Parametric Stereo (PS) is the technique that lets HE-AAC v2 deliver a stereo image at very low bitrate. Instead of coding left and right separately, the encoder transmits a mono downmix plus a compact set of parameters describing the stereo cues — inter-channel level and phase differences, and a measure of how correlated the channels are. The decoder uses those to expand the mono back into a plausible stereo field. It is the stereo analogue of SBR's spectral trick: spend almost no bits on the second channel by describing it parametrically rather than coding it. Effective down to around 24 kbps, it is a clear compromise but far better than mono.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
Parametric stereo lets HE-AAC v2 deliver a stereo image at bitrates where full stereo coding would collapse, at the price of a narrower, sometimes smeared sound. This volume covers parametric stereo in detail within the AAC family, with SBR, joint stereo and channel layouts, so you can set low-bitrate profiles knowingly. Written by Nikolay Sapunov, CEO at Fora Soft.
Using HE-AAC v2 parametric stereo and hearing a smeared stereo image?
Send us test clips and target bitrates and we will compare parametric stereo with other options and recommend profiles per rendition. 250+ video products shipped since 2005.
