I have just downloaded the evaluation copy of Easy Audio File Converter. Two of the output formats are MPEG-2 AAC (.aac) and MPEG-4 AAC (.mp4).

The first few bytes are not quite the same between these. After those bytes, the content looks like the same. Anyone can tell me the differences between these?

P.S. I can play the mp4 music with QuickTime, but not ok with aac

The .mp4 file is the .aac file inside an MP4 container.

(The MP4 format is a media container, similar to AVI or MOV)

And the naming is wrong. MPEG2 AAC and MPEG4 AAC refer to MPEG versioning only.

In that specific case, the files should be named "AAC" and "MP4" (or "AAC inside MP4"), because the way it is now, it's misleading.

QuickTime, as well as most other MPEG4 players, can only play back AAC tracks inside the MP4 container.

BTW, just for trivia, Easy Audio File Converter uses FAAC as encoder.

Ha?! How come it can encode so fast!?

Faac became a very fast encoder after Krzysztof Nikiel started tuning it for speed and quality.

you mean he's the engineer in the software house?
or the open-source one is now sped up by him?

can i get the source code of that "fast" version?

The open source one.

It's LGPL'd, by the way, so even if someone tunes it, this person will have to (theoretically) make his improvements sources available.

You can download them at www.audiocoding.com

And latest binary is available at RareWares.