MPEG-2 Elementary Video

stream structure

Structure of an MPEG-2 program stream.

video streaming:

 
The video stream consists of several consecutive sequences.

Sequence:

 
The sequence can or should contain further sequence headers. Usually before each Group of Picture (GOP). The sequence headers are needed for creating entry points (chapter markers) and for fast scrolling. At the end there should be a sequence decode. However, this does not seem to be common with MPEG-2.

Group of Picture (GOP):

 
The GOP contains the different picture types. There are I, P, B and D frames.

Intra picture (I-frames) are full images. Predicted picture (P frames) are partial pictures and refer to the previous I/P frames. Bidirectional pictures (B frames) are also partial pictures and refer to the preceding and following I/B frame. Direct Coded Picture (D-Frames) are not used in MPEG-2.

Picture:

 
The order of the images does not correspond to the order in which they are displayed. The images are composed of slices.

Slice:

 
Slices are a vertical and horizontal aggregation of macroblocks.

macro block:

 
A macroblock comprises an image section with a total of 16x16 picture elements (pixels). It stores the YUV 4:2:0 (YCbCr) color information. The Y components of all pixels are contained in the first four blocks b0 to b3. In the fifth block b4, the blue chrominance values ​​(Cb) are stored in one point for each 4 pixels. The corresponding red chrominance values ​​(Cr) are in the sixth block b5.

MPEG-2 Elementary Video

Sequence headers

Each header starts with the PACK_START_CODE_PREFIX, which consists of the three bytes 0, 0 and 1. As a hexadecimal number, it is represented as $000001. This is followed by the ID. For the sequence header, this is the value $B3. The length of the header has different lengths according to the matrices used.

Each sequence header represents a possible entry point. This means that a chapter can only be created where there is a sequence header.

It should also happen that sequence headers are not marked with $000001B3 but with $000000B3.

Construction

1. 4 bytes: SEQUENCE_HEADER_CODE = $000001B3
2. 12 Bit: Width - Image width in pixels
3. 12 Bit: Height - image height in pixels
4. 4 bit: Aspect Ratio - aspect ratio
5. 4 bit: frame rate - refresh rate
6. 18 bit: bit rate
7. 1 bit: markers
8. 10 bits: VBV
9. 1 bit: Constrained parameter flag
10. 1 bit: load intra matrix (0) or standard (1)?
11. 64 bytes: Intra matrix if non-standard matrix is ​​used.
12. 1 Bit: Load Non Intra Matrix (0) or Standard (1)?
13. 64 bytes: Non Intra Matrix, unless Standard Matrix is ​​used.

As a scheme:

 
The MPEG type can only be taken directly from the sequence header if the aspect ratio has a value above 4. But then it is an MPEG-1 video.

Explanations

The values ​​for the aspect ratio:

The values ​​for the frame rate (refresh rate):

The bit rate is given in 400 bits per second. The value $3FFFF is supposed to mean a variable bitrate. In the test, TMPGEnc and CCEB each indicated the maximum bit rate there.

The marker bit is intended to detect errors. It must always have the value 1.

The VBV is the memory required to decode the images. It is specified in 16 kB blocks.

The constrained parameter flag is always set to 0 here. The TMPG Encoder 2.5* uses the 10 bits for the VBV and the bit of the constrained parameter flag for the VBV specification, so that only half of the VBV is displayed here.

The intra or non intra matrix load bit indicates whether the standard matrix is ​​used or whether the corresponding matrix is ​​saved at this point and must be loaded. The matrix is ​​stored in Zig Zag Scan.

extensions

At least one extension follows the sequence header. An MPEG-2 is identified via this. The ID of the extension is $B5. This is followed by the ID of the extension. These extensions should be considered for the sequence header:

MPEG-2 Elementary Video

Sequence Extension

Extensions have the ID $B5. This is followed by the ID of the extension, here $1.

Construction

1. 4 bytes: EXTENSION_START_CODE = $000001B5
2. 4 bits: Start Code Identifier - Sequence Extension = $1
3. 4 bit: profile
4. 4 bits: levels
5. 1 bit: progressive sequence
6. 2-bit: chroma format
7. 2 bit: wide extension
8. 2 bits: height extension
9. 12-bit: bit rate extension
10. 1 bit: markers
11. 8-bit: VBV Buffer Extension
12. 1 bit: low delay
13. 2 bits: Frame rate extension numerator
14. 5 bits: Frame rate extension denominator

As a scheme:

 
Note: In a source, four bits are used for the profile. In Andrew Duncan, the first bit of this is called 'Profile/Level Escape', with the value 0 being reserved.

Explanations

The profiles are:

The profiles Multiview and 4:2:2 are sometimes also specified, but without coding.

The levels are:

Allowed combinations of level and profile:

The chrominance values:

Low Delay means that no B-frames are used. "Frame reordering delay is not present i the VBV description, skipped pictures (VBV underflow) may occur." The Framerate Extension Numerator and Denominator are multiplied by the base framerate.