Transmission Format

Picture Layer

    Data for each picture consists of a picture header followed by data for Group of Blocks, eventually followed by an end-of-sequence code and stuffing bits. The structure is shown in Figure 5.

    PSBI is only present if indicated by CPM. TRB and DBQUANT are only present if PTYPE indicates "PB-frame".Combinations of PSPARE and PEI may not be present. EOS may not be present, while ESTUF may be present only if EOS is present. Picture headers for dropped pictures are not transmitted.
 
Picture Start Code (PSC)
    PSC is a word of 22 bits. Its value is 0000 0000 0000 0000 1 00000. All picture start codes shall be byte aligned. This shall be achieved by inserting PSTUF before the start code such that the first bit of the start code is the first (most significant) bit of a byte.
Temporal Reference (TR)
    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 the previously transmitted one. The arithmetic is performed with only the eight LSBs. In the optional PB-frames mode, TR only addresses P-pictures; for the temporal reference for B-pictures.
Type Information (PTYPE)
    Information about the complete picture:
–     Bit 1: Always "1" in order to avoid start code emulation.
–     Bit 2: Always "0" for distinction with Recommendation H.261.
–     Bit 3: Split screen indicator, "0" off, "1" on.
–     Bit 4: Document camera indicator, "0" off, "1" on.
–     Bit 5: Freeze Picture Release, "0" off, "1" on.
–     Bits 6-8: Source Format, "000" forbidden, "001" sub-QCIF, "010" QCIF,
                     "011" CIF, "100" 4CIF, "101" 16CIF, "110" reserved, "111" reserved.
–     Bit 9: Picture Coding Type, "0" INTRA (I-picture), "1" INTER (P-picture).
–     Bit 10: Optional Unrestricted Motion Vector mode, "0" off, "1" on.
–     Bit 11: Optional Syntax-based Arithmetic Coding mode, "0" off, "1" on.
–     Bit 12: Optional Advanced Prediction mode, "0" off, "1" on.
–     Bit 13: Optional PB-frames mode, "0" normal I- or P-picture, "1" PB-frame.

    Split screen indicator is a signal that indicates that the upper and lower half of the decoded picture could be displayed side by side. This bit has no direct effect on the encoding or decoding of the picture.
    Freeze Picture Release is a signal from an encoder which responds to a request for packet retransmission (if not acknowledged) or fast update request (Reference also Annex C of Recommendation H.263) and allows a decoder to exit from its freeze picture mode and display decoded picture in the normal manner.
    If bits 6-8 indicate a different source format than in the previous picture header, the current picture shall be an I-picture.
Bits 10-13 refer to optional modes that are only used after negotiation between encoder and decoder (see also the Annexes D, E, F and G of Recommendation H.263, respectively). If bit 9 is set to "0", bit 13 shall be set to "0" as well.
 
Quantizer Information (PQUANT)
    A fixed length codeword of 5 bits which indicates the quantizer QUANT to be used for 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.
Continuous Presence Multipoint (CPM)
    A codeword of 1 bit that signals the use of the optional Continuous Presence Multipoint mode (CPM); "0" is off, "1" is on.
Picture Sub-Bit stream Indicator (PSBI)
    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 picture header and all following information until the next Picture or GOB start code.
Temporal Reference for B-picture (TRB)
    TRB is present if PTYPE indicates "PB-frame" and indicates the number of non-transmitted pictures (at 29.97 Hz) since the last P- or I-picture and before the B-picture. The codeword is the natural binary representation of the number of non-transmitted pictures plus one. The maximum number of non-transmitted pictures is 6.
Quantization information for B-pictures (DBQUANT)
    DBQUANT is present if PTYPE indicates "PB-frame". In the decoding process a quantization parameter QUANT is obtained for each macroblock. With PB-frames QUANT is used for the P-block, while for the B-block a different quantization parameter BQUANT is used. QUANT ranges from 1 to 31. DBQUANT indicates the relation between QUANT and BQUANT as defined follow. In this table, "/" means division by truncation. BQUANT ranges from 1 to 31; if the value for BQUANT resulting from the following table  is greater than 31, it is clipped to 31.
 
DBQUANT
BQUANT
00
(5*QUANT)/4
01
(6*QUANT)/4
10
(7*QUANT)/4
11
(8*QUANT)/4
 
Extra Insertion Information (PEI)
    A bit which when set to "1" signals the presence of the following optional data field.
Spare Information (PSPARE)
    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 shall not insert PSPARE until specified by the ITU. Decoders shall be designed to discard PSPARE if PEI is set to 1. This will allow the ITU to specify future backward compatible additions in PSPARE. If PSPARE is followed by PEI = 0, PSPARE = xx000000 is prohibited in order to avoid start code emulation.
Stuffing (ESTUF)
    A codeword of variable length consisting of less than 8 zero-bits. Encoders may insert this codeword directly before an EOS codeword. If ESTUF is present, the last bit of ESTUF shall be the last (least significant) bit of a byte, so that the start of the EOS codeword is byte aligned. Decoders shall be designed to discard ESTUF.
End Of Sequence (EOS)
    A codeword of 22 bits. Its value is 0000 0000 0000 0000 1 11111. It is up to the encoder to insert this codeword or not. EOS may be byte aligned.
Stuffing (PSTUF)
    A codeword of variable length consisting of less than 8 zero-bits. Encoders shall insert this codeword for byte alignment of the next PSC. The last bit of PSTUF shall be the last (least significant) bit of a byte, so that the video bit stream including PSTUF is a multiple of 8 bits from the first bit in the H.263 bit stream. Decoders shall be designed to discard PSTUF.
    If for some reason the encoder stops encoding pictures for a certain time-period and resumes encoding later, PSTUF shall be transmitted before the encoder stops, to prevent that the last up to 7 bits of the previous picture are not sent until the coder resumes coding.