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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.
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.

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

Reading Dolby AC-4 frames from an .ac4 file

In 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.

Procedure

To locate the start of an AC-4 raw frame, search for the 16-bit sync word. The value of the sync word can be either 0xAC40 or 0xAC41.
Read the 16 bits immediately succeeding the sync word.
- If the 16-bit value is not equal to 0xFFFF, the value is the frame size.
- If the 16-bit value is equal to 0xFFFF, read the next 24 bits as the frame size.
According to the frame size, read the next bits succeeding the frame size as the raw AC-4 frame.
Optional : If the sync word is 0xAC41, the AC-4 simple transport frame includes a 16-bit CRC word based on the frame_size element and the raw_ac4_frame element. Read the 16 bits immediately succeeding the raw AC-4 frame, and verify that the value matches the CRC calculated using the IBM-CRC-16 polynomial: x¹⁶+x¹⁵+x²+1.
Read the next 16 bits.
- If the sync word is 0xAC40, they are the 16 bits immediately succeeding the raw AC-4 frame.
- If the sync word is 0xAC41, they are the 16 bits immediately succeeding the CRC word.
Verify that the 16-bit word is also a sync word; otherwise, this is not a proper AC-4 bitstream.