Dolby Digital Plus
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 & 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 leverage 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.
Dolby Digital Plus Decoder Operating Modes:
|Mode||Reference Loudness (LKFS)||Application|
|Line||-31 LKFS||Home Theatre Playback - Provides Full "cinema" Dynamic Range|
|RF||-20 LKFS||TV Speaker Playback - Provides Typical "broadcast" Dynamic Range|
|Portable||-11 LKFS||Portable Device Speaker & Headphone Playback - Provides Minimum Dynamic Range (similar to music production/mixing/mastering techniques)|
NOTE: All of the decoder operating modes (listed above) are available in every Dolby Digital Plus decoder. The default operating mode is governed by device category and application. In some devices, users may also have a choice (via menu) to select an alternate mode that suits their particular taste and/or application.
Dolby Digital Plus also adds many new bitstream metadata parameters to support new applications:
- Carriage of multiple programs via substreams
- Support for more than 5.1 channels per program
- Mixing of multiple audio programs (substreams) to support worldwide media accessibility regulations for the hearing and visually impaired audiences
Additional Coding Tools
Dolby Digital Plus is largely based on the original Dolby Digital codec, but includes several enhancements in the form of new coding tools to improve coding efficiency:
- Spectral Extension - Provides efficient coding of high frequency information based on the harmonic relationship with low frequency content
- Transient pre-noise processing - Removes artifacts prior to transient events introduced by low data rate coding
- Adaptive Hybrid Transform - Provides more efficient coding of steady-state signals
- Advanced channel coupling - Allows efficient coding of common elements across channels to begin at a lower frequency
Dolby claims that the additional coding tools result in efficiency improvements of up to 50% while still allowing for the signal to be efficiently converted to Dolby Digital for backwards compatibility.
Dolby Digital Plus is capable of the following:
- Coded Bitrate: 0.032 to 6.144 Mbit/s
- Audio Channels: 1.0 to 15.1 (i.e. from mono to 15 full range channels and a low frequency effects channel)
- Number of Audio Programs per bitstream: 8
- Sample Rate: 32, 44.1 or 48 kHz
- Bit Depth: up to 24 bits per channel.
Physical transport for Consumer Devices
S/PDIF interface cannot directly transport Dolby Digital Plus (E-AC-3) bitstreams. But, IEC 61937-3: defines how to transmit Dolby Digital (AC-3) and Dolby Digital Plus (E-AC-3) bitstreams via an IEC 60958/61937 (S/PDIF) interface. So, a number of methods of transcoding exist to convert a Dolby Digital Plus (E-AC-3) bitstream into a S/PDIF compatible bitstream.
Physical transport for Professional Devices & Applications
SMPTE 340-2008 - Format for Non-PCM Audio and Data in AES3—ATSC A/52B Digital Audio Compression Standard for AC-3 and Enhanced AC-3 Data Types. This standard specifies data type specific format requirements for both Dolby Digital (AC-3) and Dolby Digital Plus (E-AC-3) data bursts carried within an AES3 interface according to SMPTE 337M. The combination of SMPTE 340-2008 and 337M allow the Dolby Digital Plus bitstream to be stored and transported within professional production, contribution and distribution workflows prior to emission to consumers.
For broadcast (emission) to consumers, the Dolby Digital Plus bitstream is packetized and multiplexed (with video) into an MPEG-2 Transport Stream. In ATSC systems, the specification for carrying Dolby Digital Plus is described in ATSC A/53 Part 3 & Part 6. In DVB systems, the specification for carrying Dolby Digital Plus is described in ETSI TS 101 154 and ETSI EN 300 468
HD DVD and Blu-ray Disc
The maximum number of discrete coded channels is the same for both formats: 7.1. However, HD DVD and Blu-ray Disc impose different technical constraints on the supported audio-codecs. Hence, the usage of DD+ differs substantially between HD DVD and Blu-ray Disc.
|Codec||HD DVD||Blu-ray Disc|
|AC-3||mandatory||1 to 5.1||504 kbit/s||mandatory||1 to 5.1||640 kbit/s|
|E-AC-3||mandatory||1 to 7.1||3.0 Mbit/s||optional, available for rear channels only||6.1 to 7.1||1.7 Mbit/s|
|1 or 2
3 to 8
|optional||1 to 8||18.0 Mbit/s|
On HD DVD, DD+ is designated a mandatory audio codec. An HD DVD movie may use DD+ as the primary (or only) audio track. An HD DVD player is required to support DD+ audio by decoding and outputting it to the player's output jacks. As stored on disc, the DD+ bitstream can carry for any number of audio channels up to the maximum allowed, at any bitrate up to 3.0 Mbit/s.
On Blu-ray Disc, DD+ is an optional codec, and is deployed as an extension to a "core" AC-3 5.1 audiotrack. The AC-3 core is encoded at 640 kbit/s, carries 5 primary channels (and 1 LFE), and is independently playable as a movie audio track by any Blu-ray Disc player. The DD+ extension bitstream is used on players that support it by replacing the rear channels in the 5.1 setup with higher fidelity versions, along with providing a possible channel extension to 6.1 or 7.1. The complete audio track is allowed a combined bitrate of 1.7 Mbit/s: 640 kbit/s for the AC-3 5.1 core, and 1 Mbit/s for the DD+ extension. During playback, both the core and extension bitstreams contribute to the final audio-output, according to rules embedded in the bitstream metadata.
Media players and downmixing
Generally, a Dolby Digital Plus bitstream can only be transported over an HDMI 1.3 or greater link. Older receivers support earlier versions of HDMI, or only have support for the S/PDIF system for digital audio, or analog inputs.
For non-HDMI 1.3 links, the player can decode the audio and then transmit it via a variety of different methods.
- Earlier versions of HDMI, such as HDMI 1.1, support PCM audio, where the player decodes the audio and transmits it losslessly as PCM over HDMI to the receiver.
- Some receivers and players support analog surround sound, and the player can decode the audio, and transmit it to the receiver as analog audio.
Most receivers and players support S/PDIF. This lower bandwidth digital connection is not capable of transmitting lossless audio with more than two channels, but a player can transmit a S/PDIF compatible audio stream to the receiver in one of the following ways:
- Blu-ray Disc players can take advantage of the legacy 5.1 AC-3 bitstream embedded in the E-AC-3 bitstream, transmitting just the AC-3 bitstream with no modifications.
- Players supporting the HD DVD standard can transcode the decoded audio into another format. Depending upon the method and options available to the player, this can be done with relatively little quality loss. Dolby's reference decoder, available to all licensees, exploits the common heritage between AC-3 and DD+ by performing the operation in the frequency domain. Hybrid re-compression avoids unnecessary end-to-end decompression and subsequent recompression (DD+ → LPCM → AC-3.) In addition to AC-3, some HD DVD players transcode audio compatible with S/PDIF into 1.5 Mbit/s DTS audio.
Should the player need to decode the audio for a non-HDMI 1.3 receiver, the results should be predictable. The DD+ specification explicitly defines downmixing modes and mechanics, so any source soundfield (up to 14.1) can be reproduced predictably for any listening environment (down to a single channel).
- Dolby Digital Plus technical specification at atsc.org