In fact, it is the most widely used codec for broadcast. The first three parts of the international standard cover the system layer, video coding scheme, and coded representation of the audio, respectively. The other documents each cover a significant component of the coding. As these documents need to remain current with the industry they serve, please note that there are numerous amendments and corrigenda to some of the current editions. Amendments alter materials in the existing standards, and corrigenda repair editorial errors in existing standards. This standard has three corrigendum documents: Cor , Cor , and Cor
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The first phase, MPEG-1, was dealing with mono and two-channel stereo sound coding, at sampling frequencies commonly used for high quality audio 48, This extension is easily added to a MPEG-1 audio decoder because it mainly implies inclusion of some more tables. MPEG-2 BC supports up to 5 full bandwidth channels plus one low frequency enhancement channel such an ensemble of channels is referred to as '5. This multichannel extension is both forward and backward compatible with MPEG An AAC bitstream is not backward compatible, i.
The original MPEG-2 audio standard contained only the first workitems and was finalized in In order to improve coding efficiency for the 5-channel case, a non-backward-compatible audio coding scheme was defined AAC and finalized in The main application area of MPEG-2 is digital television.
It produces the video quality needed in HDTV. MPEG-2 AAC comes in 3 different "flavors", called profiles, which are coder configurations with different complexity and performance. They are derived from each other by different parametrization of certain algorithmic parts of the AAC coder.
It enables coding at even lower sampling frequencies 8 kHz, This enables fully compatible decoding with an MPEG-1 decoder. In addition, the need to transfer two separate bitstreams, called simulcast one for two-channel stereo and another one for the multichannel audio programme is avoided, at some cost in coding efficiency for the multichannel audio signal, compared to AAC which is a Non Backward Compatible NBC coding algorithm.
The left and right channel of the downmix together contain components of all the channels, according to the equations in the compatibility matrix. What are the reasons behind this revision? While implementing the MPEG-2 Audio standard, as published in , it was discovered that a certain combination of functionalities could not function properly.
Although this combination was not considered to be of great practical importance, it was felt necessary to correct the standard in this respect.
Since this necessitated a revision of the document, the opportunity was then taken to improve the standard in some other fields as well. In the Second Edition, these combinations are explicitly prohibited. In the first publication, a low-pass filter was to be applied to the monophonic surround signal in matrix mode 2 analogue surround mode.
This filter is omitted in the Second Edition, greatly simplifying the decoder and improving coding efficiency. The description of the syntax of the LFE channel was ambiguous. This description has been clarified. In addition to these technical changes, many editorial changes have been made, improving readability and clarity.
Also, an amendment concerning copyright registration has been incorporated in the standard. What are the impacts of the technical changes in the revision to the Technical Report and Conformance documents? There are no impacts on the Conformance document. There is only a minor impact on the Technical Report: one possible embodiment of a lowpass filter was implemented in the Technical Report. This filter has to be removed and the dematrix operations adapted.
An amendment to the Technical Report was prepared. Up to 16 programs can be described, each consisting of any number of the audio and data elements. Some of the improvements implemented by AAC are a filter bank with a higher frequency resolution, better entropy coding and better stereo coding. Two new coding tools are an optional backward prediction used only in the Main Profile and noise shaping in the time domain which mainly improves quality of encoded speech at low bit-rates.
The Main profile is intended for use when processing power, and especially memory, are not at a premium. The Low Complexity profile is intended for use when cycles and memory use are constrained, and the SSR profile when a scalable decoder is required. What are the reasons behind this revision and what are the the technical changes? Each layer in MPEG-1 standarizes its format for the bitstream containing the encoded sound data.
All three have the same basic layout, consisting of a sequence of audio frames with a header and sound data. The frame rate is constant. MPEG-2 AAC on the contrary leaves the choice of audio transport syntax to the application, standardizing only the format of the encoded audio data. The main application of ADIF is exchange of audio files.
The encoded audio data of one frame is always contained between two sync words. The number of bits in a frame however can be variable. When using ADTS as transport syntax, break-in is enabled. The complexity of break-in support depends on the profile. For MPEG-2 AAC Main Profile, when prediction is enabled, break-ins are more tricky, as break-ins can only occur when there is a predictor reset across all frequency bands.
This only happens in case of "attacks" when the bitstream switches from long to short windows, so the easiest way to break in a Main Profile bitstream is to start with a short block. For long windows the predictors are reset in a frequency-cyclic way, which may require up to frames before all predictors are reset.
So if you break in with long windows, some distortions might appear in the first few frames. The encoder can be set-up to reset the predictors more frequently which reduces the required number of frames needed before all predictors are reset. There is reference software for both an AAC example encoder and reference decoder.
It is a general multi-channel decoder capable of decoding up to 48 audio channels, 15 auxiliary low frequency enhancement channels and 15 data streams. Furthermore, it is quite efficient in that the compiled reference source coder decodes a stereo bitstream in real-time on a MHz Pentinum. The encoder software supports multi-channel encoding, implementing essentially all of the AAC coding tools. As usual, the encoder code is designed to demonstrate how to generate compliant AAC bitstreams rather than achieving optimum audio quality.
When purchasing the standard, you will get access to the MPEG reference software source for encoder and decoder. However, the encoder will not be optimized for quality or speed. To get state-of-the-art encoder source, you need to contact one of the companies which works on AAC. Since many of the services based on MPEG-2 AAC are currently in their introductory phase into the market, numbers are changing constantly. Main areas of application are Internet Audio Audio for digital television and radio both AM and FM radio successors Portable playback devices A current estimation amounts to several million decoders both software and hardware-based and a lower number of encoders.
MP3 is the current choice for near-CD quality digital audio. However, AAC is its designated successor as it is able to provide the same sound quality with a larger compression rate. In addition it enables higher quality encoding and playback for high definition audio at 96 kHz sampling rate. So AAC is the most promising candidate e.
AAC is a state-of-the-art audio compression algorithm that provides compression superior to that provided by older algorithms such as AC AAC and AC-3 are both transform coders, but AAC uses a filterbank with a finer frequency resolution that enables superior signal compression.
AAC also uses a number of new tools such as temporal noise shaping, backward adaptive linear prediction, joint stereo coding techniques and Huffman coding of quantized components, each of which provide additional audio compression capability. Furthermore, AAC is much more flexible than AC-3, in that AAC supports a wide range of sampling rates and bitrates, from one to 48 audio channels, up to 15 low frequency enhancement channels, multilanguage capability and up to 15 embedded data streams.
Both provide 5-channel audio coding capability, however AAC provides a factor of two better audio compression relative to MPEG-2 BC, and is appropriate in all situations in which backward compatibility is not required or can be accomplished with simulcast. This was shown in an independent and impartial test conducted by the Communications Research Centre G.
Soulodre, T. Grusec, M. Lavoie and L. Therefore, the results of this test do not indicate the actual performance of a commercial AAC system.
Can you propose more detailed information in the literature? International Standard, Bosi, K. Brandenburg, S. Quackenbush, L. Fielder, K. Akagiri, H. Fuchs, M.
Dietz, J. Herre, G. Davidson, and Y. Davidson, Y. Soulodre et al. Audio Enc. Organisational details What is the status of the standardisation process? The information above is provided for convenience of the reader. MPEG, however, is not in a position to guarantee the validity of any claim made by a party with respect to IPR ownership.
MPEG Audio FAQ
The encoder supports Layer 1 and Layer 2 audio. It does not support the multichannel extensions. It cannot generate packetized elementary streams, program streams, or transport streams. The output type must be set first, before the input type. The following table lists the required and optional attributes for the output media type.
MPEG-2 Part 3