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 |
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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.
R (5 bits): This value MUST be zero.
I (1 bit): Picture coding type.
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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.