Home
HTTP Live Streaming and Dolby AC-4 overviewIntroduction about HLS and Dolby AC-4
Sync frames A Dolby AC-4 bitstream in the AC-4 sync frame transport format (as specified in ETSI TS 103 190) consists of Dolby AC-4 synchronization frames. The AC-4 sync frame transport format must be used for MPEG packed audio.

Introduction to Dolby AC-4 and HTTP Live Streaming specificationThis information set specifies the required data formatting and signaling between the server (sender) and the clients (receivers) to enable Dolby AC-4 to be used as an audio format within HTTP Live Streaming A/V delivery applications in conjunction with packed audio or ISO base media file format.
Dolby AC-4 packetization into a fragmented MP4 file The process of analyzing and preparing a Dolby AC-4 bitstream for storage within a fragmented MP4 file is described. AC-4 sync frames must be converted to raw AC-4 frames before packetization.
Dolby AC-4 packetization into HTTP Live Streaming compliant audio-only elementary stream segmentsWhen multiplexing a Dolby AC-4 audio elementary stream into HTTP Live Streaming compliant audio-only elementary stream segments, the product must meet certain conditions.
HTTP Live Streaming playlist files with Dolby AC-4 Requirements and recommendations for a playlist that lists entries for a presentation that includes Dolby AC-4 content are described.
Demultiplexing Dolby AC-4 bitstreams from an ISO base media file segmentBuffering must be considered when product designers implement an ISO base media file format file demultiplexer that supports Dolby AC-4 bitstreams.
HTTP Live Streaming and Dolby AC-4 overviewIntroduction about HLS and Dolby AC-4
- HTTP Live Streaming HLS is a protocol developed by Apple for transferring unbounded streams of multimedia data. HLS works by breaking the content into a sequence of small HTTP-based file segments, each of which can be downloaded independently. Alternative segments at different data rates are available, allowing streaming to adapt to different network conditions.
- Raw AC-4 frameEach raw AC-4 frame is composed of a table of contents and at least one substream.
- Random access pointThe Dolby AC-4 decoder features a seamless switch between Dolby AC-4 elementary streams of the same media content and different configuration options.
- Sync frames A Dolby AC-4 bitstream in the AC-4 sync frame transport format (as specified in ETSI TS 103 190) consists of Dolby AC-4 synchronization frames. The AC-4 sync frame transport format must be used for MPEG packed audio.
Glossary
Notices

Sync frames

A Dolby AC-4 bitstream in the AC-4 sync frame transport format (as specified in ETSI TS 103 190) consists of Dolby AC-4 synchronization frames. The AC-4 sync frame transport format must be used for MPEG packed audio.

Each frame begins with a sync word and optionally ends with a CRC word. The sync word allows a decoder to easily identify frame boundaries and begin decoding. The CRC word allows a decoder to detect the occurrence of bitstream errors and to perform error concealment when it detects an error. The actual codec frame is referred to as the raw AC-4 frame.

This figure shows the general structure of the AC-4 frames as specified in ETSI TS 103 190.

Figure 1. The AC-4 sync frame transport format

Possible values for the sync word are:

0xAC40: No CRC field is included.
0xAC41: A CRC field is included.

The CRC is based on the raw AC-4 frame. The frame size and sync word fields are not part of the CRC calculation.

The frame size signals the size of the raw AC-4 frame and can be either two or five bytes in size. When a five-byte frame size field is used, the first two bytes are set as 0xFF and the actual size value is written into the remaining three bytes.

Each AC-4 sync frame element with the sync word 0xAC41 can contain a 16-bit CRC word. The CRC applies to the complete AC-4 sync frame element, not including the sync word. Decoding of the CRC word allows errors to be detected.