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Difference Between AAC and M4A

mp3-mp4AAC vs. M4A

In lossy compression codecs that are used for encoding audio into much smaller file sizes, MP3 has held the throne for a considerable length of time. AAC, which stands for Advanced Audio Coding, is the intended replacement for MP3, due to its improved sound quality. However, unlike MP3, which has a unified .mp3 extension, AAC encoded audio files can have a variety of extensions that include .aac and .m4a. With that information, we can say that AAC is the actual audio encoding scheme, while M4A is simply a file extension.

Due to the better sound quality that it produces, especially at very low bitrates, AAC is beginning to gain widespread acceptance among the general population. As support for the encoding scheme gradually appears in mobile phones and portable media players, it is only a matter of time before it totally renders MP3s obsolete. Apple was one of the biggest motivators towards the shift to AAC. This is because they’ve made AAC the main format for their iPods, and even for the songs that are being sold in iTunes. A lot of iPod users are also ripping their audio CDs to AAC, to maintain standard file formats.

The AAC encoding gained prominent use as the main audio encoding for lossy MP4 videos. When files that only contain an audio stream are created, they still carry the MP4 file extension. To create a distinction between files that have both video and audio, and those that only contain audio, the M4A extension was created as a subtype of the MP4 extension. Even though the file extensions are different, they are literally identical, and there should be no problems with playing both files in a device that is capable of playing either one.

The confusion between these two file format extensions stem from the immediate belief that a different extension means a different codec. Although this is mostly true for other formats, this is not the case with AAC and M4A. This belief is aggravated even further by some players that list one file type, but not the other, despite being capable of playing both files. One of the more common recommendations to solve this problem is to replace the extension of the file so that the player will recognize it, and list and play it.


1. AAC is an audio encoding scheme ,while M4A is only a file extension.

2. An AAC encoded audio can have the AAC, MP4 and M4A extensions.

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  1. Very helpful. Can you clarify a little further ?
    My aac encoder (Pystel built 2002) has both mp2 and mp4 encoding options (?), and I presume these result in very different AAC files that may or maynot be compatible with decoders / devices ?
    It also appears to offer 2 different header formats as well as no header. (and other options)
    Are these all valid variant AAC files ?
    Can I assume that M4A can only be the mp4 type of AAC encoded file (if that makes sense) ?

    I used this encoder to convert WAV to AAC, and it played in my mediaplayer fine, but my son’s Nintendo DSi did not recognise the .aac file.
    I’ve read elsewhere that it requires .m4a – so that might only mean renaming the extension (this article implies that is the case)
    OR it might mean that I have to be careful to use parameters for mp4 type AAC encode ?, for the right header ?, and stay within the bitrate that will be compatible.

    • 2 years later. I’ll make a travel in time. I might be wrong, but we need to remember that .mp4 and .mp2 are container, thus it encapsulated the actual, raw data in its own format, specification, version… Container are all holding, sorting, indexing the data in a different way.

      When AAC first arrived, I think it may have spread a bit inside MP2 container before getting their popularity inside MP4 container. If an encoder is name [Etc] 2002, then that could mean it is encapsulating songs in MP2 format. Most simple devices are only willing to read MP4 AAC song, and H.264 AVC video.

      Some simple devices are way too slow to use software codecs like we do in Windows or OSX, and they are instead using electronic “chips” which contain some very fast codecs, and a limited set of these codecs, generally H.263, H.264, proprietary codecs (ex: Windows Phone reads Windows Movie Video – WMV), and AAC, MP3, some voice compressor (AMR, uLaw, aLaw), then all these phones will just refuse to read what can’t be read by their electronic chips.

  2. (Sorry, I’ve made several typos.)

  3. Great article, explains everything in a sufficintly short way – go on so!!!

  4. Pro tip: A 192kbps VBR AAC rip of a music CD is more than enough definition for human hearing range to not detect imperfections or differences from the original – 162kbps is actually enough for most humans. So don’t bother with WAVs hogging disc space and not being user-friendly.

  5. Hi, I think many people including me, which are reading this article would be interested if AAC can include surround sound. By briefly looking into it, I don’t think so. Which might be interesting for people trying to pick the right file to download…

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