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AAC: A Profile of the AAC Format

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Introduction:

  • Format name: MPEG-4 AAC / Advanced Audio Coding.
  • Format type: Audio.
  • File extensions: .aac, .m4a, .mp4, .m4b, .m4p, .m4r, .m4v.
  • Compression Type: Lossy.

The Advanced Audio Coding (AAC) format was developed by a group of companies with the goal of improving many of the drawbacks to using the MP3 standard. The AAC format is still a lossy audio format, just like MP3, but typically produces higher sound resolution (especially at low frequencies) due to the many improvements over MP3. There are many companies that have adopted this as their format of choice; Apple for example uses it for their iTunes Store.

Popular Hardware:

  • As discussed, Apple’s iTunes Store uses AAC encoded files with DRM for most of its music catalog. Apart from a few exceptions, the iPod and iPhone are the only hardware devices that are capable of using the FairPlay DRM system.
  • In the home video console market, Sony’s PlayStation 3, PSP, and Nintendo’s Wii utilize the AAC format in favor of MP3.
  • Portable media players such as the Microsoft Zune and Cowon A3 have built-in support for AAC.
  • Other hardware that has embraced the AAC format includes, cell phones, PDAs, and wireless audio streaming devices.

Sampling Frequencies & Bit Rates:

The AAC format provides the following sampling frequency range :

  • 8 - 96 kHz and above.
As for the MP3 format, it is the 44.1 kHz sample rate that is used the most; this is the same frequency that is used for the compact disc.

The advantage of using the AAC format compared to MP3 is that it is significantly more flexible when it comes down to bitrates – especially at the lower end. The bitrate range is:

Compression Algorithm:

The AAC format uses a lossy compression algorithm. Built into AAC is a suite of modules or profiles that make it a very flexible compression system. The most popular profile that is used to encode audio is the Low Complexity setting. This is the simplest module and the most commonly used.

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