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MPEG Surround (ISO/IEC 23003-1 or MPEG-D Part 1), also known as Spatial Audio Coding (SAC), is a lossy compression audio format for surround sound that provides a method for extending mono or stereo audio services to multi-channel audio in a backwards compatible fashion. The total used for the (mono or stereo) core and the MPEG Surround data are typically only slightly higher than the used for coding of the (mono or stereo) core. MPEG Surround adds a side-information stream to the (mono or stereo) core bit stream, containing spatial image data. Legacy stereo playback systems will ignore this side-information while players supporting MPEG Surround decoding will output the reconstructed multi-channel audio.
Moving Picture Experts Group (MPEG) issued a call for proposals on MPEG Spatial Audio Coding in March 2004. The group decided that the technology that would be the starting point in standardization process, would be a combination of the submissions from two proponents - Fraunhofer IIS / Agere Systems and Coding Technologies / Philips. The MPEG Surround standard was developed by the Moving Picture Experts Group (ISO/IEC JTC 1/SC29/WG11) and published as ISO/IEC 23003 in 2007. It was the first standard of MPEG-D standards group, formally known as ISO/IEC 23003 - MPEG audio technologies.
MPEG Surround was also defined as one of the MPEG-4 Audio Object Types in 2007. There is also the MPEG-4 No Delay MPEG Surround object type (LD MPEG Surround), which was published in 2010. The Spatial Audio Object Coding (SAOC) was published as MPEG-D Part 2 - ISO/IEC 23003–2 in 2010 and it extends MPEG Surround standard by re-using its spatial rendering capabilities while retaining full compatibility with existing receivers. MPEG SAOC system allows users on the decoding side to interactively control the rendering of each individual audio object (e.g. individual instruments, vocals, human voices). There is also the Unified Speech and Audio Coding (USAC) which will be defined in MPEG-D Part 3 - ISO/IEC 23003-3 and ISO/IEC 14496-3:2009/Amd 3. MPEG-D MPEG Surround parametric coding tools are integrated into the USAC codec.
The (mono or stereo) core could be coded with any (lossy or lossless) audio codec. Particularly low bitrates (64-96 kbit/s for 5.1 channels) are possible when using HE-AAC v2 as the core codec.
MPEG Surround encoder receives a multichannel audio signal x1 to xN where the number of input channels is N. The most important aspect of the encoding process is that a downmix signal, xt1 and xt2, which is typically stereo, is derived from the multichannel input signal, and it is this downmix signal that is compressed for transmission over the channel rather than the multichannel signal. The encoder may be able to exploit the downmix process so as to be more advantageous. It not only creates a faithful equivalent of the multichannel signal in the mono or stereo downmix, but also creates the best possible multichannel decoding based on the downmix and encoded spatial cues as well. Alternatively, the downmix could be supplied externally (Artistic Downmix in before Diagram Block). The MPEG Surround encoding process could be ignored by the compression algorithm used for the transmitted channels (Audio Encoder and Audio Decoder in before Diagram Block). It could be any type of high-performance compression algorithms such as MPEG-1 Layer III, MPEG-4 AAC or MPEG-4 High Efficiency AAC, or it could even be PCM.
The spatial signals are generated and recovered in two types of filter modules. The reverse-OTT (one-to-two) generates one downmixed stream, one level difference, one coherence value, and an optional residue signal from one pair of signals. The reverse-TTT (two-to-three) element generates two downmixed streams, two level differences, one coherence value, and an optional residue signal. In both the forward (decoding) and reverse (encoding) directions, arranging these filters into a tree setup allows for arbitrary downmixing and recovery. Abstract
MPEG Surround also supports a mode in which the downmix is compatible with popular matrix surround decoders, such as Dolby Pro-Logic.
DRM+ was designed to be fully capable of transmitting MPEG Surround and such broadcasting was also successfully demonstrated.
MPEG Surround's backward compatibility and relatively low overhead provides one way to add multichannel sound to DAB without severely reducing audio quality or impacting other services.
Applications
Digital Audio Broadcasting
Due to the relatively small channel bandwidth, the relatively large cost of transmission equipment and transmission licenses and the desire to maximize user choices by providing many programs, the majority of existing or planned digital broadcasting systems cannot provide multichannel sound to the users.
Digital TV Broadcasting
Currently, the majority of digital TV broadcasts use stereo audio coding. MPEG Surround could be used to extend these established services to surround sound, as with DAB.
Music download service
Currently, a number of commercial music download services are available and working with considerable commercial success. Such services could be seamlessly extended to provide multichannel presentations while remaining compatible with stereo players: on computers with 5.1 channel playback systems the compressed sound files are presented in surround sound while on portable players the same files are reproduced in stereo.
Streaming music service / Internet radio
Many Internet radios operate with severely constrained transmission bandwidth, such that they can offer only mono or stereo content. MPEG Surround Coding technology could extend this to a multichannel service while still remaining within the permissible operating range of bitrates. Since efficiency is of paramount importance in this application, compression of the transmitted audio signal is vital. Using recent MPEG compression technology (MPEG-4 High Efficiency Profile coding), full MPEG Surround systems have been demonstrated with bitrates as low as 48 kbit/s.
See also
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