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Dolby AC-4 packetization into MPEG-DASH compliant ISO base media files The process of analyzing and preparing a Dolby AC-4 bitstream for storage within an MPEG-DASH compliant ISO base media file is described. AC-4 sync frames must be converted to raw AC-4 frames before packetization.
Packetizing the Dolby AC-4 bitstreamA multiplexer packetizes only one raw AC-4 frame as an MP4 sample.

Introduction to the Dolby AC-4 in MPEG-DASH for online delivery specificationThis documentation is for online delivery (over-the-top services), specifying the required data formatting and signaling between a server (sender) and clients (receivers) to enable Dolby AC-4 to be used as an audio format within the MPEG‑DASH standard in conjunction with the ISO base media file format.
Dolby AC-4 packetization into MPEG-DASH compliant ISO base media files The process of analyzing and preparing a Dolby AC-4 bitstream for storage within an MPEG-DASH compliant ISO base media file is described. AC-4 sync frames must be converted to raw AC-4 frames before packetization.
- Constraints on Dolby AC-4 elementary streams within MPEG-DASH ISO base media file format segmentsA Dolby AC-4 elementary stream must meet certain conditions to be delivered via MPEG-DASH.
- Content of the MP4 sampleA Dolby AC-4 MP4 sample corresponds to one raw Dolby AC-4 frame.
- Reading Dolby AC-4 frames from an .ac4 fileIn an .ac4 file, each raw_ac4_frame is encapsulated in an ac4_syncframe, as defined in ETSI TS 103 190-1. An AC-4 sync frame must first be converted to a raw AC-4 frame by stripping its sync word, frame size, and optional CRC words before packetization.
- Packetizing the Dolby AC-4 bitstreamA multiplexer packetizes only one raw AC-4 frame as an MP4 sample.
- Signaling bitstreams in an MPEG‑DASH–compliant ISO base media file format uses the basic structures defined within ISO/IEC 14496-12 to signal audio tracks and uses specific extensions within compliance to ISO base media file format to provide detailed information on the characteristics of a bitstream.
- File-type boxAn ISO base media file that contains the Dolby AC-4 stream must indicate that the file complies with Dolby extensions by using the file-type box (ftyp).
- Contents of AC4SpecificBox The AC4SpecificBox contains the AC-4 decoder-specific information (ac4_dsi). The content of the AC4SpecificBox is derived from the AC-4 table of contents (ac4_toc) included in the Dolby AC-4 frame.
- A/V alignment and segmentationTo achieve seamless transition for certain use cases (such as ad insertion) without introducing artifacts through audio gaps or additional audio, it is important that audio and video I-frames are aligned at the beginning of segments, and it is also important to have compatible video and audio segment durations so that the segment ends are aligned time wise.
Media Presentation Description with Dolby AC-4 Use proper values in an MPD file to describe a Dolby AC-4 bitstream. Examples of MPD files for different scenarios are provided.
Demultiplexing a Dolby AC-4 bitstream from an MPEG-DASH compliant ISO base media file segmentBuffering and parameter conflicts must be considered when product designers implement an ISO base media file format file demultiplexer that supports Dolby AC-4 bitstreams.
MPEG‑DASH and Dolby AC-4 overviewIntroduction about MPEG‑DASH and Dolby AC-4.
Glossary
Notices

Packetizing the Dolby AC-4 bitstream

A multiplexer packetizes only one raw AC-4 frame as an MP4 sample.

About this task

A seamless switch can be accomplished only at a random access point. The first sample of each MPEG-DASH segment must contain a random access point. A Dolby AC-4 elementary stream contains I-frames to indicate random access points. Similar to the length of GOP for video streams, the I-frame interval for a Dolby AC-4 stream indicates the number of frames between two I-frames.

The multiplexer must ensure that the first MP4 sample of each Dolby AC-4 media segment is an I-frame. A Dolby AC-4 media segment may contain one or more random access points.

Procedure

The multiplexer scans the bitstream to find the first I-frame (a frame with the b_iframe_global value of 1). This I-frame begins a Dolby AC-4 media segment.
The multiplexer continues to scan the bitstream for preceding I-frames and keeps track of the number of frames between two I-frames( I-frame interval).

The I-frame interval should be constant.
When the multiplexer reaches the end of an I-frame interval and accumulates a media segment size worth of AC-4 frames, the multiplexer adds the AC-4 frames to an MP4 segment.

The media segment size is determined by packager settings and A/V alignment and segmentation rules.
The multiplexer begins the next media segment with the next I-frame.