MPEG-4 Audio

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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.

Subparts

  • Subpart 0: Overview
  • Subpart 1: Main (Systems Interaction)
  • Subpart 2: Speech coding - HVXC
  • Subpart 3: Speech coding - CELP
  • Subpart 4: General Audio coding (GA) - AAC, TwinVQ, BSAC
  • Subpart 5: Structured Audio (SA)
  • Subpart 6: Text To Speech Interface (TTSI)
  • Subpart 7: Parametric Audio Coding - HILN
  • Subpart 8: Parametric coding for high quality audio - SSC (and Parametric Stereo)
  • Subpart 9: MPEG-1/2 Audio in MPEG-4
  • Subpart 10: Lossless coding of oversampled audio - DST
  • Subpart 11: Audio lossless coding - ALS
  • Subpart 12: Scalable lossless coding - SLS

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