[mp4]The audio codec for mp4 atom

In the 'esds' atom there are a few fields of relevance to determining the codec used. The first byte of content of the esds atom is the objectTypeIndication. (This is the 11th byte from your solution.) This field is supposed to indicate the codec used but annoyingly there are a few entries used by multiple codecs. A full list of values is available from the MP4RA. From the list a few are relevant in this case.:

• 0x40 - MPEG-4 Audio
• 0x6B - MPEG-1 Audio (MPEG-1 Layers 1, 2, and 3)
• 0x69 - MPEG-2 Backward Compatible Audio (MPEG-2 Layers 1, 2, and 3)
• 0x67 - MPEG-2 AAC LC

0x6B and 0x69 denote MPEG-1 and 2 respectively layers 1, 2, and 3. 0x67 denotes MPEG-2 AAC LC but generally is unused in favor of 0x040 (0x66 and 0x68 are also MPEG-2 AAC profiles are seen even less frequently). 0x40 denotes MPEG-4 Audio. MPEG-4 Audio generally is thought of as AAC but there is a whole framework of audio codecs that can go in MPEG-4 Audio including AAC, BSAC, ALS, CELP, and something called MP3On4. MP3On4 is an MP3 variant with some new header information for multichannel.

We can figure out what audio format is actually in the MPEG-4 Audio by looking at the the AudioSpecificConfig. This is the global header for the decoder that exists at byte 13 of the content of the 'esds' atom.【ss：不是在byte 13，具体需要将esds字段解析，得到MP4DecSpecificDescr的data，video&audio是一样的，一般来说，11th为objectTypeIndication，11th+13th即24th为MP4DecSpecificDescr：起始码为0x05，后面跟的是size（data-size），然后为data。】At the begining of the AudioSpecificConfig there is a 5-bit AudioObjectType. A full list can be found on the multimedia wiki (that was linked in your post under the 'MPEG-4 Audio' article <http://wiki.multimedia.cx/index.php?title=MPEG-4_Audio> but here are the useful values:

• 00 - NULL
• 01 - AAC Main (a deprecated AAC profile from MPEG-2)
• 02 - AAC LC or backwards compatible HE-AAC (Most realworld AAC falls in one of these cases)
• 03 - AAC Scalable Sample Rate (rarely used)
• 03 - AAC LTP (a replacement for AAC Main, rarely used) 【ss：应该为04】
• 05 - HE-AAC explicitly signaled (Non-backward compatible)
• 22 - ER BSAC (A Korean broadcast codec)
• 23 - Low Delay AAC
• 29 - HE-AACv2 explicitly signaled (In one draft this was MP3On4 instead)
• 31 - ESCAPE (read 6 more bits, add 32)
• 32 - MP3on4 Layer 1
• 33 - MP3on4 Layer 2
• 34 - MP3on4 Layer 3

If you aren't worried about 'MP3On4' mp3 variant nor the other weird MPEG-4 Audio codecs then just use the objectTypeIndication.

In the MPEG specifications these details are spread across 14496-1, -12, -14, and -3. Of these only 14496-12 is freely available. <http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html>

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