Decode MPEG 2,4 AAC-LC LATM bitstream for a2dp

Question

I am trying to decode bitstream that I received from Apple IPhone for bluetooth A2DP profile.

According to Apple the structure should be

The sample bitstream I have is

36 00 47 00 | 80 60 00 01 00 06 3c ab 00 00 00 00 | 47 fc 00 00 b0 90 80 03 00 20 20 66 00 01 98 00 0d e1 20 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 1c

First four bytes are L2CAP, next 12 are AVDTP. But I am not able to decode rest of the bytes. Kindly help.

I have seen ISO 14496-3 that define AudioMuxElement but still I am not able to relate it to the bytes.

score 3 · Accepted Answer · answered Oct 01 '16 at 16:18

Recently, I've got very same problem. Since decoding bit-stream on a piece of paper is not very handy, I've created a simple decoder (for debugging purposes) based on ISO/IEC 14496-3 reference implementation.

Properly decoded stream from the question:

useSameStreamMux:1             = 0 => 0
audioMuxVersion:1              = 1 => 1
audioMuxVersionA:1             = 0 => 0
bytesForValue:2                = 00 => 0
valueTmp:8                     = 11111111 => 255 (taraBufferFullness)
allStreamsSameTimeFraming:1    = 1 => 1
numSubFrames:6                 = 000000 => 0
numProgram:4                   = 0000 => 0
numLayer:3                     = 000 => 0
bytesForValue:2                = 00 => 0
valueTmp:8                     = 00010110 => 22 (ascLen)
audioObjectType:5              = 00010 => 2
samplingFrequencyIndex:4       = 0100 => 4
channelConfiguration:4         = 0010 => 2
frameLengthFlag:1              = 0 => 0
dependsOnCoreCoder:1           = 0 => 0
extensionFlag:1                = 0 => 0
fillBits:6                     = 000000 => 0
frameLengthType:3              = 000 => 0
latmBufferFullness:8           = 11000000 => 192
otherDataPresent:1             = 0 => 0
crcCheckPresent:1              = 0 => 0
tmp:8                          = 00100000 => 32 (MuxSlotLengthBytes)
payload:8                      = 00100000 => 32
...
payload:8                      = 00011100 => 28
byteAlign:0                    = 0 => 0

score 1 · Answer 2 · answered Mar 11 '16 at 09:41

"ISO/IEC 14496-3, Subpart 1: Main, Section 1.6: Interface to 14496-1" is the right place to look.

First look into AudioSyncStream():

AudioSyncStream()
{
    while(nextbits()==0x2B7)        // 11-bits
    {
       audioMuxLengthBytes;         // 13-bit 
       AudioMuxElement(1);          // proceed to decode AudioMuxElement
    }
}

LATM should start withg 11-bit long syncword: syncword==(2B7)hex==(1010110111)bin. This does not seem to be the case with your stream: (47FC)hex==(0100011111111100)bin, first 11 bits being (01000111111)bin.

The issue I am facing also is that I cant find the syncword. Is it possible to transmit LATM without LOAS? Also I cant figure out what is first 9 bytes after AVDTP header, 47 fc 00 00 b0 90 80 03 00 20. The next byte '20' indicated length. — VinayChoudhary99, Mar 14 '16 at 05:20

score 0 · Answer 3 · edited Mar 14 '16 at 10:53

I was able to decode the header as follows:

Use same stream config       = 0 (1 bit)

StreamMuxConfig():

AudioMuxVersion              = 1 (1 bit)
AudioMuxVersionA             = 0 (1 bit)
LATM value Temp Bytes        = 00 (2 bit)
Fullness value               = 1111 1111 ( 8 Bit)
AllStreamsUseSameTimeFrame   = 1 (1 bit)
numSubFrames                 = 000000 (6 bits)
num program                  = 0000 (4 bits)
num layer                    = 000 ( 3 bits)
use same config              = 0 (1 bit)
Fill + other bits            = 0000 1010 0 (9 bits)

AudioSpecificConfig():

Object Type (aac)            = 00010 ( 5 bits)
Sample Type                  = 0100 ( 4 bits)
Channel                      = 0010 ( 4 bits)
GAS specific config          = 000 (3 bits)

Looks like I was having an older version of spec. The correct version of Spec to look for is ISO/IEC 14496-3:2005. Also there are typo errors in Apple's Bluetooth design guidelines doc (it refers to non-existent doc 13818-3:2005 !).

Decode MPEG 2,4 AAC-LC LATM bitstream for a2dp

3 Answers3