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What is AC3

Dolby Digital Plus (DD+ or E-AC-3 (Enhanced AC-3), and sometimes incorrectly as EC-3) is a digital audio compression scheme. E-AC-3 has a number of improvements including support for a wider range of data rates (32kbit/s to 6144kbit/s), increased channel count and multi-program support (via substreams) compared with Dolby Digital (AC-3). While Dolby Digital (AC-3) supports up to 5 full-bandwidth audio channels at a maximum bitrate of 640kbit/s, E-AC-3 supports up to 15 full-bandwidth audio channels at a maximum bitrate of 6.144 Mbit/s.

Dolby Digital Plus bitstreams are not directly backward compatible with legacy Dolby Digital decoders. However, Dolby Digital Plus decoders include a mandatory component that directly converts (without decoding and re-encoding) the Dolby Digital Plus bitstream to a Dolby Digital bitstream (operating at 640kbit/s) for carriage via legacy S/PDIF connections (including S/PDIF over HDMI) to external decoders (e.g. AVRs, etc.). Note: All Dolby Digital Plus decoders can decode Dolby Digital bitstreams.

Dolby Digital Plus leverages the same fundamental bitstream structure and transform length as Dolby Digital. This enables a simple conversion between Dolby Digital Plus and Dolby Digital bitstreams (as stated above). Dolby Digital Plus also includes comprehensive bitstream metadata for decoder control over output loudness (via dialnorm), downmixing, and reversible dynamic range control (via DRC).

The Dolby Digital Plus coding system is the only commercialized coding system that can create a single multichannel bitstream where the decoder can scale the number of decoded output channels and simultaneously optimize the playback loudness and dynamic range for any device type and application. Including: AVRs, TVs, STBs, PCs, Tablets, Mobile Phones, etc. As an example, a single multichannel bitstream can simultaneously provide full dynamic range for home theater playback and a compressed dynamic range 2-channel downmix at a reference output loudness tailored for internal speakers and headphones on portable/mobile devices.

This unique feature is enabled within the Dolby Digital Plus decoder (via specific decoder operating modes) which leverages specific metadata carried in the Dolby Digital Plus bitstream authored during the production process and/or within the encoder itself. This approach provides content creators and/or distributors with the ability to create a single asset (stream) that can scale and optimize the playback experience across a wide range of devices and applications. Thereby eliminating the need (and cost) to create multiple versions of the same content “pre-produced” for each device type and application. Specific playback behaviors (to ensure optimum experience) are also under direct control of the content creator and/or distributor via the use of metadata created in production and carried in the bitstream for use during decoding/playback.