MPEG-4HE- AAC, HE-AAC stands for High Efficiency Advanced Audio Coding, is an audio compression developed by Coding Technologies and standardized by the MPEG committee. HE-AAC, marketed as aacPlus, is the mandatory audio codec for the "Compresssed Zone" of DVD-Audio with which music tracks can be transmitted in the highly compressed audio format on music and multimedia players.
aacPlus is a further development of AAC compression, especially for low bit rates of 64 kbit/s and below. The process uses Parametric Stereo( PS) and Spectral Band Replication( SBR). It is characterized by very good sound quality, comparable to that of the audio CD. Several tests independently show that treble reproduction is not affected, nor is the separation of the stereo channels. According to these tests, the method is superior to all other audio compressions at 64 kbit/s.
As far as treble reproduction is concerned, frequencies of over 20 kHz are reproduced with aacPlus at a bit rate of 64 kbit/s; even at 48 kbit/s these are still over 16 kHz. The compression rate varies only slightly with the bit rate and is about 95%.