Advanced Audio Coding: How to convert AAC to MP3 and more

Audio Coding defines the compression of audio files using specific details of the signals. The human ear is not able to here all information of the acoustic signals. This is the theoretical basis of Audio Coding. There are different ways of coding: Dolby Digital (or ATSC A/52, AC-3), Advanced Audio Coding, MPEG-1 Layer 2 Audio Codec, MPEG-1 Layer 3 Audio Codec, Musepack, Digital Theater Systems, Adaptive Transform Acoustic Coding, Vorbis (or Ogg Vorbis) and Windows Media Audio.

Advanced Audio Coding (AAC) is one of those schemes. It should be a development of MPEG-1 Layer 2 Audio Codec. It was designed by the MPE-Group (Moving Pictures Experts Group). This includes Dolby, Fraunhofer, AT&T, Sony and Nokia. The idea was to create something better than the popular MPEG-3 format. Advantages of Advanced Audio Coding are an improved compression, smaller file sizes and a wider range of data rates. Furthermore multichannel audio is supported and less power is necessary for the process of decoding.

Often one can find the using of the Advanced Audio Coding in the music business. For example the iTunes Store, the techniques of the iPod or the Real Music Store are using the technology of Advanced Audio Coding. Besides mobile phones of the companies Nokia, Samsung, Ericsson and Siemens have implemented the Advanced Audio Coding as technique, that makes it possible to listen to music on the phone.

The method to radically minimize the amount of data which is needed for the audio files is twofolded. One course is to disable irrelevant signal components which are not necessary for the human perception. The other route lies in the substraction of redundancies in the coded audio signal.

All in all the sound quality of Advanced Audio Coding is better than the sound qualities of the MPEG-3 format in the same bitrate. It provides a higher coding efficiency for transient and stationary frequencies.

Will Advanced Audio Coding replace MPEG-3?

Standardly it is used for Sony´s Playstation 3, of course. Sony is a member of the MPE-Group. Advanced Audio Coding is an international standard since April in 1997 as part of the MPEG 2-family. And since 1999 it is also part of the MPEG-4. It offers more flexibility in designing codecs for the development departments. So it might be only a question of time until it becomes so popular as MPEG-3 is now. On the other side there might still be many technical doors open to create even this technology more efficient for the developer and the user. So it could happen that the creative workers of the MPE-Group create a new coding format before Advanced Audio Coding has had its chance getting 3 minutes of glory.