1 Adaptive Streaming Overview (Informative)

Transcription

1 1 Adaptive Streaming Overview (Informative) Adaptive streaming is enabled by two main components: 1. A media format that can be delivered by an HTTP server as a sequence of Segments, each in response to an HTTP: URL request by a player. 2. A manifest file (Media Presentation Description, MPD) that provides sufficient information to a player so it can make the appropriate URL requests. DASH Adaptive Streaming Standard The MPD describes Media Segments available for download as resources that behave as files or portions of files stored on an HTTP server that are identified by URL and can be downloaded by HTTP protocol. MPEG, 3GPP, and other organizations cooperated to complete an MPEG MPD standard called DASH (Dynamic Adaptive Streaming over HTTP), which is a very broad XML MPD file format that can be profiled for specific media formats and application protocols to make practical implementations possible. DASH is currently at DIS (Draft International Standard) stage, and is subject to small changes until it goes to International Standard ballot (expected in July). DASH Profiles DASH Profiles consist of constraint on both the media format, and the XML MPD format (both syntax and semantics). Since the MPD is basically a description of the media, the file format(s), encryption format(s), elementary stream encoding, indexing information, etc. in the media format determine the behavior that can be expressed by the MPD, and what functionality is required in a player to execute particular behavior (such as seamless bitrate and/or video resolution switching). DASH tries not to specify client behavior or implementation. For instance, it does not specify how a player should select optimal tracks for initial playback, or switch tracks (or multiplexed files) in response to high or low input buffer conditions. In theory, players could be built that could seamlessly switch between any different transport streams (e.g. MPEG-2 TS and ISO Base Media File Format and MP4 multiplexed AVC) and different codecs, given enough decoders, indexing information, processing capabilities, network bandwidth, etc. In practice, media formats like UltraViolet greatly simplify players, allowing them to independently select and synchronize tracks with simple sequencing (concatenation splicing) of independently decodable Segments from alternate tracks, without the bandwidth and processing penalties that results when trying to splice and resynchronize different tracks in overlapping multiplexed Segments. Player expected player behavior and interoperability can be better specified in the semantics of an UltraViolet DASH Profile, but most implementation and optimization remains out of scope of the DASH protocol and media format Profile. The UltraViolet DASH Profile In the case of UltraViolet video on demand (VOD) DASH applications, media may be stored as full length files, each containing a single track. The MPD describes Movie Fragments of each file as Segments in a Representation, and enables a player to execute an application layer protocol to: initialize a streaming session, request a sequence of Segments from multiple tracks (DASH Representations),

2 synchronize playout of multiple tracks to a common timeline, random access media at designated locations on the presentation timeline, switch tracks seamlessly (or otherwise) in response to network conditions or user request, request, or respond to server originated events, such as MPD updates, change of server location, etc. These DASH application protocols rely on HTTP network layer protocol to take advantage of existing HTTP infrastructure of servers, edge networks, load balancing, caching, routing, NAT traversal, stateless protocol, dynamic IP address allocation, scalability, etc. The structure of a media format and the adaptive streaming protocols that can be used with it are closely coupled. For instance, movie fragments in UltraViolet media profiles provide predefined file Segments that are optimized for adaptive streaming. Each UV movie fragment may be addressed as a Segment using the standard structure of the Common Container File format. Because UV stores audio, video, and subtitles in separate movie fragments, tracks may be addressed and combined independently (in contrast to multiplexed files where each movie fragment stream segment includes a mix of samples and metadata from multiple tracks, so those tracks are all downloaded or switched whenever a single track is changed by selecting a segment of a different multiplex file). UV movie fragments are further optimized to be independently decodable and switchable so that fragments from alternate files (e.g. alternate bitrates and resolutions) may be simply concatenated and decoded as a normal track without complicated repacketization, synchronization, splicing, etc. The structure of UltraViolet Common Container Format enables simplification of DASH application protocols. There are two general types of addressing schemes used in DASH: 1. Playlists of URLs in MPDs downloaded from the server 2. Templates in an MPD to allow the client to construct URLs automatically Playlists can be relatively large (one URL for each Segment of each available track), but are necessary where URLs are used with byte ranges, time parameters, or filename parameters that are not uniform or predictable. Playlist schemes may require fetching a new MPD as often as once per Segment duration (e.g. 2 seconds) when new Segments are being added or the MPD might change (e.g. a session is redirected to another server, ended, extended, chained, ads inserted, live encoding, etc.). When servers update MPD playlists on a regular cycle, such as when new Segments are added to a playlist every few seconds, players will tend to converge update requests to the instant the new MPD is published, creating a request storm. Normally most Segment and MPD requests are handled by edge network caches, but HTTP error responses (e.g. 404, etc.) require processing by the origin server, which can deteriorate overall content delivery network (CDN) performance. Templates allow the player to construct Segment URLs using parameter substitution in a string, and anticipate URLs in advance of Segments being encoded where parameters are known in advance. The template scheme works well for both live and VOD content. Parameters such as a sequence number, track identifier, quality or bitrate code, media time, etc. may be inserted in a URL resource name and each Segment treated by the player as a unique file on the server (a server may or may not store Segments as individual files). Note that URL fragment and query parameters may be added to Segment URLs, but will be ignored by generic HTTP servers. Only the resource name and (a single, optional) byte range are normally handled by generic HTTP 1.1 servers.

3 In the case of UltraViolet files, the movie fragment number provides a convenient Segment index parameter already stored and indexed in UV files. Players can find the available tracks listed in the MPD as Representations and Representation Groups (e.g. sets of alternate video tracks of the same content at different bitrates), and simply select the preferred file name based on Representation parameters (e.g. MIME type, language, rating, bitrate, etc.), and increment the Segment number by one until the maximum Segment number listed in the MPD is reached. The information in the MPD plus decoding and decryption information included in each track fragment are sufficient to initialize media playback without having to request an Initialization Segment allowed in the DASH protocol (The UV Streaming Media Format is called Self-initializing in DASH terminology). For each new Segment request, the player can determine if a higher or lower bitrate should be selected based on buffer level and other heuristics it may use, and change the root of the template name accordingly, using the same sequential Segment/movie fragment number. The MPD provides an index of Segment durations for tracks where all Segments are not exactly the same duration (similar to the mfra box in the file, but run length encoded) so that exact media time-to-segment number random access and synchronization can be performed on all tracks. If Segment durations are identical or relatively consistent, then the Segment/fragment number for any media time during a presentation can be found or approximated simply by dividing that media time by Segment duration. UltraViolet Streaming Media Profile UltraViolet Streaming Media Profile Track Files are defined in Annex E of the CFF Media Format Specification. Each Track File contains a single track that is conformant to PD, SD, or HD Media Profiles, with minor exceptions. Track Files containing tracks of the same type (i.e. audio, video, or subtitles), same duration, for the same synchronized program, etc. are called Alternate Track Files. All Track Files are described as Partial Representations and are assigned to non-zero Representation Groups. (Group zero is reserved for multitrack files, where tracks can not be independently combined and switched.) Alternate Track Files that have time aligned movie fragments and compatible encoding, encryption, etc. constitute Switchable Track File Sets. Switchable Track File Sets can be represented by a single RepresentationGroup in an MPD and used for seamless switching during playback. The primary use of seamless track switching is to adapt video bitrate by selecting between tracks and track fragments encoded at different bitrates. 2 The UV Adaptive Streaming Profile of MPEG DASH (Normative) MPEG standardized a general protocol for HTTP streaming of media files or streams called DASH (Dynamic Adaptive Streaming over HTTP). DASH specifies an XML Media Presentation Description (MPD) document format that provides clients with sufficient information to download file segments using HTTP requests, to select between available segments encoded at different bitrates in response to network throughput, and maintain continuous playback while doing so. This specification defines a profile of DASH, which is a schema subset, specific protocol options, and semantics optimized for the UltraViolet Adaptive Streaming Media Profile specified in Appendix E of the DECE Media Format specification. 3 Definitions and Acronyms DASH Dynamic Adaptive Streaming over HTTP, MPEG standard XXX (currently Draft International Standard stage). Streaming performed by players making frequent HTTP requests to download short segments of video that are decoded as a continuous presentation. MPD Media Presentation Description

4 Adaptive Streaming Continuous internet delivery and playback of video while periodically adjusting media bitrate in order to adjust to network throughput without interrupting playback. Segment A short file or segment of a file that can be requested by URL. In UV Profile, a Segment is refers to a movie fragment as defined in the CFF specification. Media Segment Contains audio, video, or subtitles; as opposed to an Initialization Segment, which is defined in other MPD Profiles to include the head of a file, including a moov box. Representation XML representation of a sequence of Segments and their URL addresses. Representation Group One or more Representations. In UV Profile, a single Track File or a Switchable Track File Set. Period The duration of a media presentation and its Representations. An MPD may contain multiple sequential Periods, such as a sequence of shows or commercials. 4 Normative References [DASH] DASH DIS [CFF] CFF Annex D, UltraViolet Adaptive Streaming Media Profile 5 The UV Profile of DASH For presentations conforming to the DASH UV Profile, the DASH MPD profiles attribute SHALL contain the following string: {NOTE: place holder} This profile places constraints on both the MPD schema and protocols, and the media files that may be referenced, which SHALL be constrained to the CFF Adaptive Streaming Media Profile [CFF, Appendix E] The following UV Profile specification specifies constraints on the DASH specification and describes the features used as they apply to the Adaptive Streaming Media Profile Track Files and movie fragments. Protection schemes UV Profile DASH MPD referencing encrypted content SHALL include at least one ContentProtection element for the cenc Common Encryption protection scheme including a SchemeInformtion element of the following form: <ContentProtection schmeiduri="urn:mpeg:mpegb:dash:mp4protection:cenc"> <SchemeInformation> <cencversion>0</cencversion> <defaultivsize>8</defaultivsize> <defaultalgorithmid>1</defaultalgorithmid> <defaultkid>9d03c19f-5c c </defaultkid> <defaultvideokid>7b03c19e-4c c </defaultvideokid> </SchemeInformation> </ContentProtection> cencversion SHALL be equal to the version cenc version stored in the schm box. defaultivsize, defaultalgorithmid, defaultkid, and defaultvideokid SHALL be equal to the values in

5 tenc boxes. If different KIDs are used for audio and video, then the KID of the video track SHALL be placed in the optional defaultvideokid attribute. ContentProtection elements for one or more DRM protection schemes MAY be included to assist in acquiring licenses prior to requesting Media Segments. Each DRM system can define one or more schemeiduri attribute URI strings and required or optional use of the SchemeInformation element. The following is an example ContentProtection element for PlayReady DRM: <ContentProtection schmeiduri="urn:microsoft.com:playready:<systemid>"> <SchemeInformation> <licenseurl> LicenseServer.com/example </licenseurl> <psshpayload> (Base64 encoded contents of a pssh box matching SystemID) </psshpayload> </SchemeInformation> </ContentProtection> SystemID SHALL be equal to the UUID value in a PlayReady pssh box. licenseurl SHALL be the URL of a PlayReady license server or server locator where a DRM license may be acquired. psshpayload optionally contains the contents of a PlayReady pssh box for the Adaptive Streaming File Set referenced by the MPD, which may be used for license acquisition or other DRM functions. Media Presentation Description features The following features of DASH MPDs and adaptive streaming protocols are constrained by the UltraViolet profile defined herein for use with media conforming to the Adaptive Streaming Media Profile defined in CFF Annex E.[CFF] Media Segment features: DASH Media Segments reference CFF movie fragments, as specified in Section 2.X in [CFF] Media Segments are self-initializing (no initialization segment is used in the MPD), Segments are independently decodable (start with RAP, contain necessary decoder configuration parameters and decryption information, etc.) Segments are randomly accessible using movie fragment indexes, fragment numbes or fragment time using a Representation SegmentTimeline or fixed duration Segments are independently selectable and switchable by Partial Representation and corresponding track, so different audio, video, and subtitle tracks may be used in any combination (normally limited to one of each media type) and changed individually and independently at Segment intervals Representation features: DASH Representations reference CFF Track Files All CFF Track Files are defined as Partial Representations, so one each separate audio, video, and subtitle Representations must be selected by a player to form a complete Presentation. Multitrack Representations with Group = 0 are not allowed.

6 Segments from alternate Representations of the same Switching Track Set in the same Representation Group may be sequenced by simple concatenation to produce valid tracks and seamlessly decodable elementary streams Corresponding Segments in different Representations in the same Representation Group SHALL have the same duration (they are time aligned and bitstreamswitchable ) Segments may have different durations within a track using a SegmentTimeline, but a Representation Group may only have one SegmentTimeline. Segments in different Representation Groups may have different durations (e.g. audio track Segment durations different from video track Segment durations, or a different audio Group) Signaling for content protection, accessibility, rating, and viewpoint annotations on Representations Switching between Representations in different Groups may or may not result in muting, etc. depending on differences in time alignment, codecs, device capabilities (e.g. multiple decoders), etc. A device MAY request more than one representation of a media type simultaneously e.g. stereo for headphones plus multichannel bitstream for external surround sound. Left and right video streams would be another example. Requesting multiple streams of a type requires that a device has sufficient bandwidth and multiple decoders available. File features: Track Files described by Representations in the same Representation Group SHALL share the same CFF TrackID, timebase, track duration, encryption keys (KID), display size (if video), codec and channels (if audio). sidx, ssix, tfad, and styp boxes in Segment data are not specified in CFF Adaptive Streaming Media Profile, and may be ignored if present tfdt is required in each traf to provide accumulated basemediadecodetime for each fragment. MPD features: This DASH Profile is intended for video on demand applications, not live encoding and streaming, multicast tuning, etc., although the protocols selected are compatible with those applications Periodic download of MPDs to check for updates is not required Segment URL templates SHALL be used for Segment address generation Fragment number or fragment time parameters may be used in Segment URL templates (SegmentTimeline allows players to translate between duration and fragment number for tracks with variable Segment durations) An MPD Period SHALL be equal to, or shorter than the duration of the tracks referenced within it Multiple CFF files may be sequenced in a single MPD as multiple Periods

7 XLink is not used in order to reduce device complexity, and not needed because large playlists are not used (only small templates) Support for source diversity using multiple BaseURL elements Media Presentation Description subset The following list shows the MPD elements and attributes required for this profile along with any value or cardinality constraints. type (must be OnDemand ) profiles (must contain UV profile URI) mediapresentationduration minbuffertime BaseURL ProgramInformation moreinformationurl Title Source Copyright Period id start duration minbuffertime segmentalignmentflag (must be true for UV) bitstreamswitchingflag (must be true for UV) SegmentInfoDefault duration startindex sourceurltemplateperiod BaseURL SegmentTimeline (must be present for variable duration track fragments in a track) Group (must be at least one) group width height parx pary lang mimetype startwithrap (must be true ) framerate numberofchannels samplingrate minbandwidth maxbandwidth minwidth maxwidth minheight maxheight minframerate maxframerate

8 ContentProtection Accessibility Rating Viewpoint MultipleViews stereoid FramePacking SegmentInfoDefault duration startindex sourceurltemplateperiod BaseURL SegmentTimeline Representation id bandwidth group width height parx pary lang mimetype startwithrap (must be true ) framerate numberofchannels samplingrate ContentProtection Accessibility Rating Viewpoint SegmentInfo duration startindex BaseURL SegmentTimeline UrlTemplate Media Presentation Description Schema Subset The following complete MPD schema shows those attributes and elements that are not used by the UV profile as marked out: <?xml version="1.0" encoding="utf-8"?> <xs:schema targetnamespace="urn:mpeg:mpegb:schema:dash:mpd:dis2011" attributeformdefault="unqualified" elementformdefault="qualified" xmlns:xs=" xmlns:xlink=" xmlns="urn:mpeg:mpegb:schema:dash:mpd:dis2011"> <xs:import namespace=" schemalocation="

9 <xs:annotation> <xs:appinfo>media Presentation Description</xs:appinfo> <xs:documentation xml:lang="en"> This Schema defines Media Presentation Description for MPEG DASH. The namespace identifier is for this draft version of the specification. The final version will use a different namespace identifier to align with 3GPP. However, changes from the draft to the final version are not expected to change the information model, even if the schema changes, and trial implementation and comment on the schema presented here are both encouraged. </xs:documentation> </xs:annotation> <!-- MPD: main element --> <xs:element name="mpd" type="mpdtype"/> <!-- MPD Type --> <xs:complextype name="mpdtype"> <xs:element name="programinformation" type="programinformationtype" minoccurs="0"/> <xs:element name="period" type="periodtype" maxoccurs="unbounded"/> <xs:element name="baseurl" type="baseurltype" <xs:attribute name="profiles" type="urivectortype"/> <xs:attribute name="type" type="presentationtype" default="ondemand"/> <xs:attribute name="availabilitystarttime" type="xs:datetime"/> <xs:attribute name="availabilityendtime" type="xs:datetime"/> <xs:attribute name="mediapresentationduration" type="xs:duration"/> <xs:attribute name="minimumupdateperiodmpd" type="xs:duration"/> <xs:attribute name="minbuffertime" type="xs:duration"/> <xs:attribute name="timeshiftbufferdepth" type="xs:duration"/> <!-- Type of presentation - live or on-demand --> <xs:simpletype name="presentationtype"> <xs:restriction base="xs:string"> <xs:enumeration value="ondemand"/> <xs:enumeration value="live"/> </xs:restriction> </xs:simpletype> <!-- Program information for a presentation --> <xs:complextype name="programinformationtype"> <xs:element name="title" type="xs:string" minoccurs="0"/> <xs:element name="source" type="xs:string" minoccurs="0"/> <xs:element name="copyright" type="xs:string" minoccurs="0"/> <xs:attribute name="moreinformationurl" type="xs:anyuri"/> <!-- Period of a presentation --> <xs:complextype name="periodtype"> <xs:element name="segmentinfodefault" type="segmentinfodefaulttype" minoccurs="0"/> <xs:element name="representation" type="representationtype" <xs:element name="group" type="grouptype" <xs:element name="subset" type="subsettype" <xs:attribute ref="xlink:href"/> <xs:attribute ref="xlink:actuate" default="onrequest"/> <xs:attribute name="start" type="xs:duration"/>

10 <xs:attribute name="id" type="xs:string" /> <xs:attribute name="duration" type="xs:duration"/> <xs:attribute name="minbuffertime" type="xs:duration"/> <xs:attribute name="segmentalignmentflag" type="xs:boolean" default="false"/> <xs:attribute name="bitstreamswitchingflag" type="xs:boolean" default="false"/> <!-- RepresentationBase type; extended by other Representation-related types --> <xs:complextype name="representationbasetype"> <xs:element name="contentprotection" type="contentdescriptortype" <xs:element name="accessibility" type="contentdescriptortype" <xs:element name="rating" type="contentdescriptortype" <xs:element name="viewpoint" type="contentdescriptortype" <xs:element name="multipleviews" type="multipleviewstype" minoccurs="0"/> <xs:attribute name="group" type="xs:unsignedint"/> <xs:attribute name="width" type="xs:unsignedint"/> <xs:attribute name="height" type="xs:unsignedint"/> <xs:attribute name="parx" type="xs:unsignedint"/> <xs:attribute name="pary" type="xs:unsignedint"/> <xs:attribute name="lang" type="langvectortype"/> <xs:attribute name="mimetype" type="xs:string"/> <xs:attribute name="startwithrap" type="xs:boolean"/> <xs:attribute name="framerate" type="xs:double"/> <xs:attribute name="maximumrapperiod" type="xs:double"/> <xs:attribute name="numberofchannels" type="stringvectortype"/> <xs:attribute name="samplingrate" type="stringvectortype"/> <!-- Representation of the presentation content for a specific Period; extends RepresentationBaseType --> <xs:complextype name="representationtype"> <xs:complexcontent> <xs:extension base="representationbasetype"> <xs:element name="subrepresentation" type="subrepresentationtype" <xs:element name="segmentinfo" type="segmentinfotype"/> <xs:element name="trickmode" type="trickmodetype" minoccurs="0"/> <xs:attribute name="id" type="xs:string" use="required"/> <xs:attribute name="bandwidth" type="xs:unsignedint" use="required"/> <xs:attribute name="qualityranking" type="xs:unsignedint"/> <xs:attribute name="dependencyid" type="stringvectortype"/> <xs:attribute name="bitstreamstructureid" type="stringvectortype"/> </xs:extension> </xs:complexcontent> <!-- SubRepresentation of the presentation content for a specific Period; extends RepresentationBaseType --> <xs:complextype name="subrepresentationtype"> <xs:complexcontent> <xs:extension base="representationbasetype"> <xs:attribute name="level" type="xs:unsignedint" use="required"/> <xs:attribute name="bandwidth" type="xs:unsignedint" use="required"/> <xs:attribute name="trickmodeapr" type="xs:double"/> </xs:extension> </xs:complexcontent>

11 <!-- Group to contain information common to a group of Representations; extends RepresentationBaseType --> <xs:complextype name="grouptype"> <xs:complexcontent> <xs:extension base="representationbasetype"> <xs:element name="representation" type="representationtype" <xs:element name="segmentinfodefault" type="segmentinfodefaulttype" minoccurs="0"/> <xs:attribute ref="xlink:href"/> <xs:attribute ref="xlink:actuate" default="onrequest"/> <xs:attribute name="minbandwidth" type="xs:unsignedint"/> <xs:attribute name="maxbandwidth" type="xs:unsignedint"/> <xs:attribute name="minwidth" type="xs:unsignedint"/> <xs:attribute name="maxwidth" type="xs:unsignedint"/> <xs:attribute name="minheight" type="xs:unsignedint"/> <xs:attribute name="maxheight" type="xs:unsignedint"/> <xs:attribute name="minframerate" type="xs:double"/> <xs:attribute name="maxframerate" type="xs:double"/> <xs:attribute name="subsegmentalignment" type="xs:boolean" default="false"/> <xs:attribute name="segmentalignmentflag" type="xs:boolean"/> <xs:attribute name="bitstreamswitchingflag" type="xs:boolean"/> </xs:extension> </xs:complexcontent> <!-- Subset selection information on Groups --> <xs:complextype name="subsettype"> <xs:element name="contains" type="containstype" minoccurs="1" maxoccurs="unbounded"/> <xs:complextype name="containstype"> <xs:attribute name="group" type="xs:unsignedint" use="required"/> <!-- Default Segment access information --> <xs:complextype name="segmentinfodefaulttype"> <xs:element name="initialisationsegmenturl" type="urltype" minoccurs="0"/> <xs:element name="baseurl" type="baseurltype" <xs:element name="segmenttimeline" type="segmenttimelinetype" minoccurs="0"/> <xs:attributegroup ref="segmentinfoattrgroup"/> <xs:attribute name="sourceurltemplateperiod" type="xs:string"/> <xs:attribute name="indextemplate" type="xs:string"/> <!-- Segment access information --> <xs:complextype name="segmentinfotype"> <xs:element name="initialisationsegmenturl" type="urltype" minoccurs="0"/> <xs:element name="baseurl" type="baseurltype" <xs:element name="segmenttimeline" type="segmenttimelinetype" minoccurs="0"/> <xs:choice minoccurs="0"> <xs:element name="urltemplate" type="urltemplatetype" minoccurs="0"/> <xs:element name="url" type="urltype" maxoccurs="unbounded"/> <xs:element name="index" type="urltype" maxoccurs="unbounded"/>

12 <xs:any namespace="##other" processcontents="lax" minoccurs="0" maxoccurs="unbounded"/> <xs:element name="segmentlist" type="segmentlisttype" minoccurs="0"/> <xs:any namespace="##other" processcontents="lax" minoccurs="0" maxoccurs="unbounded"/> </xs:choice> <xs:attributegroup ref="segmentinfoattrgroup"/> <!-- grouping attributes common to SegmentInfo and SegmentInfoDefault --> <xs:attributegroup name="segmentinfoattrgroup" > <xs:attribute name="duration" type="xs:duration"/> <xs:attribute name="startindex" type="xs:unsignedint" default="1"/> </xs:attributegroup> <!-- A Segment URL --> <xs:complextype name="urltype"> <xs:attribute name="sourceurl" type="xs:anyuri"/> <xs:attribute name="range" type="xs:string"/> <!-- A URL template --> <xs:complextype name="urltemplatetype"> <xs:attribute name="sourceurl" type="xs:anyuri"/> <xs:attribute name="indexurl" type="xs:anyuri"/> <xs:attribute name="endindex" type="xs:unsignedint"/> <!-- SegmentList allows xlink in addition to list of URLs --> <xs:complextype name="segmentlisttype"> <xs:element name="url" type="urltype" <xs:element name="index" type="urltype" <xs:attribute ref="xlink:href"/> <xs:attribute ref="xlink:actuate" default="onrequest"/> <xs:attribute name="startindex" type="xs:unsignedint"/> <!-- Generic named descriptive information about the content --> <xs:complextype name="contentdescriptortype"> <xs:element minoccurs="0" name="schemeinformation" type="xs:string"/> <xs:attribute name="schemeiduri" type="xs:anyuri" use="required"/> <!-- Gives information about trick mode --> <xs:complextype name=" TrickModeType"> <xs:attribute name="alternateplayoutrate" type="xs:string"/>

13 <!-- Type for MultipleViews --> <xs:complextype name="multipleviewstype"> <xs:element name="framepacking" type="contentdescriptortype" minoccurs="0" maxoccurs="unbounded"/> <xs:attribute name="stereoid" type="stringvectortype"/> <!-- stereoid: list of strings starting with "l" or "r" followed by optional index number --> <xs:attribute name="maximumviews" type="xs:unsignedint" default="1"/> <!-- Type for space delimited list of strings --> <xs:simpletype name="stringvectortype "> <xs:list itemtype="xs:string"/> </xs:simpletype> <!-- Type for space delimited list of URIs --> <xs:simpletype name="urivectortype "> <xs:list itemtype="xs:anyuri"/> </xs:simpletype> <!-- Type for space delimited list of language codes --> <xs:simpletype name="langvectortype"> <xs:list itemtype="xs:language"/> </xs:simpletype> <!-- Supplementary URL to the one given as attribute --> <xs:complextype name="baseurltype"> <xs:simplecontent> <xs:extension base="xs:anyuri"> </xs:extension> </xs:simplecontent> </xs:schema>