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AAC audio Stream packets for A2DP: LATM Audio Mux Element.
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AAC audio Stream packets for A2DP: LATM Audio Mux Element.



I am expanding the A2DP capabilities of a bluetooth device to support AAC.

 

I am using an IPOD gen 4 with latest iOS6 to connect to the bluetooth device.

 

I’ve captured the “media payload” packets coming in from the AVDTP layer and expected to be as described on page 15 of the “Bluetooth Accessory Design Guidelines for Apple Products” which shows the following fomat:

 

|L2CAP:HDR| |AVDTP:HDR| |MPEG-LATM: AudioMuxElement| |MPEG4 AAC Audio Payload|

 

The AudioMuxElement is not what I’d expect based on in 14496-3 (MPEG4 part 3 ) “MPEG-4 Audio transport stream”, table 1.28 which describes the AudioMuxElement(muxConfigPresent=1).

 

I’d expect it to first contain MuxConfiguration information.

 

What I get is this: an 8 byte header with a fixed value, and then a payloadLengthInfo( ) element (which gives the size of the audio payload) and then the payload.

For example:

 

|FC 00 00 B0 90 80 03 00| | 07 | 20 66 00 01 98 00 0e|

 

The design Guideline document indicates “the AudioMuxElement …is defined in Section 1.7.3 Table 1.32 in ISO/IEC 13818-3:2005”  So maybe the AudioMuxElement does not match MPEG4?  I have been unable to find a 2005 version of in ISO/IEC 13818-3, (the latest version I can find is from 1998) to see what is in Table 1.32 of its section 1.7.3.  I at first thought maybe there was a typo in the document name.

 

Can anyone explain the fields of that 8 byte header?  For now I’ve just been skipping over it and getting the MPEG4 data payload and it seems to work, but I want to understand that 8 byte header in case it contains pertintant info.

 

thanks

 

-Shawn

 

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