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2.5 Program Stream bitstream requirements2.5.1 Program Stream coding structure and parametersThe ITU T Rec. H.222.0†|†ISO/IEC 13818 Program Stream coding layer allows one program of one or more elementary streams to be combined into a single stream. Data from each elementary stream are multiplexed together with information that allows synchronized presentation of the elementary streams within the program. A Program Stream consists of one or more elementary streams from one program multiplexed together. Audio and video elementary streams consist of access units. Elementary Stream data is carried in PES packets. A PES packet consists of a PES packet header followed by packet data. PES packets are inserted into Program Stream packs. The PES packet header begins with a 32-bit start-code that also identifies the stream (refer to table 2-18 on page 36) to which the packet data belongs. The PES packet header may contain just a presentation timestamp (PTS) or both a presentation timestamp and a decoding timestamp (DTS). The PES packet header also contains other optional fields. The packet data contains a variable number of contiguous bytes from one elementary stream. In a Program Stream, PES packets are organized in packs. A pack commences with a pack header and is followed by zero or more PES packets. The pack header begins with a 32-bit start-code. The pack header is used to store timing and bitrate information. The Program Stream begins with a system header that optionally may be repeated. The system header carries a summary of the system parameters defined in the stream. This standard does not specify the coded data which may be used as part of conditional access systems. The standard does however provide mechanisms for program service providers to transport and identify this data for decoder processing, and to correctly reference data which are specified by the standard. 2.5.2 Program Stream system target decoderThe semantics of the Program Stream and the constraints on these semantics require exact definitions of decoding events and the times at which these events occur. The definitions needed are set out in this Specification using a hypothetical decoder known as the Program Stream system target decoder (P STD). The P STD is a conceptual model used to define these terms precisely and to model the decoding process during the construction of Program Streams. The P STD is defined only for this purpose. Neither the architecture of the P STD nor the timing described precludes uninterrupted, synchronized play-back of Program Streams from a variety of decoders with different architectures or timing schedules.  Figure 2-7 -- Program Stream system target decoder notation
The following notation is used to describe the Program Stream system target decoder and is partially illustrated in Figure 2-7 on page 54above. i, i' are indices to bytes in the Program Stream. The first byte has index 0. j is an index to access units in the elementary streams. k, k',k'' are indices to presentation units in the elementary streams. n is an index to the elementary streams. t(i) indicates the time in seconds at which the ith byte of the Program Stream enters the system target decoder. The value t(0) is an arbitrary constant. SCR(i) is the time encoded in the SCR field measured in units of the 27 MHz system clock where i is the byte index of the final byte of the system_clock_reference_base field. An(j) is the jth access unit in elementary stream n. An(j) is indexed in decoding order. tdn(j) is the decoding time, measured in seconds, in the system target decoder of the jth access unit in elementary stream n. Pn(k) is the kth presentation unit in elementary stream n. Pn(k) is indexed in presentation order. tpn(k) is the presentation time, measured in seconds, in the system target decoder of the kth presentation unit in elementary stream n. t is time measured in seconds. Fn(t) is the fullness, measured in bytes, of the system target decoder input buffer for elementary stream n at time t. Bn the input buffer in the system target decoder for elementary stream n. BSn is the size of the system target decoder input buffer, measured in bytes, for elementary stream n. Dn is the decoder for elementary stream n. On is the reorder buffer for video elementary stream n. 2.5.2.1 System clock frequencyTiming information referenced in P STD is carried by several data fields defined in this Specification. The fields are defined in 2.5.3.3 on page 58, and 2.4.3.6 on page 33. This information is coded as the sampled value of a system clock. The value of the system clock frequency is measured in Hz and shall meet the following constraints: 27 000 000 - 810 <= system_clock_frequency <= 27 000 000 + 810 rate of change of system_clock_frequency with time <= 75 10-3 Hz/s The notation "system_clock_frequency" is used in several places in this standard to refer to the frequency of a clock meeting these requirements. For notational convenience, equations in which SCR, PTS, or DTS appear, lead to values of time which are accurate to some integral multiple of (300233/system_clock_frequency) seconds. This is due to the encoding of SCR timing information as 33 bits of 1/300 of the system clock frequency plus 9 bits for the remainder, and encoding as 33 bits of the system clock frequency divided by 300 for PTS and DTS. 2.5.2.2 Input to the Program Stream system target decoderData from the Program Stream enters the system target decoder. The ith byte enters at time t(i). The time at which this byte enters the system target decoder can be recovered from the input stream by decoding the input system clock reference (SCR) fields and the program_mux_rate field encoded in the pack header. The SCR, as defined in equation 2-18 , is coded in two parts; one, in units the period of 1/300††the system clock frequency, called system_clock_reference_base (equation 2-19), and one, called system_clock_reference_ext equation (equation 2-20), in units of the period of the system clock frequency. In the following the values encoded in these fields are denoted by SCR_base(i) and SCR_ext(i). The value encoded in the SCR field indicates time t(i), where i refers to the byte containing the last bit of the system_clock_reference_base field. Specifically:  (2-18)
where  (2-20)
The input arrival time, t(i), as given in equation 2-21, for all other bytes shall be constructed from SCR(i) and the rate at which data arrives, where the arrival rate within each pack is the value represented in the program_mux_rate field in that pack's header.  (2-21)
where:
i' is the index of the byte containing the last bit of the system_clock_reference_base field in the pack header. i is the index of any byte in the pack, including the pack header. SCR(i') is the time encoded in the system clock reference base and extension fields in units of the system clock. program_mux_rate is a field defined in 2.5.3.3 on page 58. After delivery of the last byte of a pack there may be a time interval during which no bytes are delivered to the input of the P STD. 2.5.2.3 BufferingThe PES packet data from elementary stream n is passed to the input buffer for stream n, Bn. Transfer of byte i from the system target decoder input to Bn is instantaneous, so that byte i enters the buffer for stream n, of size BSn, at time t(i). Bytes present in the pack header, system headers, Program Stream Maps, Program Stream Directories, or PES packet headers of the Program Stream such as SCR, DTS, PTS, and packet_length fields, are not delivered to any of the buffers, but may be used to control the system. The input buffer sizes BS1 through BSn are given by the P STD buffer size parameter in the syntax in equation 2-16 and equation 2-17 on page 43. At the decoding time, tdn(j), all data for the access unit that has been in the buffer longest , An(j), and any stuffing bytes that immediately precede it that are present in the buffer at the time tdn(j),are removed instantaneously at time tdn(j). The decoding time tdn(j) is specified in the DTS or PTS fields. Decoding times tdn(j+1), tdn(j+2), ... of access units without encoded DTS or PTS fields which directly follow access unit j may be derived from information in the elementary stream. Refer to Annex C of ITU T Rec. H.262†|†ISO/IEC 13818-2, ISO/IEC 13818-3, ISO/IEC 11172-2 or ISO/IEC 11172-3. Also refer to 2.7.5 on page 82. As the access unit is removed from the buffer it is instantaneously decoded to a presentation unit. The Program Stream shall be constructed and t(i) shall be chosen so that the input buffers of size BS1 through BSn neither overflow nor underflow in the program system target decoder. That is:  for all t and n
and Fn(t) = 0 instantaneously before t=t(0). Fn(t) is the instantaneous fullness of P STD buffer Bn. An exception to this condition is that the P STD buffer Bn may underflow when the low_delay flag in the video sequence header is set to '1' (refer to 2.4.2.6 on page 20) or when trick_mode status is true (refer to 2.4.3.8 on page 43). For all Program Streams the delay caused by system target decoder input buffering shall be less than or equal to 1 second except for still picture video data. The input buffering delay is the difference in time between a byte entering the input buffer and when it is decoded. Specifically: in the case of no still picture video data then the delay is constrained by 
else in the case of still picture video data the delay is constrained by: 
for all bytes contained in access unit j. For Program Streams, all bytes of each pack shall enter the P STD before any byte of a subsequent pack. When the low_delay flag in the video sequence extension is set to '1' (refer to 6.2.2.3 of ITU T Rec. H.262†|†ISO/IEC 13818-2) the VBV buffer may underflow. In this case when the P STD elementary stream buffer Bn is examined at the time specified by tdn(j), the complete data for the access unit may not be present in the buffer Bn. When this case arises, the buffer shall be re-examined at intervals of two field-periods until the data for the complete access unit is present in the buffer. At this time the entire access unit shall be removed from buffer Bn instantaneously. VBV buffer underflow is allowed to occur continuously without limit. The P STD decoder shall remove access unit data from buffer Bn at the earliest time consistent with the paragraph above and any DTS or PTS values encoded in the bitstream. The decoder may be unable to re-establish correct decoding and display times as indicated by DTS and PTS until the VBV buffer underflow situation ceases and a PTS or DTS is found in the bitstream. 2.5.2.4 PES streamsIt is possible to construct a stream of data as a contiguous stream of PES packets each containing data of the same elementary stream and with the same stream_id. Such a stream is called a PES stream. The PES STD model for a PES stream is identical to that for the Program Stream, with the exception that the Elementary Stream Clock Reference (ESCR) is used in place of the SCR, and ES_rate in place of program_mux_rate. The demultiplexor sends data to only one elementary stream buffer. Buffer sizes BSn in the PES STD model are defined as follows: For ITU T Rec. H.262†|†ISO/IEC 13818-2 video, BSn = VBVmax[profile,level] + BSoh BSoh = (1/750) seconds * Rmax[profile,level], where VBVmax[profile,level] and Rmax[profile,level] are the maximum VBV size and bit rate per profile, level, and layer as defined in tables 8-14 and 8-13, respectively, of ITU T Rec. H.262†|†ISO/IEC 13818-2. BSoh is allocated for PES packet header overhead. For ISO/IEC 11172-2 video, BSn = VBV_max + BSoh BSoh = (1/750) seconds * Rmax, where Rmax and vbv_max refer to the maximum bitrate and maximum vbv_buffer_size for a constrained parameter bitstream in ISO/IEC 11172 2 respectively. For ISO/IEC 11172-3 or ISO/IEC 13818-3 audio, BSn = 2 848 bytes. 2.5.2.5 Decoding and presentationDecoding and presentation in the Program Stream system target decoder are the same as defined for the Transport Stream system target decoder, in 2.4.2.4 on page 19, and 2.4.2.5 on page 20 respectively. |
(2-19)