MPEG-4 Audio

• Company: ISO
• Samples: http://samples.mplayerhq.hu/MPEG-4/ and http://ghostarchive.org/samples/MPEG-4
• Samples: http://samples.mplayerhq.hu/A-codecs/AAC/ and http://ghostarchive.org/samples/AAC
• Samples: sample repo at standards.iso.org
• Sample Docs: sample docs
• Sample Docs: more sample docs

Specification links:

• MPEG-4 Audio: ISO/IEC 14496-3:2009
• Conformance: ISO/IEC 14496-26:2010

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