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MPEG Surround (ISO/IEC 23003-1 or MPEG-D Part 1), also known as Spatial Audio Coding (SAC)〔(【引用サイトリンク】 title=Working documents, MPEG-D (MPEG Audio Technologies) )〕 is a lossy compression 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 bit rates used for the (mono or stereo) core and the MPEG Surround data are typically only slightly higher than the bit rates 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 JTC1/SC29/WG11) and published as ISO/IEC 23003-1 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 Low 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.〔(【引用サイトリンク】title=ISO/IEC DIS 23003-3 - Information technology -- MPEG audio technologies -- Part 3: Unified speech and audio coding )〕〔(【引用サイトリンク】title=ISO/IEC 14496-3:2009/PDAM 3 - Transport of unified speech and audio coding (USAC) )〕 MPEG-D MPEG Surround parametric coding tools are integrated into the USAC codec.〔(【引用サイトリンク】title=Unified Speech and Audio Coder Common Encoder Reference Software )〕 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. == Perception of sounds in space == MPEG Surround coding uses our capacity to perceive sound in the 3D and captures that perception in a compact set of parameters. Spatial perception is primarily attributed to three parameters, or cues, describing how humans localize sound in the horizontal plane: Interaural level difference (ILD), Interaural time difference (ITD) and Interaural coherence (IC). This three concepts are illustrated in next image. Direct, or first-arrival, waveforms from the source hit the left ear at time, while direct sound received by the right ear is diffracted around the head, with time delay and level attenuation, associated. These two effects result in ITD and ILD are associated with the main source. At last, in a reverberant environment, reflected sound from the source, or sound from diffuse source, or uncorrelated sound can hit both ears, all of them are related with IC. Figure1. Illustration of ILD, ITD & IC 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「MPEG Surround」の詳細全文を読む スポンサード リンク
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