There are three common AAC specifications: LC-AAC (most basic), HE-AAC (AACPlus v1), HE-AAC v2 (AACPlus v2). The relationship between the three of them is as follows:
HE: "High Efficiency". HE-AAC v1 (also known as AACPlusV1, SBR), with the container method to achieve the AAC (LC) and SBR technology. SBR actually represents Spectral Band Replication. Briefly, the main spectrum of music is concentrated in the low frequency band, the high frequency range is very small, but it is important to determine the sound quality. For the entire band coding, if in order to protect the high frequency will cause well low-band coding so that the file is huge; to save the main components of low frequency lead high-frequency components loss which will lose sound quality. SBR cut the spectrum, low-frequency coding alone to save the main components, high-frequency amplification code to save the sound quality, to reducing the size of the file also save the sound quality, the perfect solution to this contradiction.
HEv2: Contains HE-AAC v1 and PS technology in a container approach. PS refers to "parametric stereo". The original stereo file size is twice the size of a channel. However, there is some similarity in the sound of the two channels. According to the Shannon information entropy coding theorem, the correlation should be removed to reduce the file size. So the PS technology stores all the information for a channel, and then, with very few bytes, describe the other channel and its different places with parameters.
LC-AAC, HE-AAC, HE-AAC v2 bit rate and subjective quality. As can be seen from the figure, in the case of low bit rate, HE-AAC, HE-AAC v2 encoded sound quality is significantly better than LC-AAC
The relationship between the subjective quality of HE-AAC (160kbps), Dolby Digital (384kbps), WMA (192kbps) obtained using different test audio material. The figure shows HE-AAC in the 160kbps bit rate under the sound quality has more than Dolby Digital at 384kbps bit rate sound quality, but also more than the WMA in 193kbps bit rate sound quality. Enough to see its tough performance.
LC-AAC, HE-AAC, HE-AAC v2 bit rate and subjective quality. As can be seen from the figure, in the case of low bit rate, HE-AAC, HE-AAC v2 encoded sound quality is significantly better than LC-AAC. And after the larger bit rate (128kbps), the subjective quality gradually the same.