H.261 Datasheet, PDF (13/31 Pages) List of Unclassifed Manufacturers – LINE TRANSMISSION OF NON-TELEPHONE SIGNALS

GBS C
GN
GQU AN T
GEI
G SPA RE
G EI
FIGURE 7/H.261
Structure of group of blocks layer
M B d ata
T 151 42 40- 93/d0 6
FIGURE 7/H.261...[D06] = 3 CM
4.2.2.1 Group of blocks start code (GBSC) (16 bits)
A word of 16 bits, 0000 0000 0000 0001.
4.2.2.2 Group number (GN) (4 bits)
Four bits indicating the position of the group of blocks. The bits are the binary representation of the number in Figure 6.
Group numbers 13, 14 and 15 are reserved for future use. Group number 0 is used in the PSC.
4.2.2.3 Quantizer information (GQUANT) (5 bits)
A fixed length codeword of 5 bits which indicates the quantizer to be used in the group of blocks until overridden by any
subsequent MQUANT. The codewords are the natural binary representations of the values of QUANT (see 4.2.4) which,
being half the step sizes, range from 1 to 31.
4.2.2.4 Extra insertion information (GEI) (1 bit)
A bit which when set to “1” signals the presence of the following optional data field.
4.2.2.5 Spare information (GSPARE) (0/8/16 . . . bits)
If GEI is set to “1”, then 9 bits follow consisting of 8 bits of data (GSPARE) and then another GEI bit to indicate if a further
9 bits follow and so on. Encoders must not insert GSPARE until specified by the CCITT. Decoders must be designed to
discard GSPARE if GEI is set to 1. This will allow the CCITT to specify future “backward” compatible additions in
GSPARE.
NOTE – Emulation of start codes may occur if the future specification of GSPARE has no restrictions on the final GSPARE
data bits.
4.2.3 Macroblock layer
Each GOB is divided into 33 macroblocks as shown in Figure 8. A macroblock relates to 16 pels by 16 lines of Y and the
spatially corresponding 8 pels by 8 lines of each of C_B and C_R.
Data for a macroblock consists of an MB header followed by data for blocks (see Figure 9). MQUANT, MVD and CBP are
present when indicated by MTYPE.
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Recommendation H.261 (03/93)