Flows reference media content stored in media object containers in TAMS.
The containers may have content for multiple Flows.
This application note will describe how the container and container_mapping properties are used to reference the Flow content in containers.
Container functionality changed in TAMS v5.0. This Application Note applies to TAMS v5.0+.
The Flow container property serves 2 purposes.
The container property identifies the container type for the media objects and its presence flags whether the Flow directly references media objects.
The container property is a MIME type.
Example values are video/mp2t for MPEG-TS, video/mp4 for MP4 and application/json for JSON.
A Flow references media objects either directly using Flow Segments associated with the Flow or via Flow Segments associated with a multi-essence Flow that collects (via the flow_collection) the Flow.
The diagram below shows a sequence of MPEG-TS media objects that contains video for a Flow.
The video Flow references the media objects via a sequence of Flow Segments.
The presence of the container property indicates that the Flow has Flow Segments (the list could be empty).
The diagram below shows a sequence of MPEG-TS media objects that contains a multplex of audio and video. The multi-essence Flow:
- has a
containerproperty that indicates that this Flow has Flow Segments. - uses its sequence of Flow Segments to reference the media objects.
The video and audio Flows:
- are referenced by a Collection Item in the
flow_collectionlist. - don't themselves have a sequence of Flow Segments and therefore the
containerproperty is not set.
The media content for these Flows are accessed using the multi-essence Flow's Flow Segments.
The diagram below shows a sequence of MPEG-TS media objects that contains a multiplex of audio, video and subtitles.
The subtitles in this example are very sparse compared to the audio and video.
In addition to the multi-essence Flow, the subtitle Flow references the media objects via a sequence of Flow Segments and has a container property.
This allows for a more optimised access to the subtitle media content.
The subtitle Flow Segments have different a timerange to the Flow Segments from the multi-essence Flow.
The subtitle Flow Segments don't reference every media object because not every media object contains subtitles.
The container_mapping property is used to specify the location of the media content in the container.
It is needed if the mapping is ambiguous because there are multiple tracks of the same media format in the container.
The track_index property provides an index (starting from 0) for the container tracks.
The track_index can be used in applications where the set of container tracks are detectable by all readers and have a consistent ordering.
The format_track_index property separates the track indexing for each Flow formats.
The format property of the target Flow determines which index sequence is in use.
For example, a media container with a video and 2 audio tracks has a video track index 0 and audio track indexes 0 and 1.
The format_track_index also requires the set of container tracks to be detectable by all readers and that the tracks, per format, have a consistent ordering.
There are container specific mappings that can be used to identify the track more precisely using information that is specific to the container type.
The mp2ts_container can be used for MPEG-TS and provides a pid property to identity the packet ID for the elementary stream.
The mxf_container can be used for MXF and has a package_uid and track_id property.
The isobmff_container can be used for formats based on the ISO Base Media File Format container formats (e.g. MP4 and Quicktime) and it has a track_id property.
The container_mapping may contain multiple mapping properties to help identify the container track.
This ranges from the more precise container specific mappings like mp2ts_container, to format_track_index and then track_index.
Basic reader implementations may not have have access to the container specific metadata, e.g. the MPEG-TS PID.
In that case they can still use the information from the format_track_index or track_index properties.
The diagram below illustrates this for a container with video and audio tracks.
Only the Collection Item and Flow for the left audio channel in the flow_collection is shown.
The container_mapping object has track_index 1 (video is 0), format_track_index 0 (the first audio) and a mp2t_container object with the PID for the audio elementary stream set to 257.
The audio_track object property has channel_numbers and channel_range properties that can be used to specify use of individual channels and reordering for multi-channel audio tracks.
This mapping requires readers to have the capability to extract and reorder individual audio channels.
This audio channel mapping capability may be restricted in a TAMS, e.g. only allow uncompressed audio.
The container_mapping property can be used in the Flow or Collection Item objects.
If a Flow is collected by a multi-essence Flow then the container_mapping should be specified in the Collection Item; it may be duplicated in the Flow as well.
If a Flow is not collected by a multi-essence Flow then the container_mapping in the Flow should be used.
The diagram below shows a Flow referencing a stereo audio track in a media container that also includes an unused alternative stereo audio track.
Only one of the stereo audio is of interest and therefore just a single audio Flow is defined to reference the media objects.
The audio Flow has container property to identify the container type and container_mapping to identify the target stereo audio track.
There is no need to define a multi-essence Flow because only one of the container tracks is used.
