Parametric stereo (PS) is the first employment of spatial audio coding technology in international standards and commercially available audio codecs. Within MPEG-4, PS is supported in aacPlus v2 (also known as the high-efficiency AAC (HE-AAC) profile), a codec based on (mono) AAC, spectral band replication (SBR) and PS. This codec is currently regarded as the most efficient (stereo) audio coder available today, delivering ‘good’ quality at bitrates as low as 24–32 kbps, and ‘excellent’ quality around 48 kbps.
AacPlus v2 was developed roughly between 2001 and 2004, shortly after the finalization of MPEG-4 Audio version 1 and 2 in 1999–2000. At that point in time, one of the most efficient audio codecs was MPEG-4 AAC, a codec based on transform-domain (MDCT) coefficient quantization, enhanced with perceptual noise substitution (PNS) and long-term prediction (LPT) tools. The performance (or perceptual quality) of MPEG-4 AAC for stereo signals is excellent for bitrates of 96 kbps and higher, but drops rapidly for lower bitrates.
In 2001, MPEG started work to develop new technology to enable further bitrate reduction. Two areas of improved audio coding technology were identified;