Parametric Stereo
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Parametric Stereo is a feature used in Advanced Audio Coding to further enhance efficiency in low bandwidth stereo media. It, along with Spectral Band Replication, is part of HE-AAC v2. An HE-AAC v1 decoder will only give mono sound.
Parametric Stereo works by taking the two mono streams that make up a stereo stream and combining them together to form a new single mono audio stream, but with 2-3 kbit/s of side info (the Parametric Stereo) added. By having the Parametric Stereo side info along with the mono audio stream, the decoder (player) can approximate the original stereo.
Because only one real audio signal is sent, along with the side info, a 24 kbit/s stereo stream of audio and a 24 kbit/s stream of audio with Parametric Stereo will be drastically different in quality, since there is more audio data to encode with a standard stereo stream. The additional bitrate made available to the single mono channel combined with the PS side info can improve the perceived quality of the audio compared to a standard stereo stream.
However, this technique introduces artifacts to the audio stream, so while it can improve perceived quality at very low bitrates, it generally does not achieve transparency.
