Background Advanced Audio Coding Low Complexity (AAC LC) combined with
Spectral Band Replication (SBR) and Parametric Stereo (PS) was defined as
HE-AAC v2. A HE-AAC v1 decoder will only give a mono output when decoding a HE-AAC v2 bitstream. Parametric Stereo performs sparse coding in the spatial domain, somewhat similar to what SBR does in the frequency domain. An AAC HE v2 bitstream is obtained by downmixing the stereo audio to mono at the encoder along with 2–3 kbit/s of side info (the Parametric Stereo information) in order to describe the spatial intensity stereo generation and ambience regeneration at the decoder. By having the Parametric Stereo side info along with the mono audio stream, the decoder (player) can regenerate a faithful spatial approximation of the original stereo panorama at very low bitrates. Because only one audio channel is transmitted, along with the parametric side info, a 24 kbit/s coded audio signal with Parametric Stereo will be substantially improved in quality relative to discrete stereo audio signals encoded with conventional means. The additional bitrate spent on the single mono channel (combined with some PS side info) will substantially improve the perceived quality of the audio compared to a standard stereo stream at similar bitrate. However, this technique is only useful at the lowest bitrates (approx. 16–48 kbit/s and down to 14.4 kbps in
xHE-AAC used in
DRM) to give a good stereo impression, so while it can improve perceived quality at very low bitrates, it generally does not achieve
transparency, since simulating the stereo dynamics of the audio with the technique is limited and generally deteriorates perceived quality regardless of the bitrate.
Development The development of Parametric Stereo was as a result of necessity to further enhance the coding efficiency of audio in low bandwidth stereo media. It has gone through various iterations and improvements, however, it was first standardized as an algorithm when included in the feature set of
MPEG-4 Audio. == Approaches ==