为 a2dp 解码 MPEG 2,4 AAC-LC LATM 比特流

Question

我正在尝试解码从 Apple IPhone 收到的用于蓝牙 A2DP 配置文件的比特流。

根据 Apple 的说法，结构应该是

我的样本比特流是

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

前四个字节是L2CAP，接下来的12个是AVDTP。但我无法解码其余字节。请帮忙。

我看过定义 AudioMuxElement 的 ISO 14496-3，但我仍然无法将它与字节相关联。

Answer 1

"ISO/IEC 14496-3, Subpart 1: Main, Section 1.6: Interface to 14496-1" 是正确的地方看。

先看看AudioSyncStream():

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

LATM 应以 11 位长同步字开头：syncword==(2B7)hex==(1010110111)bin。您的流似乎不是这种情况：(47FC)hex==(0100011111111100)bin，前 11 位是 (01000111111)bin。

Answer 2

我能够解码 header 如下：

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)

看来我使用的是旧版本的规范。要查找的 Spec 的正确版本是 ISO/IEC 14496-3:2005。 Apple 的蓝牙设计指南文档中也存在拼写错误（它指的是 non-existent doc 13818-3:2005 ！）。

Answer 3

最近，我遇到了同样的问题。由于在一张纸上解码比特流不是很方便，我创建了一个简单的 decoder（用于调试目的）基于 ISO/IEC 14496-3 参考实现。

来自问题的正确解码流：

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

为 a2dp 解码 MPEG 2,4 AAC-LC LATM 比特流

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

bluetooth

aac

mpeg-4

a2dp