AAC_LC用LATM封裝header信息解析

    一般來講AAC的頭信息在編解碼過程當中是能夠獲取到的，但今天須要根據音頻參數生成相應的AAC頭。項目中使用的是AAC_LC，今天先對它的結構進行分析。

    項目中使用ffmpeg進行音頻編碼，音頻編碼庫爲FAAC，好吧，直接看代碼吧。

    ffmpeg調用Faac_encode_init()初始化編碼器；在Faac_encode_init()調用faacEncGetDecoderSpecificInfo()獲取AAC_LC頭信息內容及長度。

int FAACAPI faacEncGetDecoderSpecificInfo(faacEncHandle hEncoder,unsigned char** ppBuffer,unsigned long* pSizeOfDecoderSpecificInfo)
{
    BitStream* pBitStream = NULL;  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;
    }
}

    從代碼中能夠看出，頭數據長度固定爲2；

    數據內容由高位到低位依次爲：aacObjectType（5bits）,sampleRateIdx(4bits),numChannels(4bits)

例如：音頻編碼參數爲：

aacObjectType:AAC_LC,對應值爲2，用5bit二進制表示爲00010;

sampleRate:44100KHz, 對應的IDX值爲4, 用4bit二進制表示爲0100; 

numChannels:2，對應的值爲2，用4bit二進制表示爲0010；

將它們由高位到低位串起來：0001,0010,0001,0000，

則，對應的十六進制值爲:0x1220

引一些參考資料：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