2.2.3.1 Mode A

The H.263 mode A payload header, which consists of 4 bytes, and is present before the actual compression of the H.263 video bitstream in a packet. It allows for fragmentation at group of blocks (GOB) boundaries.


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F

P

SBIT

EBIT

SRC

R

I

A

S

DBQ

TRB

TR

F (1 bit): A flag that indicates the optional PB-frames mode, as defined by H.263. For a mode A packet this value MUST be zero.

P (1 bit): A flag that indicates the optional PB-frames mode, as defined by 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

R (5 bits): This value MUST be zero.

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.

DBQ (2 bits): Differential quantization parameter used to calculate the quantizer for the B frame based on the quantizer for the P frame, when PB-Frames option is used. The PB-Frames option is not supported in H26XPF. This value MUST be zero.

TRB (3 bits): Temporal Reference for the B frame as defined by [H263]. The PB-Frames option is not supported in H26XPF. This value MUST be zero.

TR (1 byte): Temporal Reference for the P frame as defined by [H263]. The PB-Frames option is not supported in H26XPF. This value MUST be ignored.