The high level structure of Dolby bitstreams has evolved since Dolby Digital was introduced. Dolby Digital bitstreams are independent and self-contained providing a single presentation varying from mono through to 5.1 channels. Dolby Digital Plus provides a super-set of Dolby Digital functionality with the introduction of independent and dependent substreams. Each Dolby Digital Plus bitstream is labelled as either an independent or dependent substream. A Dolby Digital Plus Independent substream is similar to a Dolby Digital bitstream in that it is self-contained and independently decodable. Dependent substream are sometimes transmitted in combination with an independent substream to provide additional functionality to decoders which can process the dependent substream. An example is carriage of 7.1 channels which is achieved by combining a 5.1 channel independent substream with a 4 channel dependent substream.

High-Level Bitstream Syntax (AC-4 Part 2)

Dolby AC-4 builds on the substream concept to provide next generation services and experiences. New to Dolby AC-4 is the introduction of the TOC (Table of Contents). The TOC contains the inventory of the bitstream. Each audio substream can carry either one or more audio channels or an individual audio object. This structure provides flexibility and extensibility that allows the Dolby AC-4 format to meet future requirements.

Dolby AC-4 also allows multiple Presentations to be carried in a single bitstream. Each Presentation defines a way of mixing a set of audio substreams to create a unique rendering of the program. Instructions for which substreams to use and how to combine them for each Presentation are specified in a Presentation info element carried in the TOC.

Presentations enable multiple versions of the audio experience, such as different languages or commentary, to be delivered in a single bitstream in a convenient, bandwidth-efficient manner. An example is shown in the figure below, where four versions of a live 5.1 sports broadcast—the original English version, two alternate languages (Spanish and Mandarin Chinese), and a commentary-free version—are combined into a single Dolby AC-4 bitstream.

Live 5.1 Sports Broadcast with Four Presentations

Please see the Dolby AC-4 White Paper for more information.