High-Efficiency Advanced Audio Coding
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MPEG -4 High Efficiency Advanced Audio Coding ( HE- AAC short ), also known as AAC v1, a rights-managed audio codec the Moving Picture Experts Group ( MPEG). The standard provides relatively good results at low bit rates and is therefore particularly suitable for live streams.
Basics
HE- AAC is based on MPEG -4 AAC, but was extended by a technique called spectral band replication (SBR ). This technique was already at the mp3PRO codec used. Thanks SBR provides HE- AAC, especially at low bit rates ( 96 kbit / s and below) better sound quality than the "normal" AAC or other (older) audio codecs. Low bit rates, such as those used in live streaming or mobile, are therefore the main area of application of HE- AAC. At higher bit rates (above ~ 96 kbps) is because of the better quality continues in the Main or AAC Low Complexity ( LC) profile used.
HE- AAC supports up to 48 channels of audio and DRM. HE-AAC can be used with a constant or variable bit rate. HE- AAC is limited backward compatible with AAC. This means that all the AAC decoder can also decode HE-AAC stream; SBR without special support, however, only with half the sampling rate. Since SBR high frequencies produced synthetically, can with HE -AAC as opposed to AAC ( Main or LC profile ) no transparency be achieved.
AAC v2
In addition to AAC v1 (HE -AAC ), there is now also a new version of the codec, which is referred to as AAC v2 or eAAC. AAC v2 codec used in contrast to the AAC V1, during the compression of stereo signals in addition, a technique called parametric stereo (PS). Thereby can be compressed substantially more effective stereo signals again, which further improves the sound quality at low bit rates. For mono signals PS comes accordingly not used. Also, AAC v2 is backward compatible. Decoder without special support for PS can decode only as mono AAC v2 streams, however.
Only for data rates below 48 kbps AAC v2 provides the manufacturer better sound results. For data rates above 48 kbps for quality reasons also recommended the use of AAC v1 by the manufacturer, however.
DVD-Audio
HE -AAC and aacPlus v1 4 of the DVD Forum was standardized by the Working Group as the official audio codec for the so-called " Compressed zone" of the DVD-Audio in July 2004. In this area a highly compressed version of the musical content of the DVD - Audio is stored, which can be secured by DRM transferred to portable playback devices.
Abbreviations
There are for the " AAC " variants several shortcuts with congruent meanings:
- AAC SBR = HE- AAC = aacPlus v1 ( = AAC )
- AAC SBR PS = HE- AAC v2 = aacPlus v2 ( enhancedAAC = = eAAC )