Data for each Group of Blocks (GOB) consists of a GOB
header followed by data for macroblocks. The structure is shown in Figure
6. Each GOB contains one or more rows of macroblocks. For the first GOB
in each picture (with number 0), no GOB header shall be transmitted. For
all other GOBs, the GOB header may be empty, depending on the encoder strategy.
A decoder can signal the remote encoder to transmit only non-empty GOB
headers by external means, for example Recommendation H.245. GSTUF may
be present when GBSC is present. GN, GFID and GQUANT are present when GBSC
is present. GSBI is present when Continuous Presence Multipoint mode is
on, as indicated in the Picture header.
Stuffing (GSTUF)
A codeword of variable length consisting of less than
8 zero-bits. Encoders may insert this codeword directly before a GBSC codeword.
If GSTUF is present, the last bit of GSTUF shall be the last (least significant)
bit of a byte, so that the start of the GBSC codeword is byte aligned.
Group of Block Start Code (GBSC)
A word of 17 bits. Its value is 0000 0000 0000 0000
1. GOB start codes may be byte aligned.
Group Number (GN)
A fixed length codeword of 5 bits. The bits are the
binary representation of the number of the Group of Blocks. For the GOB
with number 0, the GOB header including GSTUF, GBSC, GN, GSBI, GFID and
GQUANT is empty; group number 0 is used in the PSC. Group number 31 is
used in the EOS and the values from 18 to 30 are reserved for future use
by the ITU.
GOB Sub-Bit stream Indicator (GSBI)
A fixed length codeword of 2 bits that is only present
if Continuous Presence Multipoint mode is indicated by CPM. The codewords
are the natural binary representation of the sub-bit stream number for
the GOB header and all following information until the next Picture or
GOB start code.
GOB Frame ID (GFID)
A fixed length codeword of 2 bits. GFID shall have the
same value in every GOB header of a given picture. Moreover, if PTYPE as
indicated in a picture header is the same as for the previous transmitted
picture, GFID shall have the same value as in that previous picture. However,
if PTYPE in a certain picture header differs from the PTYPE in the previous
transmitted picture header, the value for GFID in that picture shall differ
from the value in the previous picture.
Quantizer Information (GQUANT)
A fixed length codeword of 5 bits which indicates the
quantizer QUANT to be used for the remaining part of the picture until
updated by any subsequent GQUANT or DQUANT. The codewords are the natural
binary representations of the values of QUANT which, being half the stepsizes,
range from 1 to 31.
