The MPEG-1 Codec - A Practical Guide to Video and Audio Compression - page 196

Table 10-1 Bit-Stream Syntax Organization

Layer Description

1 Video sequence

2 Group of pictures (GOP)

3 Picture

4 Slice

5 Macroblock

6 Block

7 Sample

similarly in all the MPEG encoders although some naming conventions change. Table 10-1
summarizes the hierarchical nature of a bit stream.
Each of the layers, apart from the individual sample, has a header and an alignment
bit pattern so they can be discerned within a bit stream. These headers carry various frag-
ments of helpful metadata. The aspect ratio, frame rate, and interlacing method are all
described in the header for the sequence.

10.4 The Sequence Structure

There is a nested structure to MPEG with increasingly detailed descriptions of the video
being contained within each layer.
At the outermost level is the sequence. This contains your entire movie or program or
just an individual segment of it. Figure 10-1 shows how a movie is broken into sequences.

10.5 The Group of Pictures

Sequences still contain a lot of video content, so they must be subdivided into smaller
components. The MPEG-1 compression standard describes a series of frames as a group of
pictures (GOP). This GOP will consist of a run of usually more than 10 but less than 30 pic-
tures. Such a structure is also relevant to MPEG-2, MPEG-4, and H.264, the later codecs we
will be discussing.

Frames

Sequence 2

Sequence 1

Figure 10-1 A movie segmented into sequences.