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DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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Group of pictures<br />

(GOP header)<br />

the sequence, to enable rapid channel acquisition by<br />

decoders.<br />

The start of each interlaced video sequence establishes<br />

an immutable sequence of field pairs, ordered either<br />

{top, bottom, …}, typical of 480i, or {bottom, top, …},<br />

typical of 576i and 1080i. Within a sequence, any<br />

individual field may be field-coded, and any two adjacent<br />

fields may be frame-coded; however, field parity<br />

must alternate in strict sequence.<br />

The GOP is MPEG’s unit of random access. The GOP<br />

layer is optional in MPEG-2; however, it is a practical<br />

necessity for most applications. A GOP starts with an<br />

I-frame. (Additional I-frames are allowed.) The GOP<br />

header contains SMPTE timecode, and closed GOP and<br />

broken link flags.<br />

A GOP header contains 23 bits of coded SMPTE timecode.<br />

If present, this applies to the first frame of the<br />

GOP (in display order). It is unused within MPEG.<br />

If a GOP is closed, no coded B-frame in the GOP may<br />

reference the first I-frame of the following GOP. This is<br />

inefficient, because the following I-frame ordinarily<br />

contains useful prediction information. If a GOP is open,<br />

or the GOP header is absent, then B-pictures in the<br />

GOP may reference the first I-frame of the following<br />

GOP. To allow editing of an MPEG bitstream, GOPs<br />

must be closed.<br />

A device that splices bitstreams at GOP boundaries can<br />

set broken link; this signals a decoder to invalidate<br />

B-frames immediately following the GOP’s first I-frame.<br />

Picture layer The picture header specifies picture structure (frame,<br />

top field, or bottom field), and picture coding type (I, P,<br />

or B). The picture header can specify quantizer matrices<br />

and quantizer scale type. The VBV delay parameter is<br />

used for buffer management.<br />

Slice layer A slice aggregates macroblocks as you would read, left<br />

to right and top to bottom. No slice crosses the edge of<br />

the picture. All defined profiles have “restricted slice<br />

494 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES

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