关于MPEG-4 Audio中的Audio Specific Config的理解

MPEG-4 Audio有一个非常重要的header叫做Audio Specific Config,该header包含了Audio编码器的重要信息，比如编码器类别，音频频率，音频通道数。

例如：AAC LC,双通道48K编码的header：0001000110010

          Object Type = 2, 用5bit的二进制表示为：00010

          frequency index = 3，用4bit的二进制表示为：0011

          channel configuration = 2，用4bit的二进制表示为：0010

 

例如：AAC ELD,双通道48K编码的header：1111100011100110010

          Object Type = 39, 39无法用5位二进制表示，因此需要借助if (object type == 31)这个条件，所以前5个bit是11111，即31

                                  然后用6bit+32来表示，因为这object type是39，所以6bit需要表示7，用二进制表示为：000111.

                                   所以object type的二进制最终为：11111000111

          frequency index = 3，用4bit的二进制表示为：0011

          channel configuration = 2，用4bit的二进制表示为：0010

 

WIKI：https://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