AAC_LC用LATM封装header信息解析

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本文详细介绍了如何根据音频参数生成AAC_LC头信息，包括使用ffmpeg和FAAC库的具体实现过程，以及AAC_LC头信息的结构组成。

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通常来说AAC的头信息在编解码过程中是可以获取到的，但今天需要根据音频参数生成相应的AAC头。项目中使用的是AAC_LC，今天先对它的结构进行分析。

项目中使用ffmpeg进行音频编码，音频编码库为FAAC，好吧，直接看代码吧。

ffmpeg调用Faac_encode_init()初始化编码器；在Faac_encode_init()调用faacEncGetDecoderSpecificInfo()获取AAC_LC头信息内容及长度。

        int 
        FAACAPI faacEncGetDecoderSpecificInfo(faacEncHandle hEncoder,unsigned 
        char
        ** ppBuffer,unsigned 
        long
        * pSizeOfDecoderSpecificInfo)
       
        {
       
        BitStream* pBitStream = NULL; <span style=
        "font-family:Times New Roman;"
        ></span> 
        if
        ((hEncoder == NULL) || (ppBuffer == NULL) || (pSizeOfDecoderSpecificInfo == NULL)) {
       
        return 
        -1;
       
        }
       
        if
        (hEncoder->config.mpegVersion == MPEG2){
       
        return 
        -2; 
        /* not supported */
       
        }
       
        *pSizeOfDecoderSpecificInfo = 2;
       
        *ppBuffer = 
        malloc
        (2);
       
        if
        (*ppBuffer != NULL){
       
        memset
        (*ppBuffer,0,*pSizeOfDecoderSpecificInfo);
       
        pBitStream = OpenBitStream(*pSizeOfDecoderSpecificInfo, *ppBuffer);
       
        PutBit(pBitStream, hEncoder->config.aacObjectType, 5);
       
        PutBit(pBitStream, hEncoder->sampleRateIdx, 4);
       
        PutBit(pBitStream, hEncoder->numChannels, 4);
       
        CloseBitStream(pBitStream);
       
        return 
        0;
       
        } 
        else 
        {
       
        return 
        -3;
       
        }
       
        }

从代码中可以看出，头数据长度固定为2；

数据内容由高位到低位依次为：aacObjectType（5bits）,sampleRateIdx(4bits),numChannels(4bits)

例如：音频编码参数为：

aacObjectType:AAC_LC,对应值为2，用5bit二进制表示为00010;

sampleRate:44100KHz, 对应的IDX值为4, 用4bit二进制表示为0100;

numChannels:2，对应的值为2，用4bit二进制表示为0010；

将它们由高位到低位串起来：0001,0010,0001,0000，

则，对应的十六进制值为:0x1220

引一些参考资料：http://wiki.multimedia.cx/index.php?title=MPEG-4_Audio

Audio Specific Config

The Audio Specific Config is the global header for MPEG-4 Audio:

5 bits: object type
if (object type == 31)
    6 bits + 32: object type
4 bits: frequency index
if (frequency index == 15)
    24 bits: frequency
4 bits: channel configuration
var bits: AOT Specific Config

Audio Object Types

MPEG-4 Audio Object Types:

0: Null
1: AAC Main
2: AAC LC (Low Complexity)
3: AAC SSR (Scalable Sample Rate)
4: AAC LTP (Long Term Prediction)
5: SBR (Spectral Band Replication)
6: AAC Scalable
7: TwinVQ
8: CELP (Code Excited Linear Prediction)
9: HXVC (Harmonic Vector eXcitation Coding)
10: Reserved
11: Reserved
12: TTSI (Text-To-Speech Interface)
13: Main Synthesis
14: Wavetable Synthesis
15: General MIDI
16: Algorithmic Synthesis and Audio Effects
17: ER (Error Resilient) AAC LC
18: Reserved
19: ER AAC LTP
20: ER AAC Scalable
21: ER TwinVQ
22: ER BSAC (Bit-Sliced Arithmetic Coding)
23: ER AAC LD (Low Delay)
24: ER CELP
25: ER HVXC
26: ER HILN (Harmonic and Individual Lines plus Noise)
27: ER Parametric
28: SSC (SinuSoidal Coding)
29: PS (Parametric Stereo)
30: MPEG Surround
31: (Escape value)
32: Layer-1
33: Layer-2
34: Layer-3
35: DST (Direct Stream Transfer)
36: ALS (Audio Lossless)
37: SLS (Scalable LosslesS)
38: SLS non-core
39: ER AAC ELD (Enhanced Low Delay)
40: SMR (Symbolic Music Representation) Simple
41: SMR Main
42: USAC (Unified Speech and Audio Coding) (no SBR)
43: SAOC (Spatial Audio Object Coding)
44: LD MPEG Surround
45: USAC

Sampling Frequencies

There are 13 supported frequencies:

0: 96000 Hz
1: 88200 Hz
2: 64000 Hz
3: 48000 Hz
4: 44100 Hz
5: 32000 Hz
6: 24000 Hz
7: 22050 Hz
8: 16000 Hz
9: 12000 Hz
10: 11025 Hz
11: 8000 Hz
12: 7350 Hz
13: Reserved
14: Reserved
15: frequency is written explictly

Channel Configurations

These are the channel configurations:

0: Defined in AOT Specifc Config
1: 1 channel: front-center
2: 2 channels: front-left, front-right
3: 3 channels: front-center, front-left, front-right
4: 4 channels: front-center, front-left, front-right, back-center
5: 5 channels: front-center, front-left, front-right, back-left, back-right
6: 6 channels: front-center, front-left, front-right, back-left, back-right, LFE-channel
7: 8 channels: front-center, front-left, front-right, side-left, side-right, back-left, back-right, LFE-channel
8-15: Reserved