AAC_LC用LATM封装header信息解析

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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。

 

 

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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

 

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