This specification extends HTMLMediaElement [HTML51] to allow JavaScript to generate media streams for playback. Allowing JavaScript to generate streams facilitates a variety of use cases like adaptive streaming and time shifting live streams.
Status of This Document
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
By publishing this Recommendation, W3C expects the functionality specified in this Recommendation will not be affected by changes to File API. The Working Group will continue to track these specifications.
In September 2016, the Working Group used an implementation
report to move this document to Recommendation.
This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.
This specification allows JavaScript to dynamically construct media streams for <audio> and <video>. It defines a MediaSource object that can serve as a source of media data for an HTMLMediaElement. MediaSource objects have one or more SourceBuffer objects. Applications append data segments to the SourceBuffer objects, and can adapt the quality of appended data based on system performance and other factors. Data from the SourceBuffer objects is managed as track buffers for audio, video and text data that is decoded and played. Byte stream specifications used with these extensions are available in the byte stream format registry [MSE-REGISTRY].
1.1 Goals
This specification was designed with the following goals in mind:
Allow JavaScript to construct media streams independent of how the media is fetched.
Define a splicing and buffering model that facilitates use cases like adaptive streaming, ad-insertion, time-shifting, and video editing.
Minimize the need for media parsing in JavaScript.
Leverage the browser cache as much as possible.
Provide requirements for byte stream format specifications.
Not require support for any particular media format or codec.
This specification defines:
Normative behavior for user agents to enable interoperability between user agents and web applications when processing media data.
Normative requirements to enable other specifications to define media formats to be used within this specification.
A presentation timestamp range used to filter out coded frames while appending. The append window represents a single continuous time range with a single start time and end time. Coded frames with presentation timestamp within this range are allowed to be appended to the SourceBuffer while coded frames outside this range are filtered out. The append window start and end times are controlled by the appendWindowStart and appendWindowEnd attributes respectively.
The duration of a coded frame. For video and text, the duration indicates how long the video frame or text SHOULD be displayed. For audio, the duration represents the sum of all the samples contained within the coded frame. For example, if an audio frame contained 441 samples @44100Hz the frame duration would be 10 milliseconds.
The decode timestamp indicates the latest time at which the frame needs to be decoded assuming instantaneous decoding and rendering of this and any dependant frames (this is equal to the presentation timestamp of the earliest frame, in presentation order, that is dependant on this frame). If frames can be decoded out of presentation order, then the decode timestamp MUST be present in or derivable from the byte stream. The user agent MUST run the append error algorithm if this is not the case. If frames cannot be decoded out of presentation order and a decode timestamp is not present in the byte stream, then the decode timestamp is equal to the presentation timestamp.
Initialization Segment
A sequence of bytes that contain all of the initialization information required to decode a sequence of media segments. This includes codec initialization data, Track ID mappings for multiplexed segments, and timestamp offsets (e.g., edit lists).
A sequence of bytes that contain packetized & timestamped media data for a portion of the media timeline. Media segments are always associated with the most recently appended initialization segment.
These URLs are the same as a Blob URI, except that anything in the definition of that feature that refers to File and Blob objects is hereby extended to also apply to MediaSource objects.
The presentation start time is the earliest time point in the presentation and specifies the initial playback position and earliest possible position. All presentations created using this specification have a presentation start time of 0.
Note
For the purposes of determining if HTMLMediaElement.buffered contains a TimeRange that includes the current playback position, implementations MAY choose to allow a current playback position at or after presentation start time and before the first TimeRange to play the first TimeRange if that TimeRange starts within a reasonably short time, like 1 second, after presentation start time. This allowance accommodates the reality that muxed streams commonly do not begin all tracks precisely at presentation start time. Implementations MUST report the actual buffered range, regardless of this allowance.
Presentation Interval
The presentation interval of a coded frame is the time interval from its presentation timestamp to the presentation timestamp plus the coded frame's duration. For example, if a coded frame has a presentation timestamp of 10 seconds and a coded frame duration of 100 milliseconds, then the presentation interval would be [10-10.1). Note that the start of the range is inclusive, but the end of the range is exclusive.
Presentation Order
The order that coded frames are rendered in the presentation. The presentation order is achieved by ordering coded frames in monotonically increasing order by their presentation timestamps.
Presentation Timestamp
A reference to a specific time in the presentation. The presentation timestamp in a coded frame indicates when the frame SHOULD be rendered.
Random Access Point
A position in a media segment where decoding and continuous playback can begin without relying on any previous data in the segment. For video this tends to be the location of I-frames. In the case of audio, most audio frames can be treated as a random access point. Since video tracks tend to have a more sparse distribution of random access points, the location of these points are usually considered the random access points for multiplexed streams.
A specific set of tracks distributed across one or more SourceBuffer objects owned by a single MediaSource instance.
Implementations MUST support at least 1 MediaSource object with the following configurations:
A single SourceBuffer with 1 audio track and/or 1 video track.
Two SourceBuffers with one handling a single audio track and the other handling a single video track.
MediaSource objects MUST support each of the configurations above, but they are only required to support one configuration at a time. Supporting multiple configurations at once or additional configurations is a quality of implementation issue.
A Track ID is a byte stream format specific identifier that marks sections of the byte stream as being part of a specific track. The Track ID in a track description identifies which sections of a media segment belong to that track.
2. MediaSource Object
The MediaSource object represents a source of media data for an HTMLMediaElement. It keeps track of the readyState for this source as well as a list of SourceBuffer objects that can be used to add media data to the presentation. MediaSource objects are created by the web application and then attached to an HTMLMediaElement. The application uses the SourceBuffer objects in sourceBuffers to add media data to this source. The HTMLMediaElement fetches this media data from the MediaSource object when it is needed during playback.
Terminates playback and signals that a network error has occured.
Note
JavaScript applications SHOULD use this status code to terminate playback with a network error. For example, if a network error occurs while fetching media data.
decode
Terminates playback and signals that a decoding error has occured.
Note
JavaScript applications SHOULD use this status code to terminate playback with a decode error. For example, if a parsing error occurs while processing out-of-band media data.
SourceBuffer objects in this list MUST appear in the same order as they appear in the sourceBuffers attribute; e.g., if only sourceBuffers[0] and sourceBuffers[3] are in activeSourceBuffers, then activeSourceBuffers[0] MUST equal sourceBuffers[0] and activeSourceBuffers[1] MUST equal sourceBuffers[3].
When this method is invoked, the user agent must run the following steps:
If type is an empty string then throw a TypeError exception and abort these steps.
If type contains a MIME type that is not supported or contains a MIME type that is not supported with the types specified for the other SourceBuffer objects in sourceBuffers, then throw a NotSupportedError exception and abort these steps.
If the user agent can't handle any more SourceBuffer objects or if creating a SourceBuffer based on type would result in an unsupported SourceBuffer configuration, then throw a QuotaExceededError exception and abort these steps.
Note
For example, a user agent MAY throw a QuotaExceededError exception if the media element has reached the
HAVE_METADATA readyState. This can occur if the user agent's media engine does not support adding more tracks during playback.
Create a new SourceBuffer object and associated resources.
Set the generate timestamps flag on the new object to the value in the "Generate Timestamps Flag" column of the byte stream format registry [MSE-REGISTRY] entry that is associated with type.
The start of the range, in seconds measured from presentation start time. While set, and if duration equals positive Infinity, HTMLMediaElement.seekable will return a non-empty TimeRanges object with a lowest range start timestamp no greater than start.
The end of range, in seconds measured from presentation start time. While set, and if duration equals positive Infinity, HTMLMediaElement.seekable will return a non-empty TimeRanges object with a highest range end timestamp no less than end.
Check to see whether the MediaSource is capable of creating SourceBuffer objects for the specified MIME type.
Note
If true is returned from this method, it only indicates that the MediaSource implementation is capable of creating SourceBuffer objects for the specified MIME type. An addSourceBuffer() call SHOULD still fail if sufficient resources are not available to support the addition of a new SourceBuffer.
Note
This method returning true implies that HTMLMediaElement.canPlayType() will return "maybe" or "probably" since it does not make sense for a MediaSource to support a type the HTMLMediaElement knows it cannot play.
If the resource fetch algorithm was invoked with a media provider object that is a MediaSource object or a URL record whose object is a MediaSource object, then let mode be local, skip the first step in the resource fetch algorithm (which may otherwise set mode to remote) and add the steps and clarifications below to the "Otherwise (mode is local)" section of the resource fetch algorithm.
Note
The resource fetch algorithm's first step is expected to eventually align with selecting local mode for URL records whose objects are media provider objects. The intent is that if the HTMLMediaElement's src attribute or selected child <source>'s src attribute is a blob: URL matching a MediaSource object URL when the respective src attribute was last changed, then that MediaSource object is used as the media provider object and current media resource in the local mode logic in the resource fetch algorithm. This also means that the remote mode logic that includes observance of any preload attribute is skipped when a MediaSource object is attached. Even with that eventual change to [HTML51], the execution of the following steps at the beginning of the local mode logic is still required when the current media resource is a MediaSource object.
Note
Relative to the action which triggered the media element's resource selection algorithm, these steps are asynchronous. The resource fetch algorithm is run after the task that invoked the resource selection algorithm is allowed to continue and a stable state is reached. Implementations may delay the steps in the "Otherwise" clause, below, until the MediaSource object is ready for use.
An attached MediaSource does not use the remote mode steps in the resource fetch algorithm, so the media element will not fire "suspend" events. Though future versions of this specification will likely remove "progress" and "stalled" events from a media element with an attached MediaSource, user agents conforming to this version of the specification may still fire these two events as these [HTML51] references changed after implementations of this specification stabilized.
2.4.2 Detaching from a media element
The following steps are run in any case where the media element is going to transition to NETWORK_EMPTY and queue a task to fire a simple event named emptied at the media element. These steps SHOULD be run right before the transition.
Going forward, this algorithm is intended to be externally called and run in any case where the attached MediaSource, if any, must be detached from the media element. It MAY be called on HTMLMediaElement [HTML51] operations like load() and resource fetch algorithm failures in addition to, or in place of, when the media element transitions to NETWORK_EMPTY. Resource fetch algorithm failures are those which abort either the resource fetch algorithm or the resource selection algorithm, with the exception that the "Final step" [HTML51] is not considered a failure that triggers detachment.
2.4.3 Seeking
Run the following steps as part of the "Wait until the user agent has established whether or not the media data for the new playback position is available, and, if it is, until it has decoded enough data to play back that position" step of the seek algorithm:
If the readyState attribute is "ended" and the new playback position is within a TimeRange currently in HTMLMediaElement.buffered, then the seek operation must continue to completion here even if one or more currently selected or enabled track buffers' largest range end timestamp is less than new playback position. This condition should only occur due to logic in buffered when readyState is "ended".
The media element resets all decoders and initializes each one with data from the appropriate initialization segment.
Having enough data to ensure uninterrupted playback is an implementation specific condition where the user agent determines that it currently has enough data to play the presentation without stalling for a meaningful period of time. This condition is constantly evaluated to determine when to transition the media element into and out of the HAVE_ENOUGH_DATA ready state. These transitions indicate when the user agent believes it has enough data buffered or it needs more data respectively.
Note
An implementation MAY choose to use bytes buffered, time buffered, the append rate, or any other metric it sees fit to determine when it has enough data. The metrics used MAY change during playback so web applications SHOULD only rely on the value of
HTMLMediaElement.readyState to determine whether more data is needed or not.
Note
When the media element needs more data, the user agent SHOULD transition it from HAVE_ENOUGH_DATA to
HAVE_FUTURE_DATA early enough for a web application to be able to respond without causing an interruption in playback. For example, transitioning when the current playback position is 500ms before the end of the buffered data gives the application roughly 500ms to append more data before playback stalls.
Playback may resume at this point if it was previously suspended by a transition to HAVE_CURRENT_DATA.
Abort these steps.
If HTMLMediaElement.buffered contains a TimeRange that includes the current playback position and some time beyond the current playback position, then run the following steps:
Playback may resume at this point if it was previously suspended by a transition to HAVE_CURRENT_DATA.
Abort these steps.
If HTMLMediaElement.buffered contains a TimeRange that ends at the current playback position and does not have a range covering the time immediately after the current position:
If an audio track becomes disabled and the SourceBuffer associated with this track is not associated with any other enabled or selected track, then run the following steps:
If a text track mode becomes "disabled" and the SourceBuffer associated with this track is not associated with any other enabled or selected track, then run the following steps:
Duration reductions that would truncate currently buffered media are disallowed. When truncation is necessary, use remove() to reduce the buffered range before updating duration.
This algorithm gets called when the application signals the end of stream via an endOfStream() call or an algorithm needs to signal a decode error. This algorithm takes an error parameter that indicates whether an error will be signalled.
This allows the duration to properly reflect the end of the appended media segments. For example, if the duration was explicitly set to 10 seconds and only media segments for 0 to 5 seconds were appended before endOfStream() was called, then the duration will get updated to 5 seconds.
Notify the media element that it now has all of the media data.
The timestamps in the media segment determine where the coded frames are placed in the presentation. Media segments can be appended in any order.
sequence
Media segments will be treated as adjacent in time independent of the timestamps in the media segment. Coded frames in a new media segment will be placed immediately after the coded frames in the previous media segment. The timestampOffset attribute will be updated if a new offset is needed to make the new media segments adjacent to the previous media segment. Setting the timestampOffset attribute in "sequence" mode allows a media segment to be placed at a specific position in the timeline without any knowledge of the timestamps in the media segment.
Indicates whether the asynchronous continuation of an appendBuffer() or remove() operation is still being processed. This attribute is initially set to false when the object is created.
Text track-buffers are included in the calculation of highest end time, above, but excluded from the buffered range calculation here. They are not necessarily continuous, nor should any discontinuity within them trigger playback stall when the other media tracks are continuous over the same time range.
If readyState is "ended", then set the end time on the last range in track ranges to highest end time.
Let new intersection ranges equal the intersection between the intersection ranges and the track ranges.
Replace the ranges in intersection ranges with the new intersection ranges.
If intersection ranges does not contain the exact same range information as the current value of this attribute, then update the current value of this attribute to
intersection ranges.
Controls the offset applied to timestamps inside subsequent media segments that are appended to this SourceBuffer. The timestampOffset is initially set to 0 which indicates that no offset is being applied.
On getting, Return the initial value or the last value that was successfully set.
On setting, run the following steps:
Let new timestamp offset equal the new value being assigned to this attribute.
Each track buffer has a last decode timestamp variable that stores the decode timestamp of the last coded frame appended in the current coded frame group. The variable is initially unset to indicate that no coded frames have been appended yet.
Each track buffer has a track buffer ranges variable that represents the presentation time ranges occupied by the coded frames currently stored in the track buffer.
Note
For track buffer ranges, these presentation time ranges are based on presentation timestamps, frame durations, and potentially coded frame group start times for coded frame groups across track buffers in a muxed SourceBuffer.
These coded frame group start times differ slightly from those mentioned in the coded frame processing algorithm in that they are the earliest presentation timestamp across all track buffers following a discontinuity. Discontinuities can occur within the coded frame processing algorithm or result from the coded frame removal algorithm, regardless of mode. The threshold for determining disjointness of track buffer ranges is implementation-specific. For example, to reduce unexpected playback stalls, implementations MAY approximate the coded frame processing algorithm's discontinuity detection logic by coalescing adjacent ranges separated by a gap smaller than 2 times the maximum frame duration buffered so far in this track buffer. Implementations MAY also use coded frame group start times as range start times across track buffers in a muxed SourceBuffer to further reduce unexpected playback stalls.
The append or remove has successfully completed. updating transitions from true to false.
updateend
Event
The append or remove has ended.
error
Event
An error occurred during the append. updating transitions from true to false.
abort
Event
The append or remove was aborted by an abort() call. updating transitions from true to false.
3.5 Algorithms
3.5.1 Segment Parser Loop
All SourceBuffer objects have an internal append state variable that keeps track of the high-level segment parsing state. It is initially set to WAITING_FOR_SEGMENT and can transition to the following states as data is appended.
The input buffer is a byte buffer that is used to hold unparsed bytes across appendBuffer() calls. The buffer is empty when the SourceBuffer object is created.
The buffer full flag keeps track of whether appendBuffer() is allowed to accept more bytes. It is set to false when the SourceBuffer object is created and gets updated as data is appended and removed.
The generate timestamps flag is a boolean variable that keeps track of whether timestamps need to be generated for the
coded frames passed to the coded frame processing algorithm. This flag is set by addSourceBuffer() when the SourceBuffer object is created.
When the segment parser loop algorithm is invoked, run the following steps:
Loop Top: If the input buffer is empty, then jump to the need more data step below.
The frequency at which the coded frame processing algorithm is run is implementation-specific. The coded frame processing algorithm MAY be called when the input buffer contains the complete media segment or it MAY be called multiple times as complete coded frames are added to the input buffer.
This is the signal that the implementation was unable to evict enough data to accommodate the append or the append is too big. The web application SHOULD use remove() to explicitly free up space and/or reduce the size of the append.
3.5.5 Buffer Append Algorithm
When appendBuffer() is called, the following steps are run to process the appended data.
Each SourceBuffer object has an internal first initialization segment received flag that tracks whether the first initialization segment has been appended and received by this algorithm. This flag is set to false when the SourceBuffer is created and updated by the algorithm below.
Update the duration attribute if it currently equals NaN:
If the initialization segment contains a duration:
Run the duration change algorithm with new duration set to the duration in the initialization segment.
User agents MAY consider codecs, that would otherwise be supported, as "not supported" here if the codecs were not specified in the type parameter passed to addSourceBuffer(). For example, MediaSource.isTypeSupported('video/webm;codecs="vp8,vorbis"') may return true, but if
addSourceBuffer() was called with 'video/webm;codecs="vp8"' and a Vorbis track appears in the
initialization segment, then the user agent MAY use this step to trigger a decode error.
Let audio byte stream track ID be the
Track ID for the current track being processed.
Let audio language be a BCP 47 language tag for the language specified in the initialization segment for this track or an empty string if no language info is present.
If audio language equals the 'und' BCP 47 value, then assign an empty string to audio language.
Let audio label be a label specified in the initialization segment for this track or an empty string if no label info is present.
Let audio kinds be a sequence of kind strings specified in the
initialization segment for this track or a sequence with a single empty string element in it if no kind information is provided.
For each value in audio kinds, run the following steps:
Let current audio kind equal the value from audio kinds for this iteration of the loop.
Let video byte stream track ID be the
Track ID for the current track being processed.
Let video language be a BCP 47 language tag for the language specified in the initialization segment for this track or an empty string if no language info is present.
If video language equals the 'und' BCP 47 value, then assign an empty string to video language.
Let video label be a label specified in the initialization segment for this track or an empty string if no label info is present.
Let video kinds be a sequence of kind strings specified in the
initialization segment for this track or a sequence with a single empty string element in it if no kind information is provided.
For each value in video kinds, run the following steps:
Let current video kind equal the value from video kinds for this iteration of the loop.
Let text byte stream track ID be the
Track ID for the current track being processed.
Let text language be a BCP 47 language tag for the language specified in the initialization segment for this track or an empty string if no language info is present.
If text language equals the 'und' BCP 47 value, then assign an empty string to text language.
Let text label be a label specified in the initialization segment for this track or an empty string if no label info is present.
Let text kinds be a sequence of kind strings specified in the
initialization segment for this track or a sequence with a single empty string element in it if no kind information is provided.
For each value in text kinds, run the following steps:
Let current text kind equal the value from text kinds for this iteration of the loop.
Let presentation timestamp be a double precision floating point representation of the coded frame's presentation timestamp in seconds.
Note
Special processing may be needed to determine the presentation and decode timestamps for timed text frames since this information may not be explicitly present in the underlying format or may be dependent on the order of the frames. Some metadata text tracks, like MPEG2-TS PSI data, may only have implied timestamps. Format specific rules for these situations SHOULD be in the byte stream format specifications or in separate extension specifications.
Let decode timestamp be a double precision floating point representation of the coded frame's decode timestamp in seconds.
Note
Implementations don't have to internally store timestamps in a double precision floating point representation. This representation is used here because it is the represention for timestamps in the HTML spec. The intention here is to make the behavior clear without adding unnecessary complexity to the algorithm to deal with the fact that adding a timestampOffset may cause a timestamp rollover in the underlying timestamp representation used by the byte stream format. Implementations can use any internal timestamp representation they wish, but the addition of timestampOffset SHOULD behave in a similar manner to what would happen if a double precision floating point representation was used.
Let frame duration be a double precision floating point representation of the coded frame's duration in seconds.
Jump to the Loop Top step above to restart processing of the current coded frame.
Otherwise:
Continue.
Let frame end timestamp equal the sum of presentation timestamp and frame duration.
If presentation timestamp is less than appendWindowStart, then set the need random access point flag to true, drop the coded frame, and jump to the top of the loop to start processing the next coded frame.
Note
Some implementations MAY choose to collect some of these coded frames with presentation timestamp less than appendWindowStart and use them to generate a splice at the first coded frame that has a presentation timestamp greater than or equal to appendWindowStart even if that frame is not a random access point. Supporting this requires multiple decoders or faster than real-time decoding so for now this behavior will not be a normative requirement.
If frame end timestamp is greater than appendWindowEnd, then set the need random access point flag to true, drop the coded frame, and jump to the top of the loop to start processing the next coded frame.
Note
Some implementations MAY choose to collect coded frames with presentation timestamp less than appendWindowEnd and frame end timestamp greater than appendWindowEnd and use them to generate a splice across the portion of the collected coded frames within the append window at time of collection, and the beginning portion of later processed frames which only partially overlap the end of the collected coded frames. Supporting this requires multiple decoders or faster than real-time decoding so for now this behavior will not be a normative requirement. In conjunction with collecting coded frames that span appendWindowStart, implementations MAY thus support gapless audio splicing.
Let remove window timestamp equal the overlapped framepresentation timestamp plus 1 microsecond.
If the presentation timestamp is less than the remove window timestamp, then remove overlapped frame from track buffer.
Note
This is to compensate for minor errors in frame timestamp computations that can appear when converting back and forth between double precision floating point numbers and rationals. This tolerance allows a frame to replace an existing one as long as it is within 1 microsecond of the existing frame's start time. Frames that come slightly before an existing frame are handled by the removal step below.
Remove all coded frames from track buffer that have a presentation timestamp greater than or equal to
presentation timestamp and less than frame end timestamp.
If highest end timestamp for track buffer is set and less than or equal to presentation timestamp:
Remove all possible decoding dependencies on the coded frames removed in the previous two steps by removing all coded frames from track buffer between those frames removed in the previous two steps and the next
random access point after those removed frames.
Note
Removing all coded frames until the next random access point is a conservative estimate of the decoding dependencies since it assumes all frames between the removed frames and the next random access point depended on the frames that were removed.
If spliced audio frame is set:
Add spliced audio frame to the track buffer.
If spliced timed text frame is set:
Add spliced timed text frame to the track buffer.
Otherwise:
Add the coded frame with the presentation timestamp, decode timestamp, and frame duration to the
track buffer.
The greater than check is needed because bidirectional prediction between coded frames can cause
presentation timestamp to not be monotonically increasing even though the decode timestamps are monotonically increasing.
For each track buffer in this source buffer, run the following steps:
Let remove end timestamp be the current value of duration
If this track buffer has a random access point timestamp that is greater than or equal to
end, then update remove end timestamp to that random access point timestamp.
Note
Random access point timestamps can be different across tracks because the dependencies between coded frames within a track are usually different than the dependencies in another track.
Remove all media data, from this track buffer, that contain starting timestamps greater than or equal to start and less than the remove end timestamp.
Remove all possible decoding dependencies on the coded frames removed in the previous step by removing all coded frames from this track buffer between those frames removed in the previous step and the next
random access point after those removed frames.
Note
Removing all coded frames until the next random access point is a conservative estimate of the decoding dependencies since it assumes all frames between the removed frames and the next random access point depended on the frames that were removed.
Let removal ranges equal a list of presentation time ranges that can be evicted from the presentation to make room for the
new data.
Note
Implementations MAY use different methods for selecting removal ranges so web applications SHOULD NOT depend on a specific behavior. The web application can use the buffered attribute to observe whether portions of the buffered data have been evicted.
For each range in removal ranges, run the coded frame removal algorithm with start and end equal to the removal range start and end timestamp respectively.
Update presentation timestamp and decode timestamp to the nearest audio sample timestamp based on sample rate of the audio in overlapped frame. If a timestamp is equidistant from both audio sample timestamps, then use the higher timestamp (e.g.,
floor(x * sample_rate + 0.5) / sample_rate).
Some implementations MAY apply fades to/from silence to coded frames on either side of the inserted silence to make the transition less jarring.
Return to caller without providing a splice frame.
Note
This is intended to allow new coded frame to be added to the track buffer as if
overlapped frame had not been in the track buffer to begin with.
Let frame end timestamp equal the sum of presentation timestamp and frame duration.
Let splice end timestamp equal the sum of presentation timestamp and the splice duration of 5 milliseconds.
Let fade out coded frames equal overlapped frame as well as any additional frames in track buffer that have a presentation timestamp greater than presentation timestamp and less than splice end timestamp.
Remove all the frames included in fade out coded frames from track buffer.
Return a splice frame with the following properties:
The fade out coded frames equals fade-out coded frames.
The fade in coded frame equal new coded frame.
Note
If the new coded frame is less than 5 milliseconds in duration, then coded frames that are appended after the
new coded frame will be needed to properly render the splice.
The splice timestamp equals presentation timestamp.
Let splice end timestamp equal splice timestamp plus five milliseconds.
Let fade out samples be the samples generated by decoding fade out coded frames.
Trim fade out samples so that it only contains samples between presentation timestamp and splice end timestamp.
Let fade in samples be the samples generated by decoding fade in coded frames.
If fade out samples and fade in samples do not have a common sample rate and channel layout, then convert
fade out samples and fade in samples to a common sample rate and channel layout.
Let output samples be a buffer to hold the output samples.
Apply a linear gain fade out with a starting gain of 1 and an ending gain of 0 to the samples between
splice timestamp and splice end timestamp in fade out samples.
Apply a linear gain fade in with a starting gain of 0 and an ending gain of 1 to the samples between splice timestamp and
splice end timestamp in fade in samples.
Copy samples between presentation timestamp to splice timestamp from fade out samples into output samples.
For each sample between splice timestamp and splice end timestamp, compute the sum of a sample from fade out samples and the corresponding sample in fade in samples and store the result in output samples.
Copy samples between splice end timestamp to end timestamp from fade in samples into output samples.
Render output samples.
Note
Here is a graphical representation of this algorithm.
Let frame end timestamp equal the sum of presentation timestamp and frame duration.
Let first overlapped frame be the coded frame in track buffer with a presentation interval that contains presentation timestamp.
Let overlapped presentation timestamp be the presentation timestamp of the first overlapped frame.
Let overlapped frames equal first overlapped frame as well as any additional frames in track buffer that have a presentation timestamp greater than presentation timestamp and less than frame end timestamp.
Remove all the frames included in overlapped frames from track buffer.
Update the coded frame duration of the first overlapped frame to presentation timestamp - overlapped presentation timestamp.
Add first overlapped frame to the track buffer.
Return to caller without providing a splice frame.
Note
This is intended to allow new coded frame to be added to the track buffer as if it hadn't overlapped any frames in track buffer to begin with.
4. SourceBufferList Object
SourceBufferList is a simple container object for SourceBuffer objects. It provides read-only array access and fires events when the list is modified.
This algorithm is intended to mirror the behavior of the createObjectURL()[FILE-API] method, which does not auto-revoke the created URL. Web authors are encouraged to use revokeObjectURL()[FILE-API] for any MediaSource object URL that is no longer needed for attachment to a media element.
Return a single range with a start time equal to the earliest start time in union ranges and an end time equal to the highest end time in union ranges and abort these steps.
Let source ranges equal the ranges returned by the buffered attribute on the current SourceBuffer.
If readyState is "ended", then set the end time on the last range in source ranges to
highest end time.
Let new intersection ranges equal the intersection between the intersection ranges and the source ranges.
Replace the ranges in intersection ranges with the new intersection ranges.
If the current value of this attribute has not been set by this algorithm or
intersection ranges does not contain the exact same range information as the current value of this attribute, then update the current value of this attribute to
intersection ranges.
Return the current value of this attribute.
7. AudioTrack Extensions
This section specifies extensions to the HTML AudioTrack definition.
The bytes provided through appendBuffer() for a SourceBuffer form a logical byte stream. The format and semantics of these byte streams are defined in byte stream format specifications. The byte stream format registry [MSE-REGISTRY] provides mappings between a MIME type that may be passed to addSourceBuffer() or
isTypeSupported() and the byte stream format expected by a SourceBuffer created with that MIME type. Implementations are encouraged to register mappings for byte stream formats they support to facilitate interoperability. The byte stream format registry [MSE-REGISTRY] is the authoritative source for these mappings. If an implementation claims to support a MIME type listed in the registry, its SourceBuffer implementation MUST conform to the
byte stream format specification listed in the registry entry.
Note
The byte stream format specifications in the registry are not intended to define new storage formats. They simply outline the subset of existing storage format structures that implementations of this specification will accept.
Note
Byte stream format parsing and validation is implemented in the segment parser loop algorithm.
This section provides general requirements for all byte stream format specifications:
If the byte stream format covers a format similar to one covered in the in-band tracks spec [INBANDTRACKS], then it SHOULD try to use the same attribute mappings so that Media Source Extensions playback and non-Media Source Extensions playback provide the same track information.
It MUST be possible to identify segment boundaries and segment type (initialization or media) by examining the byte stream alone.
The user agent MUST run the append error algorithm when any of the following conditions are met:
The number and type of tracks are not consistent.
Note
For example, if the first initialization segment has 2 audio tracks and 1 video track, then all initialization segments that follow it in the byte stream MUST describe 2 audio tracks and 1 video track.
Track IDs are not the same across initialization segments, for segments describing multiple tracks of a single type (e.g., 2 audio tracks).
For example, a byte stream that starts with an initialization segment that specifies a single AAC track and later contains an initialization segment that specifies a single AMR-WB track is not allowed. Support for multiple codecs is handled with multiple SourceBuffer objects.
Video frame size changes. The user agent MUST support seamless playback.
Note
This will cause the <video> display region to change size if the web application does not use CSS or HTML attributes (width/height) to constrain the element size.
Audio channel count changes. The user agent MAY support this seamlessly and could trigger downmixing.
Note
This is a quality of implementation issue because changing the channel count may require reinitializing the audio device, resamplers, and channel mixers which tends to be audible.
The following rules apply to all media segments within a byte stream. A user agent MUST:
Support seamless playback of media segments having a timestamp gap smaller than the audio frame size. User agents MUST NOT reflect these gaps in the buffered attribute.
Note
This is intended to simplify switching between audio streams where the frame boundaries don't always line up across encodings (e.g., Vorbis).
The number and type (audio, video, text, etc.) of all tracks in the media segments are not identified.
The decoding capabilities needed to decode each track (i.e., codec and codec parameters) are not provided.
Encryption parameters necessary to decrypt the content (except the encryption key itself) are not provided for all encrypted tracks.
All information necessary to decode and render the earliest random access point in the sequence of media segments and all subsequence samples in the sequence (in presentation time) are not provided. This includes in particular,
Information that determines the intrinsic width and height of the video (specifically, this requires either the picture or pixel aspect ratio, together with the encoded resolution).
Information necessary to convert the video decoder output to a format suitable for display
For example, if I1 is associated with M1, M2, M3 then the above MUST hold for all the combinations I1+M1, I1+M2, I1+M1+M2, I1+M2+M3, etc.
Byte stream specifications MUST at a minimum define constraints which ensure that the above requirements hold. Additional constraints MAY be defined, for example to simplify implementation.
11. Conformance
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, SHOULD, and SHOULD NOT are to be interpreted as described in [RFC2119].
12. Examples
Example use of the Media Source Extensions
<script>functiononSourceOpen(videoTag, e) {
var mediaSource = e.target;
if (mediaSource.sourceBuffers.length > 0)
return;
var sourceBuffer = mediaSource.addSourceBuffer('video/webm; codecs="vorbis,vp8"');
videoTag.addEventListener('seeking', onSeeking.bind(videoTag, mediaSource));
videoTag.addEventListener('progress', onProgress.bind(videoTag, mediaSource));
var initSegment = GetInitializationSegment();
if (initSegment == null) {
// Error fetching the initialization segment. Signal end of stream with an error.
mediaSource.endOfStream("network");
return;
}
// Append the initialization segment.var firstAppendHandler = function(e) {
var sourceBuffer = e.target;
sourceBuffer.removeEventListener('updateend', firstAppendHandler);
// Append some initial media data.
appendNextMediaSegment(mediaSource);
};
sourceBuffer.addEventListener('updateend', firstAppendHandler);
sourceBuffer.appendBuffer(initSegment);
}
functionappendNextMediaSegment(mediaSource) {
if (mediaSource.readyState == "closed")
return;
// If we have run out of stream data, then signal end of stream.if (!HaveMoreMediaSegments()) {
mediaSource.endOfStream();
return;
}
// Make sure the previous append is not still pending.if (mediaSource.sourceBuffers[0].updating)
return;
var mediaSegment = GetNextMediaSegment();
if (!mediaSegment) {
// Error fetching the next media segment.
mediaSource.endOfStream("network");
return;
}
// NOTE: If mediaSource.readyState == “ended”, this appendBuffer() call will// cause mediaSource.readyState to transition to "open". The web application// should be prepared to handle multiple “sourceopen” events.
mediaSource.sourceBuffers[0].appendBuffer(mediaSegment);
}
functiononSeeking(mediaSource, e) {
var video = e.target;
if (mediaSource.readyState == "open") {
// Abort current segment append.
mediaSource.sourceBuffers[0].abort();
}
// Notify the media segment loading code to start fetching data at the// new playback position.
SeekToMediaSegmentAt(video.currentTime);
// Append a media segment from the new playback position.
appendNextMediaSegment(mediaSource);
}
functiononProgress(mediaSource, e) {
appendNextMediaSegment(mediaSource);
}
</script><videoid="v"autoplay></video><script>var video = document.getElementById('v');
var mediaSource = new MediaSource();
mediaSource.addEventListener('sourceopen', onSourceOpen.bind(this, video));
video.src = window.URL.createObjectURL(mediaSource);
</script>
13. Acknowledgments
The editors would like to thank Alex Giladi, Bob Lund, Chris Poole, Cyril Concolato, David Dorwin, David Singer, Duncan Rowden, Frank Galligan, Glenn Adams, Jer Noble, Joe Steele, John Simmons, Kevin Streeter, Mark Vickers, Matt Ward, Matthew Gregan, Michael Thornburgh, Philip Jägenstedt, Pierre Lemieux, Ralph Giles, Steven Robertson, and Tatsuya Igarashi for their contributions to this specification.
A. VideoPlaybackQuality
This section is non-normative.
The video playback quality metrics described in previous revisions of this specification (e.g., sections 5 and 10 of the Candidate Recommendation) are now being developed as part of [MEDIA-PLAYBACK-QUALITY]. Some implementations may have implemented the earlier draft VideoPlaybackQuality object and the HTMLVideoElement extension method getVideoPlaybackQuality() described in those previous revisions.
