MPEG-4 Audio
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- Company: ISO
- Samples: http://samples.mplayerhq.hu/MPEG-4/
- Samples: http://samples.mplayerhq.hu/A-codecs/AAC/
- Samples: sample repo at standards.iso.org
- Sample Docs: sample docs
Specification links:
- MPEG-4 Audio: ISO/IEC 14496-3:2005 plus many addenda and corrigenda (soon to be 14496-3:2009)
- Conformance: ISO/IEC 14496-4:2004 (soon to be 14496-26:2009)
MPEG-4 Audio
MPEG-4 includes a system for handling a diverse group of audio formats in a uniform matter. Each format is assigned a unique Audio Object Type (AOT) to represent it. The common format Global header shared by all AOTs is called the Audio Specific Config.
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
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