Video Bit Stream Syntax

In order to support different applications, the syntax of video bit stream must be flexible. This is achieved by having a hierarchy of different layers that each start with a header. Each layer performs a different logical function. Most headers can be uniquely identified in the bit stream since they begin with a start code and a start code identifier.
* Sequence and VOL for MPEG-4 Visual
Sequence header and end-of-sequence code. Global parameters are contained in the sequence header and its extension.

* Group of Picture (GOP)
GOP allows random access and interaction with video sequence since GOP is coded independent of each other. For MPEG-4 Visual, it is GVOP and for H.26x, no GOP since they are for realtime interactive applications. The first picture in GOP needs to be coded independently. Closed GOP means the B picture in the end of GOP could not use the I picture of the following GOP as reference. It is only used for video edition.

* Picture
The basic coding unit of a video sequence. It consists of three components: Y, Cb and Cr. The picture header indicates the picture type, picture structure and others. A VOP is the coding unit in MPEG-4 Visual.

* GOB, Slice and Video Packet: Groups of MBs
- GOB has fixed structure with three lines of MB and eleven MBs in one line. H.26x.
- Slice has one line of MB with variable length. Note its definition in MPEG-1 and MPEG-2 is different. DC components of intra MBs would be coded predicatively within one slice and MV for non-intra MBs are coded predicatively within one slice too.
- Video packet is based on the bit rate threshold as defined by the encoder instead of MB.
All these three structures are used to do the resync and error recovery.

* Macro Block (MB)
Eight blocks for 4:2:2 and six blocks for 4:2:0. Basic unit for motion compensation. No start code for MB.

* Block
Transform and entropy coding unit. No start code for blocks.