AAC – audio compression and file format

AAC can mean a file format for audio files, but also a so-called audio data compression method. Because the abbreviation stands for "Advanced Audio Coding", and this occurs in file formats or containers such as MPEG or MP4, among other things. Although it is a lossy audio compression, contrary to this designation, high quality can be achieved even at low bit rates. You can find more details on AAC audio compression and the associated file format in this guide. Do you have any questions or additions? Then please leave a comment.

History of AAC as a compression method

The Advanced Audio Coding was developed in the mid-1990s by the Moving Pictures Experts Group (MPEG) and from 1997 z. B. used for the video container MPEG. The encoding is used for lossy audio compression, although this technical term does not mean that the audio compressed with AAC is of poor quality. 

On the contrary: "With less complexity than MP3, better quality can be achieved with the same data rate", is it [called at Wikipedia. Multi-channel coding and the use of tools to create more complex audio tracks are also possible. That is why AAC audio data compression is used in commonly used video containers.

From 1997, AAC was used in the MPEG-2 standard. It is interesting to see what significance this still has for audio playback over the Internet to this day. Because the three profiles of the new standard defined at that time include e.g. B. SSR for audio streaming:

• Low Complexity (LC)
• Main Profile
• Scalable Sample Rate (SSR)

From 1999 MPEG-4 came along and with it the containers and file formats MP4 and M4A. AAC was recorded as General Audio Coding for video files of the new standard and divided into different object types. Since then, the combination of the original AAC with the procedure known as Perceptual Noise Substitution has been regarded as main or object type 1. AAC Low Delay (LD) has also been specified. 

HE-AAC and HE-AAC 2 also emerged as new profiles in which several object types were used in combination. HE-AAC or AAC+ or aacPlus has been expanded to include spectral band replication and has been optimized for low bit rates of 32 to 80 kbit/s. HE-AAC v2 or eAAC+, AAC+ v2 or aacPlus v2 brought the combination with Parametric Stereo (PS) and offered optimization to even lower bit rates of 16 to 40 kbit/s.

Use of AAC in different areas

While AAC is used in widespread video containers such as MPEG or MPG, the AAC file format has not been able to establish itself as widely as MP3. However, it will still be available in file formats like 3GP, MP4, M4A and M4B used. Audio compression is also used in digital audio broadcasting, e.g. B. in the realization of the digital radio DAB +. The following comparisons are made on the Wikipedia page linked above to assess the AAC quality at different bit rates:

• From 64 kbit/s - acceptable, albeit limited, stereo
• From 96 kbit/s - quality comparable to VHF transmission
• From 128 kbit/s - High transparency for better audio quality
• From 192 or 224 kbit/s – Comparable with CD format of 1.411 kbit/s (low-loss)
• With HE-AAC and HE-AAC v2 already good quality from 16, 32, 40 and 80 kbit/s

Play AAC files on Mac

If you want to play back an AAC file or a video file in 3GP, MP4 or MPEG-2 format on the Apple Mac, then e.g. B. the preinstalled one QuickTime Player. This still supports MPEG-4 files in various formats (MP3, MP4, M4V, M4A) and a few more. Plus you can do almost anything with it free VLC player play back In addition to macOS, there are also versions for iOS and iPadOS on the iPhone and iPad as well as for Windows, Linux and Android. If you haven't had enough of the topic yet, there is more information on the website of the Moving Pictures Experts Group.

Johannes Domke

After graduating from high school, Johannes completed an apprenticeship as a business assistant specializing in foreign languages. But then he decided to research and write, which resulted in his independence. For several years he has been working for Sir Apfelot, among others. His articles include product introductions, news, manuals, video games, consoles, and more. He follows Apple keynotes live via stream.