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. 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)
• 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?

This section documents the ObjectTypeIndication used in MPEG-4 systems to indicate the type of streams. Applications for a new codec type will also automatically receive an object type indication.

This table also occurs in ISO/IEC 14496-1 (various versions), the MPEG-4 systems specification. The intent is that the table maintained by this registration authority is the most up-to-date and definitive version.

There is also an informative table of the current StreamType allocations.

    ValueObjectTypeIndication DescriptionDefined in/by0x00Forbidden 0x01Systems ISO/IEC 14496-1 (a)MPEG-40x02Systems ISO/IEC 14496-1 (b)MPEG-40x03Interaction StreamMPEG-40x04Extended BIFS (h)MPEG-40x05AFX Stream (i)MPEG-40x06Font Data StreamMPEG-40x07Synthetised TextureMPEG-40x08Text StreamMPEG-40x09LASeR StreamMPEG-40x0ASimple Aggregation Format (SAF) StreamMPEG-40x20Visual ISO/IEC 14496-2 (c)MPEG-40x21Visual ITU-T Recommendation H.264 | ISO/IEC 14496-10 (g)MPEG-40x22Parameter Sets for ITU-T Recommendation H.264 | ISO/IEC 14496-10 (g)MPEG-40x40Audio ISO/IEC 14496-3 (d)MPEG-40x60Visual ISO/IEC 13818-2 Simple ProfileMPEG-40x61Visual ISO/IEC 13818-2 Main ProfileMPEG-40x62Visual ISO/IEC 13818-2 SNR ProfileMPEG-40x63Visual ISO/IEC 13818-2 Spatial ProfileMPEG-40x64Visual ISO/IEC 13818-2 High ProfileMPEG-40x65Visual ISO/IEC 13818-2 422 ProfileMPEG-40x66Audio ISO/IEC 13818-7 Main ProfileMPEG-40x67Audio ISO/IEC 13818-7 LowComplexity ProfileMPEG-40x68Audio ISO/IEC 13818-7 Scaleable Sampling Rate ProfileMPEG-40x69Audio ISO/IEC 13818-3MPEG-40x6AVisual ISO/IEC 11172-2MPEG-40x6BAudio ISO/IEC 11172-3MPEG-40x6CVisual ISO/IEC 10918-1MPEG-40x6DPortable Network Graphics (f)PNG0x6EVisual ISO/IEC 15444-1 (JPEG 2000)MPEG-40xA0EVRC Voice3GPP20xA1SMV Voice3GPP20xA23GPP2 Compact Multimedia Format (CMF)3GPP20xA3SMPTE VC-1 VideoSMPTE0xA4Dirac Video CoderDirac0xA5AC-3 AudioETSI0xA6Enhanced AC-3 audioETSI0xA7DRA AudioDRA0xA8ITU G.719 AudioITU G.7190xA9DTS Coherent Acoustics audioDTS0xAADTS-HD High Resolution AudioDTS0xABDTS-HD Master AudioDTS0xACDTS Express low bit rate audio, also known as DTS LBRDTSE0xC0 - 0xE0user private 0xE113K Voice3GPP20xE2 - 0xFEuser private 0xFFno object type specified (e)MPEG-4

1. This type is used for all 14496-1 streams unless specifically indicated to the contrary. Scene Description scenes, which are identified with StreamType=0x03 (see Table 9 ), using this object type value shall use the BIFSConfig specified in section 9.3.5.2.2  of this specification.
2. Used, with StreamType=0x03 (see Table 9 ), for Scene Description streams that use the BIFSv2Config. Its use with other StreamTypes is reserved .
3. Includes associated Amendment(s) and Corrigendum(a). The actual object types are defined in ISO/IEC 14496-2 and are conveyed in the DecoderSpecificInfo as specified in ISO/IEC 14496-2, Annex K.
4. Includes associated Amendment(s) and Corrigendum(a). The actual object types are defined in ISO/IEC 14496-3 and are conveyed in the DecoderSpecificInfo as specified in ISO/IEC 14496-3 subpart 1 subclause 6.2.1.
5. Streams with this value with a StreamType indicating a systems stream (values 1,2,3, 6, 7, 8, 9) shall be treated as if the ObjectTypeIndication had been set to 0x01.
6. For PNG Streams, the following configuration/rules apply:
   • StreamType: 0x04 (visual)
   • No decoder specific info needed
   • The PNG stream shall consist in only one (1) AccessUnit
   • The PNG stream access unit shall contain the complete PNG image/file
   • MNG shall not be used inside PNG streams
7. Includes associated Amendment(s) and Corrigendum(a). The actual object types are defined in ITU-T Recommendation H.264 | ISO/IEC 14496-10 and are conveyed in the DecoderSpecificInfo.
8. Used,  with StreamType=0x03, for Scene Description streams that use the BIFSConfigEx; its use with other StreamTypes is reserved. (Was previously reserved for MUCommandStream but not used for that purpose.)
9. Used,  with StreamType=0x03, for Scene Description streams that use the AFXConfig; its use with other StreamTypes is reserved.

ValueStreamType Description0x00Forbidden0x01ObjectDescriptorStream (see 8.5)0x02ClockReferenceStream (see 10.2.5)0x03SceneDescriptionStream (see 9.2.1)0x04VisualStream0x05AudioStream0x06MPEG7Stream0x07IPMPStream (see 8.3.2)0x08ObjectContentInfoStream (see 8.4.2)0x09MPEGJStream0x0AInteraction Stream0x0BIPMPToolStream0x0CFontDataStream0x0DStreamingText0x20- 0x3Fuser private

参考：http://www.mp4ra.org/object.html