What is the difference between M4A and AAC Audio F

问题:

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

回答1:

.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.

回答2:

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.

回答3:

They are not the same thing. An m4a is basically the same thing as an mp4, and is just a container format. codec != container. It does not imply a codec, and therefor could contain mp3, ac3 or any other audio codec. A AAC file is concatenated AAC frames prepended with ADTS headers (and optionally an ID3 tag).