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GAAD (Go Advanced Audio Decoder)

Package currently provides AAC parsing capabilities. This package performs a full parse of AAC-LC and HE-AACv1 bitstreams. Bitstreams with Parametric Stereo (HE-AACv2) are not yet supported, but AAC bitstream data and SBR data will be extracted. The AAC decode from the parsed data to LPCM (.wav) is not yet implemented. Please help us expand and test this library!

AACParser

This package currently supports AAC audio data contained in an ADTS header. All available data is returned in the adts struct and can be accessed as nested objects as presented in the AAC specification. All parameter names should be verbatim from the AAC specification, if you find an issue with this please file a bug or submit a pull request.

AAC Types

Type	Description	CODEC
AACLC	AAC	mp4a.40.2
HEAAC	AAC + SBR	mp4a.40.5
HEAACv2	AAC + SBR + PS	mp4a.40.29

where:

SBR = Spectral band replication
PS = Parametric Stereo

Usage

var []byte buf
buf = <ADTS+AAC data>

// Parsing the buffer
adts, err := gaad.ParseADTS(buf)

// Looping through top level elements and accessing sub-elements
var sbr bool
if adts.Fill_elements != nil {
	for _, e := range adts.Fill_elements {
		if e.Extension_payload != nil &&
			e.Extension_payload.Extension_type == gaad.EXT_SBR_DATA {
			sbr = true
		}
	}
}

VBR vs CBR

VBR (Variable bitrate) and CBR (Constant bitrate) is derived from the bitstream_type attribute in the adif_header section. It is VBR if bitstream_type is true, and CBR otherwise.

References

ISO/IEC STANDARD 14496-3

Title         : Coding of audio-visual objects — Part 3: Audio
File          : ISO_14496-3-4th-Edition.pdf
Edition       : Fourth edition (2009-09-01)
Relevant Sections
    - Page 64   : 1.6.5 Signaling of SBR
    - Page 120  : 1.A.2 AAC Interchange formats
    - Page 489  : 4.4.2 GA bitstream payloads

Related Books

Video Demystified (5th Edition) http://my.safaribooksonline.com/book/-/9780750683951