mp4 atom - How to discriminate the audio codec? Is it AAC or MP3?

I'm working on a mp4 container parser but I'm going crazy trying to recognize the audio codecs of the streams.I used both QtAtomViewer and AtomicParsley but when I find the atom:

trak->mdia->minf->stbl->stsd

I get always "mp4a" even if the mp4 file has an mp3 stream.

Should I look for an ".mp3" fourcc?

I attach two different mp4 structures:mp4 container with AAC audio stream

     Atom trak @ 716882 of size: 2960, ends @ 719842     Atom tkhd @ 716890 of size: 92, ends @ 716982     Atom mdia @ 716982 of size: 2860, ends @ 719842         Atom mdhd @ 716990 of size: 32, ends @ 717022         Atom hdlr @ 717022 of size: 33, ends @ 717055         Atom minf @ 717055 of size: 2787, ends @ 719842             Atom dinf @ 717063 of size: 36, ends @ 717099                 Atom dref @ 717071 of size: 28, ends @ 717099             Atom stbl @ 717099 of size: 2727, ends @ 719826                 Atom stts @ 717107 of size: 24, ends @ 717131                 Atom stsz @ 717131 of size: 1268, ends @ 718399                 Atom stsc @ 718399 of size: 40, ends @ 718439                 Atom stco @ 718439 of size: 32, ends @ 718471                 Atom stss @ 718471 of size: 1264, ends @ 719735                 Atom stsd @ 719735 of size: 91, ends @ 719826                     Atom mp4a @ 719751 of size: 75, ends @ 719826                         Atom esds @ 719787 of size: 39, ends @ 719826             Atom smhd @ 719826 of size: 16, ends @ 719842

mp4 container with mp3 audio stream

Atom trak @ 1663835 of size: 4844, ends @ 1668679     Atom tkhd @ 1663843 of size: 92, ends @ 1663935     Atom mdia @ 1663935 of size: 4744, ends @ 1668679         Atom mdhd @ 1663943 of size: 32, ends @ 1663975         Atom hdlr @ 1663975 of size: 45, ends @ 1664020         Atom minf @ 1664020 of size: 4659, ends @ 1668679             Atom smhd @ 1664028 of size: 16, ends @ 1664044             Atom dinf @ 1664044 of size: 36, ends @ 1664080                 Atom dref @ 1664052 of size: 28, ends @ 1664080             Atom stbl @ 1664080 of size: 4599, ends @ 1668679                 Atom stsd @ 1664088 of size: 87, ends @ 1664175                     Atom mp4a @ 1664104 of size: 71, ends @ 1664175                         Atom esds @ 1664140 of size: 35, ends @ 1664175                 Atom stts @ 1664175 of size: 24, ends @ 1664199                 Atom stsc @ 1664199 of size: 28, ends @ 1664227                 Atom stsz @ 1664227 of size: 2228, ends @ 1666455                 Atom stco @ 1666455 of size: 2224, ends @ 1668679

ThanksFE

UPDATE:

I found a way to solve the problem:by watching the code of AtomicParsley I see that it's possible to get thecodec informations about the stream atom (mp4a), reading the 11th Byteinto the esds (Elementary Stream Description) atom.

Now I'm working in this way:

if the value of the 11th Byte is 0x40 I assume the stream is AAC, else if I read 0x69 I assume that the stream is MP3.

I don't like these "empirics" solutions so I'm looking for a more correctway, but I found onlyUnderstanding_AAC that is not complete.

Anyone know where I can get a more detailed specification of MP4 containers?