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2.4 Transport Stream bitstream requirements




2.4.1 Transport Stream coding structure and parameters

The ITU T Rec. H.222.0†|†ISO/IEC 13818 Transport Stream coding layer allows one or more programs 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 a program.


A Transport Stream consists of one or more programs. 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 Transport Stream packets. The first byte of each PES packet header is located at the first available payload location of a Transport Stream packet.
The PES packet header begins with a 32-bit start-code that also identifies the stream or stream type to which the packet data belongs. The PES packet header may contain decoding and presentation time stamps(DTS and PTS). The PES packet header also contains other optional fields. The PES packet data field contains a variable number of contiguous bytes from one elementary stream.
Transport Stream packets begin with a 4 byte prefix, which contains a 13 bit Packet ID (PID), defined in table 2-2 on page 21 . The PID identifies, via the Program Specific Information (PSI) tables, the contents of the data contained in the Transport Stream packet. Transport Stream packets of one PID value carry data of one and only one elementary stream.
The PSI tables are carried in the Transport Stream. There are four PSI tables:
Program Association Table

Program Map Table

Conditional Access Table

Network Information Table


These tables contain the necessary and sufficient information to demultiplex and present programs. The Program Map Table, in table 2-28 on page 50 , specifies, among other information, which PIDs, and therefore which elementary streams are associated to form each program. This table also indicates the PID of the Transport Stream packets which carry the PCR for each program. The Conditional Access Table shall be present if scrambling is employed. The Network Information Table is optional and its contents are not specified by this Recommendation†|†International Standard.
Transport Stream packets may be null packets. Null packets are intended for padding of Transport Streams. They may be inserted or deleted by re-multiplexing processes and, therefore, the delivery of the payload of null packets to the decoder cannot be assumed.
This standard does not specify the coded data which may be used as part of conditional access systems. This Specification does however provide mechanisms for program service providers to transport and identify this data for decoder processing, and to reference correctly data which are specified by this Specification. This type of support is provided both through Transport Stream packet structures and in the conditional access table (refer to table 2-27 on page 49 of the PSI).

2.4.2 Transport Stream system target decoder

The semantics of the Transport Stream specified in 2.4.3 on page 20 and the constraints on these semantics specified in 2.7. on page 81 require exact definitions of byte arrival and decoding events and the times at which these occur. The definitions needed are set out in this Recommendation†|†International Standard using a hypothetical decoder known as the Transport Stream system target decoder (T-STD). Informative Annex D of this Specification contains further explanation of the T-STD.


The T-STD is a conceptual model used to define these terms precisely and to model the decoding process during the construction or verification of Transport Streams. The T-STD is defined only for this purpose. There are three types of decoders in the T-STD: video, audio, and systems. Figure 2-6 illustrates an example. Neither the architecture of the T-STD nor the timing described precludes uninterrupted, synchronized play-back of Transport Streams from a variety of decoders with different architectures or timing schedules.




Figure 2-6 -- Transport Stream system target decoder notation
The following notation is used to describe the Transport Stream system target decoder and is partially illustrated in Figure 2-6 above.
i, i', i" are indices to bytes in the Transport 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.
p is an index to Transport Stream packets in the Transport Stream.
t(i) indicates the time in seconds at which the ith byte of the Transport Stream enters the system target decoder. The value t(0) is an arbitrary constant.
PCR(i) is the time encoded in the PCR field measured in units of the period of the 27 MHz system clock where i is the byte index of the final byte of the program_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) results from decoding An(j). 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 is the main buffer for elementary stream n. It is present only for audio elementary streams.
BSn is the size of buffer, Bn, measured in bytes.
Bsys is the main buffer in the system target decoder for system information for the program that is in the process of being decoded.
BSsys is the size of Bsys, measured in bytes.
MBn is the multiplexing buffer, for elementary stream n. It is present only for video elementary streams.
MBSn is the size of MBn, measured in bytes.
EBn is the elementary stream buffer for elementary stream n. It is present only for video elementary streams.
EBSn is the size of the elementary stream buffer EBn, measured in bytes.
TBsys is the transport buffer for system information for the program that is in the process of being decoded.
TBSsys is the size of TBsys, measured in bytes.
TBn is the transport buffer for elementary stream n.
TBSn is the size of TBn, measured in bytes.
Dsys is the decoder for system information in Program Stream n.
Dn is the decoder for elementary stream n.
On is the reorder buffer for video elementary stream n.
Rsys is the rate at which data are removed from Bsys.
Rxn is the rate at which data are removed from TBn.
Rbxn is the rate at which PES packet payload data are removed from MBn when the leak method is used. Defined only for video elementary streams.
Rbxn(j) is the rate at which PES packet payload data are removed from MBn when the vbv_delay method is used. Defined only for video elementary streams.
Rxsys The rate at which data are removed from TBsys.
Res The video elementary stream rate coded in a sequence header.

2.4.2.1 System clock frequency

Timing information referenced in the T-STD is carried by several data fields defined in this Specification. Refer to 2.4.3.4 on page 23, and 2.4.3.6 on page 33. In PCR fields this information is coded as the sampled value of a program's system clock. The PCR fields are carried in the adaptation field of the Transport Stream packets with a PID value equal to the PCR_PID defined in the TS_program_map_section of the program being decoded.


Practical decoders may reconstruct this clock from these values and their respective arrival times. The following are minimum constraints which apply to the program's system clock frequency as represented by the values of the PCR fields when they are received by a decoder.
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
Note - Sources of coded data should follow a tighter tolerance in order to facilitate compliant operation of consumer recorders and playback equipment.
A program's system_clock_frequency may be more accurate than required. Such improved accuracy may be transmitted to the decoder via the System clock descriptor described in 2.6.20 on page 76.
Bit rates defined in this Specification are measured in terms of system_clock_frequency. For example, a bit rate of 27†000†000 bits per second in the T STD would indicate that one byte of data is transferred every eight(8) cycles of the system clock.
The notation "system_clock_frequency" is used in several places in this Specification to refer to the frequency of a clock meeting these requirements. For notational convenience, equations in which PCR, 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 PCR 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.

2.4.2.2 Input to the Transport Stream system target decoder

Input to the Transport Stream system target decoder (T-STD) is a Transport Stream. A Transport Stream may contain multiple programs with independent time bases. However, the T-STD decodes only one program at a time. In the T-STD model all timing indications refer to the time base of that program.


Data from the Transport Stream enters the T-STD at a piecewise constant rate. The time t(i) at which the ith byte enters the T-STD is defined by decoding the program clock reference (PCR) fields in the input stream, encoded in the Transport Stream packet adaptation field of the program to be decoded and by counting the bytes in the complete Transport Stream between successive PCRs of that program. The PCR field (equation 2-1) is encoded in two parts; one, in units of the period of 1/300 times the system clock frequency, called program_clock_reference_base (equation 2-2), and one in units of the system clock frequency called program_clock_reference_extension (equation 2-3). The values encoded in these are computed by PCR_base(i) (equation 2-2) and PCR_ext(i) (equation 2-3) respectively. The value encoded in the PCR field indicates the time t(i), where i is the index of the byte containing the last bit of the program_clock_reference_base field.

Specifically:
(2-1)
where:

(2-2)



(2-3)


For all other bytes the input arrival time, t(i) shown in equation 2-4 below, is computed from PCR(i") and the transport rate at which data arrive, where the transport rate is determined as the number of bytes in the Transport Stream between the bytes containing the last bit of two successive program_clock_reference_base fields of the same program divided by the difference between the time values encoded in these same two PCR fields.
(2-4)

Where:


i is the index of any byte in the Transport Stream for i"< i < i'.

i'' is the index of the byte containing the last bit of the most recent program_clock_reference_base field applicable to the program being decoded.

PCR(i") is the time encoded in the program clock reference base and extension fields in units of the system clock.
The transport rate is given by
(2-5)

where
i' is the index of the byte containing the last bit of the immediately following program_clock_reference_base field applicable to the program being decoded.

Note: i" i i'
In the case of a timebase discontinuity, indicated by the discontinuity_indicator in the transport packet adaptation field, the definition given in equation 2-4 and equation 2-5 for the time of arrival of bytes at the input to the T-STD is not applicable between the last PCR of the old timebase and the first PCR of the new timebase. In this case the time of arrival of these bytes is determined according to equation 2-4 with the modification that the transport rate used is that applicable between the last and next to last PCR of the old timebase.
A tolerance is specified for the PCR values. The PCR tolerance is defined as the maximum inaccuracy allowed in received PCRs. This inaccuracy may be due to imprecision in the PCR values or to PCR modification during remultiplexing. It does not include errors in packet arrival time due to network jitter or other causes. The PCR tolerance is ±500 ns.
In the T-STD model, the inaccuracy will be reflected as an inaccuracy in the calculated transport rate using equation 2-5.

Transport Streams with multiple programs and variable rate.

Transport Streams may contain multiple programs which have independent time bases. Separate sets of PCRs, as indicated by the respective PCR_PID values, are required for each such independent program, and therefore the PCRs cannot be co-located. The Transport Stream rate is piecewise constant for the program entering the T-STD. Therefore, if the Transport Stream rate is variable it can only vary at the PCRs of the program under consideration. Since the PCRs, and therefore the points in the transport Stream where the rate varies, are not co-located, the rate at which the Transport Stream enters the T-STD would have to differ depending on which program is entering the T-STD. Therefore, it is not possible to construct a consistent T-STD delivery schedule for an entire Transport Stream when that Transport Stream contains multiple programs with independent time bases and the rate of the Transport Stream is variable. It is straightforward, however, to construct constant bit rate Transport Streams with multiple variable rate programs.



2.4.2.3 Buffering

Complete Transport Stream packets containing data from elementary stream n, as indicated by its PID, are passed to the transport buffer for stream n, TBn. This includes duplicate Transport Stream packets and packets with no payload. Transfer of the ith byte from the system target decoder input to TBn is instantaneous, so that the ith byte enters the buffer for stream n, of size TBSn, at time t(i).


All bytes that enter the buffer TBn are removed at the rate Rxn specified below. Bytes which are part of the PES packet or its contents are delivered to the main buffer Bn for audio elementary streams and system data, and to the multiplexing buffer MBn for video elementary streams. Other bytes are not, and may be used to control the system. Duplicate Transport Stream packets are not delivered to Bn, MBn, or Bsys.
The buffer TBn is emptied as follows: when there is no data in TBn , Rxn is equal to zero. Otherwise
for video Rxn = 1,2 x Rmax [profile,level] where
Rmax[profile,level] is specified according to the profile and level which can be found in table 8-13 of ITU T Rec. H.262†|†ISO/IEC 13818-2. Table 8-13 specifies the upper bound of the rate of each elementary video stream within a specific profile and level.
Rxn is equal to 1,2 * Rmax for ISO/IEC 11172-2 constrained parameter video streams, where Rmax refers to the maximum bitrate for a Constrained Parameters bitstream in ISO/IEC 11172-2.
for audio Rxn = 2 x 106 bits per second
for systems data Rxn = 1 x 106 bits per second
Rxn is measured with respect to the system clock frequency.
Complete Transport Stream packets containing system information, for the program selected for decoding, enter the system transport buffer, TBsys, at the Transport Stream rate. These include Transport Stream packets whose PID values are 0 or 1, and all Transport Stream packets identified via the Program Association Table (table 2-25 on page 47 ) as having the program_map_PID value for the selected program. Network Information Table (NIT) data as specified by the NIT PID is not transferred to TBsys.
Bytes are removed from TBsys at the rate Rxsys and delivered to Bsys. Each byte is transferred instantaneously.
Duplicate Transport Stream packets are not delivered to Bsys.
Transport packets which do not enter any TBn or TBsys are discarded.
The transport buffer size is fixed at 512 bytes.
The elementary stream buffer sizes EBS1 through EBSn are defined for video as equal to the vbv_buffer_size as it is carried in the sequence header. Refer to Summary of Constrained Parameters in ISO/IEC 11172-2 and table 8-14 of ITU-T Rec. H.262 | ISO/IEC 13818-2.
The multiplexing buffer size MBS1 through MBSn are defined for video as follows:
For Low and Main level
MBSn=BSmux + BSoh +VBVmax[profile,level] - vbv_buffer_size,
where BSoh, PES packet overhead buffering is defined as:
BSoh = (1/750)seconds x Rmax[profile,level]
and BSmux,, additional multiplex buffering is defined as:
BSmux = 0.004 seconds * Rmax[profile,level]
and where VBVmax[profile,level] is defined in table 8-14 and Rmax[profile,level] is defined in table 8-13 in ITU T Rec. H.262†|†ISO/IEC 13818-2, and vbv buffer size is carried in the sequence header described in 6.2.2 of ITU T Rec. H.262†|†ISO/IEC 13818-2.
For High 1440 and High level

MBSn=BSmux + BSoh


where BSoh is defined as:
BSoh = (1/750)seconds x Rmax[profile,level]
and BSmux is defined as:
BSmux = 0.004 seconds * Rmax[profile,level]
and where Rmax[profile,level] is defined in table 8-13 in ITU T Rec. H.262†|†ISO/IEC 13818-2.
For Constrained Parameters ISO/IEC 11172-2 bitstreams
MBSn=BSmux + BSoh + vbv_max - vbv_buffer_size
where BSoh is defined as:
BSoh = (1/750) seconds x Rmax
and BSmux is defined as:
BSmux = 0.004 seconds * Rmax
and where Rmax and vbv_max refer to the maximum bitrate and the maximum vbv_buffer_size for a Constrained Parameters bitstream in ISO/IEC 11172-2 respectively.
A portion BSmux = 4ms x Rmax[profile,level] of the MBSn is allocated for buffering to allow multiplexing. The remainder is available for BSoh and may also be available for initial multiplexing.
Note - Buffer occupancy by PES packet overhead is directly bounded in PES streams by the P STD which is defined in 2.5.2.4 on page 57. It is possible, but not necessary, to utilize PES streams to construct Transport Streams.

Buffer BSn

The main buffer sizes BS1 through BSn are defined as follows:



Audio

BSn = BSmux + BSdec + BSoh = 3 584 bytes


The size of the access unit decoding buffer BSdec, and the PES packet overhead buffer BSoh are constrained by
BSdec + BSoh 2 848 bytes
A portion (736 bytes) of the 3 584 byte buffer is allocated for buffering to allow multiplexing. The rest, 2†848 bytes, are shared for access unit buffering BSdec, BSoh and additional multiplexing.

Systems

The main buffer Bsys for system data is of size BSsys = 1536 bytes.



Video

For video elementary streams, data is transferred from MBn to EBn using one of two methods: the leak method or the VBV delay method.



Leak method

The leak method transfers data from MBn to EBn using a leak rate Rbx. The leak method is used whenever any of the following is true:



  1. The STD descriptor (refer to 2.6.32 on page 79) for the elementary stream is not present in the Transport Stream,

  2. the STD descriptor is present and the leak_valid flag has a value of '1',

  3. the STD descriptor is present, the leak_valid has a value of '0', and the vbv_delay fields coded in the video stream have the value 0xFFFF, or

  4. trick mode status is true (refer to 2.4.3.6 on page 33).

For Low and Main level

Rbxn = Rmax(profile,level)
For High-1440 and High level
Rbxn = Min {1.05 x Res, Rmax(profile,level)}
For Constrained Parameters bitstream in ISO/IEC 11172-2
Rbxn = 1,2 x Rmax where Rmax is the maximum bit rate for a Constrained Parameters bitstream in ISO/IEC 11172-2.
If there is PES packet payload data in MBn, and buffer EBn is not full, the PES packet payload is transferred from MBn to EBn at a rate equal to Rbx. If EBn is full, data are not removed from MBn. When a byte of data is transferred from MBn to EBn, all PES packet header bytes that are in MBn and immediately precede that byte are instantaneously removed and discarded. When there is no PES packet payload data present in MBn, no data is removed from MBn. All data that enters MBn leaves it. All PES packet payload data bytes enter EBn instantaneously upon leaving MBn.

Vbv_delay method

The vbv delay method specifies precisely the time at which each byte of coded video data is transferred from MBn to EBn, using the vbv_delay values coded in the video elementary stream. The vbv_delay method is used whenever the STD descriptor (refer to 2.6.32 on page 79) for this elementary stream is present in the Transport Stream, the leak_valid flag in the descriptor has the value '0', and vbv_delay fields coded in the video stream are not equal to 0xFFFF. If any vbv_delay values in a video sequence are not equal to 0xFFFF, none of the vbv_delay fields in that sequence shall be equal to 0xFFFF (refer to ISO/IEC 11172-2 and ITU T Rec. H.262†|†ISO/IEC 13818-2).


When the vbv_delay method is used, the final byte of the video picture start code for picture j is transferred from MBn to the EBn at the time tdn(j) - vbv_delay(j), where tdn(j) is the decoding time of picture j, as defined above, and vbv_delay(j) is the delay time, in seconds, indicated by the vbv_delay field of picture j. The transfer of bytes between the final bytes of successive picture start codes (including the final byte of the second start code), into the buffer EBn, is at a piecewise constant rate, Rbx(j), which is specified for each picture j. Specifically, the rate, Rbx(j), of transfer into this buffer is given by:
Rbx(j) = NB(j) / (vbv_delay(j) - vbv_delay(j+1) + tdn(j+1) - tdn(j)) (2-6)

where NB(j) is the number of bytes between the final bytes of the picture start codes (including the final byte of the second start code) of pictures j and j+1, excluding PES packet header bytes.


Note - vbv_delay(j+1) and tdn(j+1) may have values that differ from those normally expected for periodic video display if the low_delay flag in the video sequence extension is set to '1'. It may not be possible to determine the correct values by examination of the bit stream.
The Rbx(j) derived from equation 2-6 shall be less than or equal to Rmax[profile,level] for elementary streams of stream type 0x02 (refer to table 2-29 on page 51), where Rmax[profile,level] is defined in ITU T Rec. H.262†|†ISO/IEC 13818-2, and shall be less than or equal to the maximum bit rate allowed for constrained parameter video elementary streams of stream type 0x01, refer to ISO/IEC 11172 2.
When a byte of data is transferred from MBn to EBn, all PES packet header bytes that are in MBn and immediately precede that byte are instantaneously removed and discarded. All data that enters MBn leaves it. All PES packet payload data bytes enter EBn instantaneously upon leaving MBn.

Removal of access units

For each elementary stream buffer EBn and main buffer Bn 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 (refer to 2.4.3.6 on page 33). 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, or ISO/IEC 11172. Also refer to 2.7.5 on page 82. In the case of audio all PES packet headers that are stored immediately before the access unit or that are embedded within the data of the access unit are removed simultaneously with the removal of the access unit. As the access unit is removed it is instantaneously decoded to a presentation unit.



System data

In the case of system data, data is removed from the main buffer Bsys at a rate of Rsys whenever there is at least 1 byte available in buffer Bsys.
(2-7)

Note - The intention of increasing Rsys in the case of high transport rates is to allow an increased data rate for the Program Specific Information.

Low delay

When the low_delay flag in the video sequence extension is set to '1' (6.2.2.3 of ITU T Rec. H.262†|†ISO/IEC 13818-2) the EBn buffer may underflow. In this case when the T-STD elementary stream buffer EBn is examined at the time specified by tdn (j), the complete data for the access unit may not be present in the buffer EBn. 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 EBn instantaneously. Overflow of buffer EBn shall not occur.
When the low_delay_mode flag is set to '1', EBn underflow is allowed to occur continuously without limit. The T-STD decoder shall remove access unit data from buffer EBn at the earliest time consistent with the paragraph above and any DTS or PTS values encoded in the bit stream. Note that the decoder may be unable to re-establish correct decoding and display times as indicated by DTS and PTS until the EBn buffer underflow situation ceases and a PTS or DTS is found in the bit stream.

Trick mode

When the DSM_trick_mode flag (2.4.3.6 on page 33) is set to '1' in the PES Packet header of a packet containing the start of a B-type video access unit and the trick_mode_control field is set to '001' (slow motion) or '010' (freeze frame), or '100' (slow reverse) the B picture access unit is not removed from the video data buffer EBn until the last time of possibly multiple times that any field of the picture is decoded and presented. Repetition of the presentation of fields and pictures is defined in 2.4.3.8 on page 43 under slow motion, slow reverse, and field_id_cntrl. The access unit is removed instantaneously from EBn at the indicated time, which is dependent on the value of rep_cntrl.
When the DSM_trick_mode flag is set to '1' in the PES packet header of a packet containing the first byte of a picture start code, trick_mode status becomes true when that picture start code in the PES packet is removed from buffer EBn. Trick mode status remains true until a PES packet header is received by the T-STD in which the DSM_trick_mode flag is set to '0' and the first byte of the picture start code after that PES packet header is removed from buffer EBn. When trick mode status is true, the buffer EBn may underflow. All other constraints from normal streams are retained when trick mode status is true.

2.4.2.4 Decoding

Elementary streams buffered in B1 through Bn and EB1 through EBn are decoded instantaneously by decoders D1 through Dn and may be delayed in reorder buffers O1 through On before being presented at the output of the T-STD. Reorder buffers are used only in the case of a video elementary stream when some access units are not carried in presentation order. These access units will need to be reordered before presentation. In particular, if Pn(k) is an I-picture or a P-picture carried before one or more B-pictures, then it must be delayed in the reorder buffer, On, of the T-STD before being presented. Any picture previously stored in On is presented before the current picture can be stored. Pn(k) should be delayed until the next I picture or P picture is decoded. While it is stored in the reorder buffer, the subsequent B-pictures are decoded and presented.


The time at which a presentation unit Pn(k) is presented is tpn(k). For presentation units that do not require reordering delay, tpn(k) is equal to tdn(j) since the access units are decoded instantaneously; this is the case, for example, for B-frames. For presentation units that are delayed, tpn(k) and tdn(j) differ by the time that Pn(k) is delayed in the reorder buffer, which is a multiple of the nominal picture period. Care should be taken to use adequate re-ordering delay from the beginning of video elementary streams to meet the requirements of the entire stream. For example, a stream which initially has only I- and P-pictures but later includes B-pictures should include re-ordering delay starting at the beginning of the stream.
ITU T Rec. H.262†|†ISO/IEC 13818-2 explains reordering of video pictures in greater detail.

2.4.2.5 Presentation

The function of a decoding system is to reconstruct presentation units from compressed data and to present them in a synchronized sequence at the correct presentation times. Although real audio and visual presentation devices generally have finite and different delays and may have additional delays imposed by post-processing or output functions, the system target decoder models these delays as zero.


In the T-STD in Figure 2-6 on page 11 the display of a video presentation unit (a picture) occurs instantaneously at its presentation time, tpn(k).
In the T-STD the output of an audio presentation unit starts at its presentation time, tpn(k), when the decoder instantaneously presents the first sample. Subsequent samples in the presentation unit are presented in sequence at the audio sampling rate.

2.4.2.6 Buffer management

Transport Streams shall be constructed so that conditions defined in this section are satisfied. This section makes use of the notation defined for the System Target Decoder.


TBn and TBsys shall not overflow. TBn and TBsys shall empty at least once every second. Bn shall not overflow nor underflow. Bsys shall not overflow.
EBn shall not underflow except when the low delay flag in the video sequence extension is set to '1' (refer to 6.2.2.3 in ITU T Rec. H.262†|†ISO/IEC 13818-2) or trick_mode status is true.
When the leak method for specifying transfers is in effect, MBn shall not overflow, and shall empty at least once every second. EBn shall not overflow.
When the vbv_delay method for specifying transfers is in effect, MBn shall not overflow nor underflow, and EBn shall not overflow.
The delay of any data through the System Target Decoders buffers shall be less than or equal to one second except for still picture video data. Specifically: tdn(j)-t(i) <= 1 second for all j, and all bytes i in access unit An(j).
For still picture video data, the delay is constrained by tdn(j)-t(i) <= 60 second for all j, and all bytes I in access unit An(j).

Definition of overflow and underflow

Let Fn(t) be the instantaneous fullness of T-STD buffer Bn.

Fn(t)=0 instantaneously before t=t(0) then -.


Overflow does not occur if

for all t and n.
Underflow does not occur if

for all t and n.

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