Carriage of AV1 in MPEG-2 TS

1.1 Modal verbs terminology

In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).

1.2 Definition of mnemonics and syntax function

In the present document the mnemonics, the syntax functions, and the syntax descriptors are to be interpreted as described in [MPEG-2 TS]. The uimsbf and bslbf mnemonics are defined in Section 2.2.6 of [MPEG-2 TS]. The nextbits() function is interpreted as in [MPEG-2 TS].

3.1 General constraints

For AV1 video streams, the following constraints apply:

• An AV1 video stream conforming to a profile defined in Annex A of [AV1] shall be an element of an MPEG-2 program and the stream_type for this elementary stream shall be equal to 0x06 (MPEG-2 PES packets containing private data).
• An AV1 video stream shall have the low overhead byte stream format as defined in [AV1].
• The sequence_header_obu as specified in [AV1], that are necessary for decoding an AV1 video stream shall be present within the elementary stream carrying that AV1 video stream.
• An OBU may contain the obu_size field. For applications that need easy conversion to MP4, using the obu_size field is recommended.
• OBU trailing bits should be limited to byte alignment and should not be used for padding.
• Tile List OBUs shall not be used
• Temporal Delimiters may be removed
• Redundant Frame Headers and Padding OBUs may be used.

In addition, a start code insertion and emulation prevention process shall be performed on the AV1 Bitstream prior to its PES encapsulation. This process is described in section 3.2.

3.2 Start-code based format

Prior to carriage into PES, the AV1 open_bitstream_unit() is encapsulated into ts_open_bitstream_unit(). This is required to provide direct access to OBU through a start-code mechanism inserted prior to each OBU. The following syntax describes how to retrieve the open_bitstream_unit() from the ts_open_bitstream_unit() (tsOBU).

obu_start_code - This value shall be set to 0x000001.

open_bitstream_unit[i] - i-th byte of the AV1 open bitstream unit (As defined in section 5.3 of [AV1]).

It is the responsability of the TS muxer to prevent start code emulation by escaping all the forbidden three-byte sequences using the emulation_prevention_three_byte (always equal to 0x03). The forbidden sequences are defined below.

Within the ts_open_bitstream_unit() payload, the following three-byte sequences shall not occur at any byte-aligned position :

• 0x000000
• 0x000001
• 0x000002

Within the ts_open_bitstream_unit() payload, any four-byte sequence that starts with 0x000003 other than the following sequences shall not occur at any byte-aligned position :

• 0x00000300
• 0x00000301
• 0x00000302
• 0x00000303

3.3 The AV1 Access Unit

An AV1 Access Unit consists of all OBUs, including headers, between the end of the last OBU associated with the previous frame, and the end of the last OBU associated with the current frame. With this definition, an Access Unit sometimes maps with a Decodable Frame Group (DFG) as defined in Annex E of [AV1] and some other times to a Temporal Unit (TU) as defined in [AV1], or both, as illustrated in the figure below. An illustration is provided in the figure below for a group of pictures with frames predicted as follows :

3.4 Use of PES packets

AV1 video encapsulated as defined in clause 4.2 is carried in PES packets as PES_packet_data_bytes, using the stream_id 0xBD (private_stream_id_1).

A PES shall encapsulate one, and only one, AV1 access unit as defined in clause 4.3. All the PES shall have data_alignment_indicator set to 1. Usage of data_stream_alignment_descriptor is not specified and the only allowed alignment_type is 1 (Access unit level).

The highest level that may occur in an AV1 video stream, as well as a profile and tier that the entire stream conforms to, shall be signalled using the AV1 video descriptor.

If an AV1 video descriptor is associated with an AV1 video stream, then this descriptor shall be conveyed in the descriptor loop for the respective elementary stream entry in the program map table. This specification does not specify the presentation of AV1 streams in the context of a program stream.

3.5 Assignment of DTS and PTS

For AV1 video stream multiplexed into [MPEG-2 TS], the decoder_model_info may not be present. If the decoder_model_info is present, then the STD model shall match with the decoder model defined in Annex E of [AV1].

For synchronization and STD management, PTSs and, when appropriate, DTSs are encoded in the header of the PES packet that carries the AV1 video stream data setting the PTS_DTS_flags to '01' or '11'. For PTS and DTS encoding, the constraints and semantics apply as defined in the PES Header and associated constraints on timestamp intervals.

There are cases in AV1 bitstreams where information about a frame is sent multiple times. For example, first to be decoded, and subsequently to be displayed. In the case of a frame being decoded but not displayed, it is desired to assign a valid DTS but without need for a PTS. However, the MPEG2-TS specification prevents a DTS from being transmitted without a PTS. Hence, a PTS is always assigned for AV1 access units and its value is not relevant for frames being decoded but not displayed.

To achieve consistency between the STD model and the buffer model defined in Annex E of [AV1], the following PTS and DTS assignment rules shall be applied :

Note : The ScheduleRemovalTiming[] and PresentationTime[] are defined in the Annex E of [AV1].

3.6 Buffer considerations

3.6.2 T-STD Extensions for AV1

When there is an AV1 video stream in an [MPEG-2 TS] program, the T-STD model as described in the section "Transport stream system target decoder" is extended as specified below.

3.6.2.1 TB_n, MB_n, EB_n buffer management

The following additional notations are used to describe the T-STD extensions and are illustrated in the figure above.

Notation	Definition
t(i)	indicates the time in seconds at which the i-th byte of the transport stream enters the system target decoder
TBn	is the transport buffer for elementary stream n
TBS	is the size of the transport buffer TBn, measured in bytes
MBn	is the multiplexing buffer for elementary stream n
MBSn	is the size of the multiplexing buffer MBn, measured in bytes
EBn	is the elementary stream buffer for the AV1 video stream
EBSn	is the size of the multiplexing buffer MBn, measured in bytes
j	is an index to the AV1 access unit of the AV1 video stream
An(j)	is the j-th access unit of the AV1 video bitstream
tdn (j)	is the decoding time of An(j), measured in seconds, in the system target decoder
Rxn	is the transfer rate from the transport buffer TBn to the multiplex buffer MBn as specified below.
Rbxn	is the transfer rate from the multiplex buffer MBn to the elementary stream buffer EBn as specified below

The following apply:

• There is exactly one transport buffer TB_n for the received AV1 video stream where the size TBS is fixed to 512 bytes.
• There is exactly one multiplexing buffer MB_n for the AV1 video stream, where the size MBS_n of the multiplexing buffer MB is constrained as follows: MBS_n = BS_mux + BS_oh + 0.1 x BufferSize where BS_oh, packet overhead buffering, is defined as: BS_oh = (1/750) seconds × max{ 1100 × BitRate, 2 000 000 bit/s} and BS_mux, additional mutliplex buffering, is defined as: BS_mux = 0.004 seconds ×max{ 1100 × BitRate, 2 000 000 bit/s} BufferSize and BitRate are defined in Annex E of the [AV1]
• There is exactly one elementary stream buffer EB_n for all the elementary streams in the set of received elementary streams associated by hierarchy descriptors, with a total size EBS_n: EBS_n = BufferSize
• Transfer from TB_n to MB_n is applied as follows: When there is no data in TB_n then Rx_n is equal to zero. Otherwise: Rx_n = 1.1 x BitRate
• The leak method shall be used to transfer data from MB_n to EB_n as follows: Rbx_n = 1.1 × BitRate
• The removal of start-code and emulation prevention as defined in section 4.2 is instantaneously performed between MB_n and EB_n.

If there is PES packet payload data in MB_n, and buffer EB_n is not full, the PES packet payload is transferred from MB_n to EB_n at a rate equal to Rbx_n. If EB_n is full, data are not removed from MB_n. When a byte of data is transferred from MB_n to EB_n, all PES packet header bytes that are in MB_n and precede that byte are instantaneously removed and discarded. When there is no PES packet payload data present in MB_n, no data is removed from MB_n. All data that enters MB_n leaves it. All PES packet payload data bytes enter EB_n instantaneously upon leaving MB_n.

3.6.2.2 STD delay

The STD delay of any AV1 video through the system target decoders buffers TB_n, MB_n, and EB_n shall be constrained by td_n(j) – t(i) ≤ 10 seconds for all j, and all bytes i in access unit A_n(j).

3.6.2.3 Buffer management conditions

Transport streams shall be constructed so that the following conditions for buffer management are satisfied:

• Each TB_n shall not overflow and shall be empty at least once every second.
• Each MB_n, EB_n and Buffer Pool shall not overflow.
• EB_n shall not underflow, except when the Operating parameters info syntax has low_delay_mode_flag set to '1'. Underflow of EB_n occurs for AV1 access unit A_n(j) when one or more bytes of A_n(j) are not present in EB_n at the decoding time td_n(j).