INTERNATIONAL ORGANISATION FOR STANDARDISATION
ORGANISATION INTERNATIONALE DE NORMALISATION
ISO/IEC JTC1/SC29/WG11
CODING OF MOVING PICTURES AND AUDIO
ISO/IEC JTC1/SC29/WG11 N2431
October 1998/Atlantic City
| Source | Audio Subgroup |
| Title | MPEG Audio FAQ Version 9 |
| Authors | D. Thom, H. Purnhagen, and the MPEG Audio Subgroup |
While implementing the MPEG-2 Audio standard, as published in 1995, 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.
The technical changes in the Second Edition compared to the first publication of ISO/IEC 13818-3 (1995) are :
In addition to these technical changes, many editorial changes have been made, improving readability and clarity.
An amendment concerning copyright registration has been incorporated in the standard.
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 is now being prepared.
The MPEG-2 AAC standard is a new, state of the art audio standard that provides very high audio quality at a rate of 64 kb/s/channel for multichannel operation. It provides a capability of up to 48 main audio channels, 16 low frequency effects channels, 16 overdub/multilingual channels, and 16 data streams. Up to 16 programs can be described, each consisting of any number of the audio and data elements. There are three profiles for the AAC standard, called Main Profile, Low Complexity Profile, and Scalable Sampling Rate Profile. The Main profile is intended for use when processing, 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. The Main and LC profiles have been tested at 320 kb/s for 5-channel audio programmes, and both have demonstrated better quality than competing audio coding algorithms running at 640 kb/s for the 5-channel program.
When will ISO/IEC 13818-4,5 for MPEG-2 AAC be available?
Both parts of the standards are currently out for ballot and should be available September 1998.
Why should I use MPEG-2 AAC rather than Dolby AC-3?
AAC is a state-of-the-art audio compression algorithm that provides compression superior to that provided by older algorithms such as AC-3. 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.
When should I use AAC rather than MPEG-2 BC?
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. An MPEG-1 two channel decoder can decode an MPEG-2 BC 5-channel bitstream. AAC has no such backward compatibility requirement and, for 5-channel audio signals, has been shown in MPEG formal listening tests to provide slightly better audio quality at 320 kb/s than MPEG-2 BC can provide at 640 kb/s.
AAC and PAC are similar audio coding technologies. However AAC has a number of new coding tools, such as Temporal Noise Shaping (TNS), that permits AAC to offer performance superior to that of PAC. This was shown in an independent and impartial test conducted by the Communications Research Centre (G. Soulodre, T. Grusec, M. Lavoie and L. Thibault, "Subjective Evaluation of State-of-the-Art 2-Channel Audio Codecs," Journal of the Audio Engineering Soc., Mar., 1998, pp. 164-177). This test showed that when coding stereo signals, the quality of AAC at 96 kb/s was comparable to the quality of PAC at 128 kb/s and that AAC at 128 kb/s was significantly better than PAC at 160 kb/s.
There is another test, conducted by Moulton Laboratories, that claims to compare PAC and AAC. However the system claimed to be AAC was not the same coding system tested by the Communications Research Centre, and did not use a state-of-the-art AAC encoder. Therefore, the results of this test do not indicate the actual performance of a commercial AAC system.
Is stream splicing or "break-in" supported in MPEG-2 AAC?
In MPEG-2 AAC Low Complexity and MPEG-2 AAC SSR modes, the prediction tools are not used, so break-in support is the same as that for MPEG-1 audio. For MPEG-2 AAC Main Profile, when prediction is enabled, break-ins are a little tricker, as break-ins can only occur when there is a predictor reset across all frequency bands. This only happens in case of æattacksÆ 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 240 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.
Is there any reference software for MPEG-2 AAC?
Yes. There is reference software for both an AAC example encoder and reference decoder. The decoder source is complete and fully compliant and is capable of decoding all three AAC profiles: Main, Low Complexity and Scaleable Sampling Rate. 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 100 MHz Pentinum.
The encoder software is not yet a general multi-channel encoder, and does not yet make use of all AAC coding tools.
What kind of support does MPEG provide for implementers of MPEG Audio?
MPEG provides different kinds of support to implementers. Firstly, a Technical Report is issued that contains software that describes the decoder and an example encoder. This software can be used by implementers to analyze and to get accustomed with the algorithms, and could be used as a basis for an implementation. This encoder can be used to generate test sequences. The Technical Report is published as part 5 of the standard, i.e. ISO/IEC 11172-5 for MPEG-1 and ISO/IEC 13818-5 for MPEG-2, ISO/IEC 13818-5 for MPEG-2 AAC.
Secondly, a conformance document is issued. This document provides guidelines to test conformance to the standard of bitstreams, and conformance of decoders. It also describes the accuracy level that a decoder should meet in order to be called an MPEG audio decoder or a 'high accuracy' MPEG Audio decoder.
An important part of the conformance document is a set of bitstreams and the corresponding reference decoder output, that address several functionalities of the decoder. The conformance document is published as part 4 of the standard, i.e. ISO/IEC 11172-4 for MPEG-1 and ISO/IEC 13818-4 for MPEG-2 and ISO/IEC 13818-4 for MPEG-2 AAC.
For MPEG-2 ISO/IEC 13818-5 a CD ROM was released which contains all the reference bitstreams needed to perform the conformance test of the decoder implementations. The CD ROM can be ordered directly from the ISO/IEC Copyright Office. Their address is given above.
Who should I contact for AAC license information & availability?
Licencing Contact is:
Dolby Laboratories
AAC Licensing Administration
Fax: +1-415-863-1373
e-mail: aacla@dolby.com