2.2.3.2 Mode B

The H.263 mode B payload header, which consists of 8 bytes and starts at the luminance boundaries without the PB-frames option.


0


1


2


3


4


5


6


7


8


9

1
0


1


2


3


4


5


6


7


8


9

2
0


1


2


3


4


5


6


7


8


9

3
0


1

F

P

SBIT

EBIT

SRC

QUANT

I

A

S

GOBN

MBA

HMV1

VMV1

HMV2

VMV2

F (1 bit): The mode of the payload header. This value MUST be one.

P (1 bit): Optional PB-frames mode as defined by the H.263. This value MUST be zero.

SBIT (3 bits): The start bit position, which specifies the number of bits to be ignored in the first data byte, starting with the most significant.

EBIT (3 bits): The end bit position, which specifies the number of bits to be ignored in the last data byte, starting with the least significant.

SRC (3 bits): The source format specifies the resolution of the current picture.

Value

Meaning

1

SQCIF

2

QCIF

3

CIF

QUANT (5 bits): Quantization value for the first macro block (MB) coded at the start of the packet. Set to zero if the packet begins with a GOB header.

I (1 bit): Picture coding type.

Value

Meaning

0

Intercoded.

1

Intracoded.

A (1 bit): This value MUST be zero.

S (1 bit): This value MUST be zero.

GOBN (5 bits): The group of blocks number (GOBN) value is defined by the inherent resolution.

MBA (1 byte): The address within the GOB of the first MB in the packet, counting form zero in scan order.

HMV1 (1 byte): Horizontal vector predictor for the first MB in this packet.

VMV1 (1 byte): Vertical vector predictor for the first MB in this packet.

HMV2 (1 byte): Horizontal motion vector predictors for the third MB in this packet.

VMV2 (1 byte): Vertical motion vector predictors for the third MB in this packet.