoded using only backward prediction or intracoding.
broken_link – This is a one-bit flag which shall be set to ‘0’ during encoding. It is set to ‘1’ to indicate that the first
consecutive B-Pictures (if any) immediately following the first coded I-frame following the group of picture header may not be correctly decoded because the reference frame which is used for prediction is not available (because of the action of editing).A decoder may use this flag to avoid displaying frames that cannot be correctly decoded.
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