Parametric Stereo
Encyclopedia
Parametric Stereo is a feature and an Audio Object Type defined and used in MPEG-4 Part 3
(MPEG-4 Audio) to further enhance efficiency in low bandwidth stereo media. Advanced Audio Coding
Low Complexity (AAC LC) combined with Spectral band replication
(SBR) and Parametric Stereo (PS) was defined as HE-AAC
v2. An HE-AAC v1 decoder will only give mono sound when decoding an 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, compared to a discretely stereo coded audio signal coded with conventional means. Thus, the additional bitrate spent on the single mono channel (combined with some PS side info) will improve the perceived quality substantially of the audio compared to a standard stereo stream at similar bitrate.
However, this technique is only useful at the lowest bitrates (say 16 - 32 kbit/s, sweet-spot at 24 kbit/s) 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.
MPEG-4 Part 3
MPEG-4 Part 3 or MPEG-4 Audio is the third part of the ISO/IEC MPEG-4 international standard developed by Moving Picture Experts Group. It specifies audio coding methods...
(MPEG-4 Audio) to further enhance efficiency in low bandwidth stereo media. Advanced Audio Coding
Advanced Audio Coding
Advanced Audio Coding is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates....
Low Complexity (AAC LC) combined with Spectral band replication
Spectral band replication
Spectral band replication is a technology to enhance audio or speech codecs, especially at low bit rates and is based on harmonic redundancy in the frequency domain....
(SBR) and Parametric Stereo (PS) was defined as HE-AAC
HE-AAC
High-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
v2. An HE-AAC v1 decoder will only give mono sound when decoding an 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, compared to a discretely stereo coded audio signal coded with conventional means. Thus, the additional bitrate spent on the single mono channel (combined with some PS side info) will improve the perceived quality substantially of the audio compared to a standard stereo stream at similar bitrate.
However, this technique is only useful at the lowest bitrates (say 16 - 32 kbit/s, sweet-spot at 24 kbit/s) to give a good stereo impression, so while it can improve perceived quality at very low bitrates, it generally does not achieve transparency
Transparency (data compression)
In data compression or psychoacoustics, transparency is the ideal result of lossy data compression. If a lossy compressed result is perceptually indistinguishable from the uncompressed input, then the compression can be declared to be transparent...
, since simulating the stereo dynamics of the audio with the technique is limited and generally deteriorates perceived quality regardless of the bitrate.