Same codec, better container
An .aac file and an .m4a file often contain the same kind of audio: AAC. The difference is the packaging. Raw AAC (ADTS) is a bare stream of audio frames with no index and no place for information about the song. M4A wraps the audio in an MP4 container, which adds:
- a correct duration, so players show the right length;
- an index for instant, accurate seeking;
- tags such as title, artist, album and cover art;
- gapless playback information;
- full support in iTunes, the Apple Music app, iPhone, iPad and Windows 10 and 11.
That is why converting AAC to M4A is the best fix when a raw AAC file behaves oddly or is not accepted by Apple software.
How this converter handles the audio
Here the audio is decoded and encoded again as AAC inside M4A, with the bitrate you choose. That gives you control over size and makes every output file consistent, but it is a second lossy encode. To keep the loss inaudible, pick a bitrate at least as high as the source:
| Source AAC | Suggested M4A bitrate |
|---|---|
| 64 kbps speech | 96 kbps |
| 128 kbps | 160 to 192 kbps |
| 192 kbps | 256 kbps |
| 256 kbps | 256 or 320 kbps |
The M4A will be about the same size as the AAC at the same bitrate, plus a few kilobytes for the container.
After converting
Raw AAC has no tags, so the M4A starts without a title or artwork. Add them in the Music app with Get Info, or in iTunes on Windows. Once tagged, the files sort properly into albums and sync to an iPhone like purchased songs. If the AAC was a recording you want to share widely, consider AAC to MP3 for even broader support. To make a ringtone from it, use AAC to M4R; M4R is the same container with a different extension. Need to shorten the recording? Cut audio handles M4A directly.