ISO/IEC 14496-15:2004/Amd 2:2008
(Amendment)Information technology — Coding of audio-visual objects — Part 15: Advanced Video Coding (AVC) file format — Amendment 2: File format support for Scalable Video Coding (SVC)
Information technology — Coding of audio-visual objects — Part 15: Advanced Video Coding (AVC) file format — Amendment 2: File format support for Scalable Video Coding (SVC)
Technologies de l'information — Codage des objets audiovisuels — Partie 15: Format de fichier de codage vidéo avancé (AVC) — Amendement 2: Support de format de fichier pour codage vidéo extensible (SVC)
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Standards Content (Sample)
INTERNATIONAL ISO/IEC
STANDARD 14496-15
First edition
2004-04-15
AMENDMENT 2
2008-07-15
Information technology — Coding of
audio-visual objects —
Part 15:
Advanced Video Coding (AVC) file format
AMENDMENT 2: File format support for
Scalable Video Coding (SVC)
Technologies de l'information — Codage des objets audiovisuels —
Partie 15: Format de fichier de codage vidéo avancé (AVC)
AMENDEMENT 2: Support de format de fichier pour codage vidéo
extensible (SVC)
Reference number
ISO/IEC 14496-15:2004/Amd.2:2008(E)
©
ISO/IEC 2008
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of
ISO or IEC participate in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of technical activity. ISO and IEC
technical committees collaborate in fields of mutual interest. Other international organizations, governmental
and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
Amendment 2 to ISO/IEC 14496-15:2004 was prepared by Joint Technical Committee ISO/IEC JTC 1,
Information Technology, Subcommittee SC 29, Coding of audio, picture, multimedia and hypermedia
information
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
Information technology — Coding of audio-visual objects —
Part 15:
Advanced Video Coding (AVC) file format
AMENDMENT 2: File format support for Scalable Video Coding (SVC)
Add to the Introduction:
This International Standard defines the storage for both plain AVC and SVC video streams, where ‘plain AVC’
refers to the main part of ISO/IEC 14496-10, not including Annex G (Scalable Video Coding), and SVC refers
to ISO/IEC 14496-10 when the techniques in Annex G (Scalable Video Coding) are in use. Specific
techniques are introduced for the handling of scalable streams, enabling their use, and assisting the extraction
of subsets of scalable streams.
Add the following to the end of Clause 1 (Scope):
The file format for storage of SVC content, as defined in Annexes A-E, uses the existing capabilities of the
ISO base media file format and the AVC file format. In addition, the following new extensions to support
SVC-specific features are specified:
• Scalable Grouping: A structuring and grouping mechanism to indicate the association of NAL units
with different types and hierarchy levels of scalability.
• Aggregator: A structure to enable efficient scalable grouping of NAL units by changing irregular
patterns of NAL units into regular patterns of aggregated data units.
• Extractor: A structure to enable efficient extraction of NAL units from other tracks than the one
containing the media data.
• Temporal metadata statements: Structures for storing time-aligned information of media samples.
• AVC Compatibility: A provision for storing an SVC bitstream in an AVC compatible manner, such
that the AVC compatible base layer can be used by any existing AVC file format compliant reader.
Add the following terms (or replace when the item exists) to 3.2 maintaining alphabetical order:
AVC Advanced Video Coding. Where contrasted with SVC in this International Standard, this term
refers to the main part of ISO/IEC 14496-10, not including Annex G (Scalable Video Coding)
FF File Format
ROI Region-Of-Interest
SVC Scalable Video Coding. Refers to ISO/IEC 14496-10 when the techniques in Annex G (Scalable
Video Coding) are in use
VCL Video Coding Layer
Replace the text of 4.1 with the following:
The technologies originally documented in Clause 4 are now defined in ISO/IEC 14496-12:2008 (technically
identical to ISO/IEC 15444-12:2008).
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
Replace the text of 4.2 (file identification) with the following:
See 6.3 in ISO/IEC 14496-12:2008.
Replace the text of 4.3 (independent and disposable samples) with the following:
See 8.6.4 in ISO/IEC 14496-12:2008 for the definition of this box.
Replace the text of 4.4 (sample groups) with the following:
See 8.9 in ISO/IEC 14496-12:2008.
Replace the text of 4.5 (random access recovery points) with the following:
See 10.1 in ISO/IEC 14496-12:2008.
Replace the text of 4.6 (representation of new structures) with the following:
See 8.9.4 in ISO/IEC 14496-12:2008.
Add to the end of 5.3.3 (Track Structure):
A video stream is represented by one or more video tracks in a file.
If there is more than one track representing scalable aspects of a single stream, then they form alternatives to
each other, and the field ‘alternate_group’ should be used, or the composition system used should select
one of them, as appropriate. See 10.3 “Grouping and labelling of tracks” of ISO/IEC 14496-12:2008 for
informative labelling of why tracks are members of alternate groups.
Add this new subclause as 5.3.16 Definition of a sub-sample for AVC:
For the use of the sub-sample information box (7.7 of ISO/IEC 14496-12:2008) in an AVC stream, a sub-
sample is defined as one or more contiguous NAL units within a sample and having the same value of the
following fields; RefPicFlag, RedPicFlag and VclNalUnitFlag. Each sub-sample includes both NAL
unit(s) and their preceding NAL unit length field(s). The presence of this box is optional; however, if present in
a track containing AVC data, it shall have the semantics defined here.
The subsample_priority field shall be set to a value in accordance with the specification of this field in
ISO/IEC 14496-12.
The discardable field shall be set to 1 only if this sample can still be decoded if this sub-sample is discarded
(e.g. the sub-sample consists of an SEI NAL unit, or a redundant coded picture).
The reserved field is defined for AVC as follows:
unsigned int(1) RefPicFlag;
unsigned int(1) RedPicFlag;
unsigned int(1) VclNalUnitFlag;
unsigned int(29) reserved = 0;
RefPicFlag equal to 0 indicates that all the NAL units in the sub-sample have nal_ref_idc equal to 0.
RefPicFlag equal to 1 indicates that all the NAL units in the sub-sample have nal_ref_idc
greater than 0.
RedPicFlag equal to 0 indicates that all the NAL units in the sub-sample have redundant_pic_cnt
equal to 0. RedPicFlag equal to 1 indicates that all the NAL units in the sub-sample have
redundant_pic_cnt greater than 0.
VclNalUnitFlag equal to 0 indicates that all NAL units in the sub-sample are non-VCL NAL units.
Value 1 indicates that all NAL units in the sub-sample are VCL NAL units.
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
Insert the following Annexes A to E:
Annex A
(normative)
SVC elementary stream and sample definitions
A.1 Introduction
This annex specifies the basic storage format of SVC data. It extends the definitions of the storage format of
AVC in Clause 5.
A.2 Terms and definitions
For the purpose of Annex A to Annex E, inclusive, the following terms and definitions apply. The definitions in
ISO/IEC 14496-10 (including Annex G) also apply.
A.2.1
Aggregator
Aggregators are in-stream structures using a NAL unit header including a NAL unit header SVC extension,
with a NAL unit type equal to 30. Aggregators are used to group NAL units belonging to the same sample.
A.2.2
AVC base layer
The AVC base layer is the maximum subset of a scalable bitstream that is AVC compatible – a bitstream not
using any of the functionality of ISO/IEC 14496-10 Annex G. The AVC base layer is represented by AVC VCL
NAL units and associated non-VCL NAL units.
NOTE The AVC base layer itself may be a temporal scalable bitstream.
A.2.3
AVC NAL units
AVC VCL NALs collectively refer to AVC VCL NAL units and their associated non-VCL NAL units in a
bitstream.
A.2.4
AVC VCL NAL unit
AVC VCL NAL units are NAL units with type in the range of 1 to 5 (inclusive).
A.2.5
Extraction path
An extraction path is a set of operations on the original bitstream, each yielding a subset bitstream, ordered
such that the complete bitstream is first in the set, and the base layer is last, and all the bitstreams are in
decreasing complexity (along one of the scalability axes, such as resolution), and where every bitstream is a
valid operating point.
NOTE An extraction path may be represented by the values of priority_id in the NAL unit headers.
Alternatively an extraction path can be represented by the run of tiers or by a set of hierarchically dependent
tracks.
A.2.6
Extractor
Extractors are in-stream structures using a NAL unit header including a NAL unit header SVC extension, with
a NAL unit type equal to 31. Extractors contain instructions on how to extract data from other tracks. Logically
an Extractor can be seen as a ‘link’. While accessing a track containing Extractors, the Extractor is replaced
by the data it is referencing.
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
A.2.7
In-stream
In-stream structures reside within sample data.
A.2.8
Operating point
A subset of a scalable bitstream, representing a particular spatial resolution, temporal resolution, and quality.
Each operating point consists of all the data needed to decode this particular bitstream subset.
NOTE In an SVC stream an operating point can be represented either by (i) specific values of DTQ
(dependency_id, temporal_id and quality_id) or (ii) specific values of P (priority_id) or (iii) combinations of them
(e.g. PDTQ). Note that the usage of priority_id is defined by the application. In an SVC file a track represents one
or more operating points. Within a track tiers may be used to define multiple operating points.
A.2.9
Prefix NAL unit
Prefix NAL units are NAL units with type 14. Prefix NAL units provide scalability information about AVC VCL
NAL units and filler data NAL units. Prefix NAL units do not affect the decoding process of a legacy AVC
decoder. The behaviour of a legacy AVC file reader as a response to prefix NAL units is undefined.
A.2.10
Scalable layer
A scalable layer consists of a set of VCL NAL units with the same values of dependency_id, quality_id, and
temporal_id and the associated non-VCL NAL units. A scalable layer with any of dependency_id, quality_id,
and temporal_id not equal to 0 enhances the video by one or more scalability levels in at least one direction
(temporal, quality or spatial resolution). A scalable layer may also be simply referred to a layer.
NOTE SVC uses a “layered” encoder design which results in a bitstream representing “coding layers”. In
some publications the ‘base layer’ is the first quality layer of a specific coding layer. In some publications the
base layer is the scalable layer with the lowest priority. The SVC file format uses “scalable layer” or “layer” in a
general way for describing nested bitstreams (using terms like AVC base layer or SVC enhancement layer).
A.2.11
Scalable layer representation
A scalable layer representation refers to the bitstream subset that is required for decoding the scalable layer,
and consists of the scalable layer itself and all the scalable layers on which the scalable layer depends. A
scalable layer representation is also referred to as the representation of the scalable layer.
A.2.12
Sub-picture
A sub-picture consists of a proper subset of coded slices of a layer representation.
A.2.13
Sub-picture tier
A sub-picture tier is a tier that consists of sub-pictures. Any coded slice that is not included in the tier
representation of a sub-picture tier shall not be referred to in inter prediction or inter-layer prediction for
decoding of the sub-picture tier.
A.2.14
SVC enhancement layer
An SVC enhancement layer specifies a part of a scalable bitstream that enhances the video. An SVC
enhancement layer is represented by SVC VCL NAL units and the associated non-VCL NAL units and SEI
messages.
NOTE Usually an SVC enhancement layer represents a spatial or coarse-grain scalability (CGS) coding layer
(identified by a specific value of dependency_id).
A.2.15
SVC NAL units
SVC NAL units collectively refer to SVC VCL NAL units and their associated non-VCL NAL units in an SVC
stream.
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
A.2.16
SVC stream
Let the greatest value of dependency_id of all the operating points represented by DTQ (dependency_id,
temporal_id and quality_id) combinations be equal to mDid, and the set of all the operating points with
dependency_id equal to mDid be mOpSet. SVC stream refers to the bitstream represented by the operating
point for which dependency_id is equal to mDid, temporal_id is the greatest temporal_id value among mOpSet,
and quality_id is the greatest quality_id value among mOpSet. The term “SVC stream” is referenced by
‘decoding/accessing the entire stream’ in this document. There may be NAL units which are not required for
decoding this operating point.
A.2.17
SVC VCL NAL unit
SVC VCL NAL units are NAL units with type 20, and NAL units with type 14 when the immediately following
NAL units are AVC VCL NAL units. SVC VCL NAL units do not affect the decoding process of a legacy AVC
decoder. The behaviour of a legacy AVC file reader as a response to SVC VCL NAL units is undefined.
A.2.18
Tier
Tiers define a set of operating points within a track, providing information about the operating points and
instructions on how to access the corresponding bitstream portions (using maps and groups). A tier
represents one or more scalable layers of an SVC bitstream.
NOTE The term “tier” is used to avoid confusion with the frequently used term layer. A tier represents a
subset of a track and represents an operating point of an SVC bitstream. Tiers in a track subset the entire track,
no matter whether the track references another track by extractors.
A.2.19
Tier representation
A tier representation refers to the bitstream subset that is required for decoding the tier, and consists of the
tier itself and all the tiers on which the tier depends. A tier representation is also referred to as the
representation of the tier.
A.3 Elementary stream structure
SVC streams are stored in accordance with subclause 5.1, with the following definition of an SVC video
elementary stream:
• SVC Video Elementary Streams shall contain all video coding related NAL units (i.e. those NAL
units containing video data or signalling video structure) and may contain non-video coding related
NAL units such as SEI messages and access unit delimiter NAL units. Also Aggregators (see B.2) or
Extractors (see B.3) may be present. Aggregators and Extractors shall be processed as defined in this
International Standard (e.g. shall not directly be placed in the output buffer while accessing the file).
Other NAL units that are not expressly prohibited may be present, and if they are unrecognized they
should be ignored (e.g. not placed in the output buffer while accessing the file).
SVC streams may also be stored using associated parameter set streams, if needed.
For SVC streams, Table 1 is updated as follows; only entries where the definition for SVC differs from AVC,
are shown.
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
Table A.1 — NAL Unit Types in SVC and AVC Streams
Value of Description AVC video SVC video Parameter set
nal_unit_type elementary stream elementary stream elementary stream
No
14 Prefix NAL unit in scalable Not specified Yes
extension
prefix_nal_unit_rbsp( )
Yes
15 Subset sequence Not specified No.
parameter set If parameter set
subset_seq_parameter_set elementary stream is
_rbsp( ) not used, Subset
SPS shall be stored
in the Decoder
Specific Information.
No
20 Coded slice in scalable Not specified Yes
extension
slice_layer_in_scalable_ext
ension_rbsp( )
24 – 29 Not specified Not specified Not specified Not specified
30 Aggregator Not specified Yes No
31 Extractor Not specified Yes No
There may be AVC VCL NAL units, SVC VCL NAL units and other NAL units, i.e. non-VCL NAL units, present
in an SVC video elementary stream. Additionally, there may be Aggregator NAL units and Extractor NAL units
present in an SVC video elementary stream.
An AVC VCL NAL unit in an SVC video elementary stream conforming to one or more profiles specified in
Annex G of ISO/IEC 14496-10 shall be immediately preceded by a prefix NAL unit containing the scalability
information for the AVC VCL NAL unit. In this file format an AVC VCL NAL unit and the immediately preceding
prefix NAL unit are logically seen as one NAL unit: the prefix NAL unit provides the scalability information and
the AVC VCL NAL unit provides the NAL unit type and payload.
A.4 Use of the AVC file format
The SVC file format is an extension of the AVC file format defined in this International Standard.
Subclause 5.3.12 is defined for use with plain AVC streams. Its use with SVC streams is deprecated.
A.5 Sample and configuration definition
A.5.1 Introduction
SVC Sample: An SVC sample is also an access unit as defined in subclause 7.4.1.2 of ISO/IEC 14496-10.
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
A.5.2 Canonical order and restrictions
A.5.2.1 Restrictions
The following restrictions apply to SVC data in addition to the requirements in subclause 5.2.2:
• SVC coded slice NAL units (Coded slices in scalable extension): All SVC coded slice NAL units for
a single instant in time shall be contained in the sample whose composition time is that of the picture
represented by the access unit. An SVC sample shall contain at least one AVC or SVC VCL NAL unit.
• Prefix NAL units (Prefix NAL unit in scalable extension): Each prefix NAL unit is placed immediately
before the corresponding AVC VCL NAL unit, providing scalability information about the AVC VCL
NAL unit.
NOTE Prefix NAL units may also be associated with filler data NAL units, which are not present in
elementary streams.
• Aggregators/Extractors: The order of all NAL units included in an Aggregator or referenced by an
Extractor is exactly the decoding order as if these NAL units were present in a ‘plain’ sample. After
processing the Aggregator or the Extractor, all NAL units must be in valid decoding order as specified
in ISO/IEC 14496-10.
A.5.2.2 Decoder configuration record
When the decoder configuration record defined in subclause 5.2.4.1 is used for a stream which can be
interpreted as either an SVC or AVC stream, the AVC decoder configuration record shall reflect the properties
of the AVC compatible base layer, e.g. it shall contain only parameter sets needed for decoding the AVC base
layer.
A parameter set stream may be used with SVC streams, as with AVC streams. In that case, parameter sets
shall not be included in the decoder configuration record.
Sequence parameter sets are numbered in order of storage from 1 to numOfSequenceParameterSets or
numOfPictureParameterSets respectively. Sequence and Picture parameter sets stored in this record in
a file may be referenced using this 1-based index by the InitialParameterSetBox.
The SVCDecoderConfigurationRecord is structurally identical to an
AVCDecoderConfigurationRecord. However, the reserved bits preceding and succeeding the
lengthSizeMinusOne field are re-defined. The syntax is as follows:
aligned(8) class SVCDecoderConfigurationRecord {
unsigned int(8) configurationVersion = 1;
unsigned int(8) AVCProfileIndication;
unsigned int(8) profile_compatibility;
unsigned int(8) AVCLevelIndication;
bit(1) complete_represenation;
bit(5) reserved = ‘11111’b;
unsigned int(2) lengthSizeMinusOne;
bit(1) reserved = ‘0’b;
unsigned int(7) numOfSequenceParameterSets;
for (i=0; i< numOfSequenceParameterSets; i++) {
unsigned int(16) sequenceParameterSetLength ;
bit(8*sequenceParameterSetLength) sequenceParameterSetNALUnit;
}
unsigned int(8) numOfPictureParameterSets;
for (i=0; i< numOfPictureParameterSets; i++) {
unsigned int(16) pictureParameterSetLength;
bit(8*pictureParameterSetLength) pictureParameterSetNALUnit;
}
}
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
The semantics of the fields AVCProfileIndication, profile_compatibility, and
AVCLevelIndication differ from the AVCDecoderConfigurationRecord as follows:
The fields AVCProfileIndication, AVCLevelIndication carry the profile and level indications,
respectively, indicating the profile and level of the entire scalable stream in this track. They, and the
profile_compatibility field, must have values such that a conforming SVC decoder is able to decode
bitstreams conforming to the profile, level and profile compatibility flags indicated in any of the sequence
parameter sets or subset sequence parameter sets contained in this record.
The semantics of other fields are as follows, or are as defined for an AVCDecoderConfigurationRecord:
complete_representation is set on a minimal set of tracks that contain a portion of the original
encoded scalable stream, as defined in A.6.1. Other tracks may be removed from the file without loss
of any portion of the original encoded bitstream, and, once the set of tracks has been reduced to only
those in the complete subset, any further removal of a track removes a portion of the encoded
information.
numOfSequenceParameterSets indicates the number of SPSs and subset SPSs that are used for
decoding the SVC elementary stream. The value of numOfSequenceParameterSets shall be in the
range of 0 to 64, inclusive.
SequenceParameterSetLength indicates the length in bytes of the SPS or subset SPS NAL unit.
SequenceParameterSetNALUnit contains a SPS or subset SPS NAL unit. SPSs shall occur in order
of ascending parameter set identifier with gaps being allowed. Subset SPSs shall occur in order of
ascending parameter set identifier with gaps being allowed. Any SPS shall occur before all the subset
SPSs, if any.
A.6 Derivation from the ISO base media file format
A.6.1 SVC track structure
A scalable video stream is represented by one or more video tracks in a file. Each track represents one or
more operating points of the scalable stream. A scalable stream may, of course, be further thinned, if desired.
There is a minimal set of one or more tracks that, when taken together, contain the complete set of encoded
information. All these tracks shall have the flag “complete_representation” set in all their sample entries.
This group of tracks that form the complete encoded information are called the “complete subset”.
Let the lowest operating point be the one of all the operating points represented by DTQ (dependency_id,
temporal_id and quality_id) combinations that has the least values of dependency_id, temporal_id and
quality_id, respectively. The track that has the flag “complete_representation” set and contains the
lowest operating point shall be nominated as the ‘scalable base track’. All the other tracks that are part of the
same scalable encoded information shall be linked to this base track by means of a track reference of type
‘sbas’ (scalable base). The complete encoded information can be retained when the tracks included in the
“complete subset” are retained; all other tracks shall be extractions, subsets, copies or re-orderings of the
complete subset.
NOTE 1 An alternate group may also include completely independent bitstreams, as well as alternative
operating points of the same bitstream. The SVC tracks in the alternate group must be examined to see how
many scalable base tracks are identified.
NOTE 2 “A scalable bitstream” may require more than one track to represent it (consider a stream with a low-
resolution, low-frame-rate base layer, and a high resolution enhancement layer, and a high frame-rate
enhancement layer, but missing the data for high resolution high frame-rate). However, such a scalable bitstream
is typically a non-conforming bitstream.
All the tracks sharing the same scalable base track must share the same timescale.
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ISO/IEC 14496-15:2004/Amd.2:2008(E)
A.6.2 Data sharing and extraction
Different tracks may logically share data. This sharing can take one of the following two forms:
a) The sample data is copied from one track into another track (and possibly compacted or re-
interleaved with other data, such as audio). This creates larger overall files, but the low bit rate data
may be compacted and/or interleaved with other material, for ease of extraction.
b) There may be instructions on how to perform this copy at the time that the file is read.
For the second case, Extractors (defined in B.3) are used.
A.6.3 SVC video stream definition
A.6.3.1 Sample description name and format
A.6.3.1.1 Definition
Types: ‘avc2’, ‘avcC’, ‘svc1’,’svcC’,’seib’
Container: Sample Table Box (‘stbl’)
Mandatory: Either the avc1, or avc2 or svc1 box is mandatory.
Quantity: One or more sample entries may be present
If an SVC elementary stream contains a usable AVC compatible base layer, then an AVC visual sample entry
(‘avc1’ or ‘avc2’) shall be used. Here, the entry shall contain initially an AVC Configuration Box, possibly
followed by an SVC Configuration Box as defined below. The AVC Configuration Box documents the Profile,
Level and Parameter Set information pertaining to the AVC compatible base layer as defined by the
AVCDecoderConfigurationRecord. The SVC Configuration Box documents the Profile, Level and
Parameter Set information pertaining to the entire stream containing the SVC compatible enhancement layers
as defined by the SVCDecoderConfigurationRecord, stored in the SVCConfigurationBox.
For all sample entries except for ‘svc1’, i.e. ‘avc1’ and ‘avc2’, the width and height fields in the sample entry
document the AVC base layer. For an ‘svc1’ sample entry, the width and height document the resolution
achieved by decoding the entire stream.
If the SVC elementary stream does not contain a usable AVC base layer, then an SVC visual sample entry
(‘svc1’) shall be used. The SVC visual sample entry shall contain an SVC Configuration Box, as defined below.
This includes an SVCDecoderConfigurationRecord, as defined in this International Standard.
The lengthSizeMinusOne field in the SVC and AVC configurations in any given sample entry shall have
the same value.
A priority assignment URI provides the name (in the URI space) of a method used to assign priority_id values.
When it occurs in an AVC or SVC sample entry, exactly one URI shall be present, that documents the
priority_id assignments in the stream. The URI is treated here as a name only; it should be de-referenceable,
though this is not required. File readers may be able to recognize some methods and thereby know what
stream extraction operations based on
...
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