AAC+ and aacPlus are known as HE-AAC v1, and aacPlus v2 and eAAC+ are known as HE-AAC v2. Details
HE-AAC v2, a.k.a. aacPlus v2, eAAC+ or Enhanced aacPlus, the combination of Parametric Stereo (PS) and HE-AAC (which already uses SBR).
Both are synthetic to artificially improve the audio at the lowest possible bitrates.
Spectral band replication
SBR replicates higher frequency content by transposing up harmonics from the lower and midfrequencies at the decoder, it is based on harmonic redundancy in the frequency domain. A Swedish company Coding Technologies (acquired by Dolby in 2007) developed and pioneered the use of SBR in its MPEG-2 AAC-derived codec called aacPlus, which first appeared in 2001. This codec was submitted to MPEG and formed the basis of MPEG-4 High-Efficiency AAC (HE-AAC), standardized in 2003.
Spectral band replication (SBR) is a technology to artificially enhance audio or speech codecs, especially at low bit rates (e.g. AAC+ 48Kbps) and is based on harmonic redundancy in the frequency domain.
Parametric stereo
Parametric stereo is an artificial technology to enhance audio or speech codecs, especially at low bit rates (e.g. AAC+ 48Kbps) is part of aacPlus v2 and MPEG-4 HE-AACv2. It is based on spatial redundancy in the frequency domain. Parametric Stereo digitally codes a stereo audio signal at the source by storing the audio as a small amount of extra information (2–3 kbit/s of side info). This extra information (defined as “parametric overhead”) describes how the monaural signal will behave across both stereo channels, which allows for the signal to exist in true stereo upon playback.
Because only one real audio signal is sent, along with the negligible side info, a 24 kbit/s stereo stream of audio and a 24 kbit/s stream of audio with parametric stereo will be drastically different in quality.
This can be modelled as:
- 12 kbit/s mono + 12 kbit/s mono = 24 kbit/s
But, with parametric stereo:
- 22 kbit/s mono + 2 kbit/s PS side info = 24 kbit/s
Other details
AAC (Advanced Audio Coding) Low Complexity (AAC LC) combined with Spectral Band Replication (SBR) and Parametric Stereo (PS) is defined as HE-AAC v2.
Parametric Stereo performs sparse coding in the spatial domain, somewhat similar to what SBR does in the frequency domain.