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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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
