H.261 Datasheet, PDF (10/31 Pages) List of Unclassifed Manufacturers – LINE TRANSMISSION OF NON-TELEPHONE SIGNALS

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Video multiplex coder
4.1 Data structure
Unless specified otherwise the most significant bit is transmitted first. This is bit 1 and is the leftmost bit in the code tables in
this Recommendation. Unless specified otherwise all unused or spare bits are set to “1”. Spare bits must not be used until
their functions are specified by the CCITT.
4.2 Video multiplex arrangement
The video multiplex is arranged in a hierarchical structure with four layers. From top to bottom the layers are:
– picture;
– Group of blocks (GOB);
– Macroblock (MB);
– Block.
A syntax diagram of the video multiplex coder is shown in Figure 4. Abbreviations are defined in later subclauses.
4.2.1 Picture layer
Data for each picture consists of a picture header followed by data for GOBs. The structure is shown in Figure 5. Picture
headers for dropped pictures are not transmitted.
4.2.1.1 Picture start code (PSC) (20 bits)
A word of 20 bits. Its value is 0000 0000 0000 0001 0000.
4.2.1.2 Temporal reference (TR) (5 bits)
A 5-bit number which can have 32 possible values. It is formed by incrementing its value in the previously transmitted picture
header by one plus the number of non-transmitted pictures (at 29.97 Hz) since that last transmitted one. The arithmetic is
performed with only the five LSBs.
4.2.1.3 Type information (PTYPE) (6 bits)
Information about the complete picture:
Bit 1 Split screen indicator, “0” off, “1” on;
Bit 2
Bit 3
Document camera indicator, “0” off, “1” on;
Freeze picture release, “0” off, “1” on;
Bit 4 Source format, “0” QCIF, “1” CIF;
Bit 5
Bit 6
Optional still image mode HI_RES defined in Annex D; “0” on, “1” off;
Spare.
4.2.1.4 Extra insertion information (PEI) (1 bit)
A bit which when set to “1” signals the presence of the following optional data field.
4.2.1.5 Spare information (PSPARE) (0/8/16 . . . bits)
If PEI is set to “1”, then 9 bits follow consisting of 8 bits of data (PSPARE) and then another PEI bit to indicate if a further 9
bits follow and so on. Encoders must not insert PSPARE until specified by the CCITT. Decoders must be designed to discard
PSPARE if PEI is set to 1. This will allow the CCITT to specify future backward compatible additions in PSPARE.
Recommendation H.261 (03/93)
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