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Is there any difference between M4A audio files and AAC audio files or are they exactly the same thing but with a different file extension?

Serge Stroobandt
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alexandertr
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3 Answers3

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.M4A files typically contain audio only and are formatted as MPEG-4 Part 14 files (.MP4 container).

.AAC is not a container format and instead it is a raw MPEG-4 Part 3 bitstream with audio stream encoded.

Note that M4A does not have to contain exactly AAC audio, there are other valid options as well.

Roman R.
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    Isn't it the case that `.M4A` is a container separate from the .`MP4` container, the former being an audio container - in the same way that `.M4V` is a video container - whereas the latter, by contrast, is a container for *both audio and video*? Your answer seems to simply imply `.M4A` files use `.MP4` containers, which is not how I understand things. – Hashim Aziz Oct 30 '16 at 01:53
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    @Hashim: M4A and MP4 (and other) are extensions that suppose container format known as MP4. The structure of data is exactly the same, it's just choice of extension of M4A that suggests that the file might have audio only (which is not a strict condition, rather more of a hint). – Roman R. Oct 30 '16 at 08:51
  • So M4A is not a codec? I'm trying to understand why the FFmpeg interface in Accord.NET gives AAC and M4A as choices for audio codecs. – Kyle Delaney May 19 '17 at 15:16
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    @KyleDelaney M4A is not a codec. MP4/M4A/M4V are all interchangeable file extensions that are used for convenience to refer to the same MP4 container format, as a hint to the enclosed media type. M4A: audio only, M4V: video only, MP4 (and sometimes M4V): both. The most common codecs used for MP4 files are H.264 video and AAC audio. If M4A is listed as a codec option, it likely just gives you a choice between multiple standard MP4 audio codecs (or if not, is just AAC with different options). – Beejor Sep 05 '17 at 16:25
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    Adding to comments from @Beejor to try and help clarify for folks: M4A, MP4, and MV4 are containers. In reference to the OP, .AAC is a file extension, and AAC is a codec. Making matters more confusing (of course!), AAC is one of the few audio codecs that is capable of standing alone, and it may also be included ("contained") inside a container (such as MP4). Re-capping: AAC is a codec and can also be a File Extension. M4A is a container. M4A is not a codec. – MrPotatoHead Jan 19 '20 at 01:32
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There are raw video and audio streams, this streams cannot be played directly on most video/audio player, they need to be "encapsulated" on a transport, a raw H.264 video stream and a raw AAC audio stream need to be inside a MP4 encapsulator, it can be also inside an AVI or MOV encapsulator.

A MP4 file can contain a H.264 video stream and/or an AAC audio stream, but for some reason someone decided that a MP4 file that contains video and audio use the file extension M4V (v for video) and if it is an MP4 file that only contains audio to use the M4A extension, that is a common practice in other encapsulators like Windows Media which use WMV and WMA, or OGG which use OGV and OGA, silly as it seems.

So a file that has a M4A file extension is an MP4 file that can contain a AAC audio track but it is not always the case, that's why programs like mediainfo become handy to know what is inside a file.

Rodrigo Polo
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    Where can I read more about your statement that some audio players [as of 2013] can play only "encapsulated" files but not raw streams? – root May 27 '20 at 10:15
  • Just try to playback an AAC track on iTunes or Windows Media without using a MP4 encapsulation. – Rodrigo Polo Jun 03 '20 at 11:03
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    I don't have iTunes. Windows Media Player version 12.0.18362 can play it. To me it sounds like "ecapsulated" is raw AAC plus extra headers, i.e. more difficult to parse than raw. So I'm wondering whether I'm wrong that raw AAC is *simpler*, or otherwise why raw AAC is supported less often. – root Jun 03 '20 at 12:45
  • Here is a great article which explains the difference between codecs and containers: https://tubularinsights.com/file-formats-containers-compression/ – Rodrigo Polo Jun 08 '20 at 05:09
  • The link doesn't answer my question: why raw AAC is supported less often. But I have a theory now. Since video with audio is always in a container, programmers *have to* implement reading of video+audio containers; then from there it might be easier to implement reading of audio containers than reading of raw audio. – root Jun 09 '20 at 09:46
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They are not the same thing.

An .m4a file is basically the same thing as an mp4; it is only a container format. codec != container It does not imply a codec, and therefore it can only contain mp3, ac3 or any other audio codec.

An .aac file contains concatenated AAC frames pre-pended with ADTS headers (and optionally an ID3 tag).

Serge Stroobandt
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szatmary
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