What are AAC+ and HE-AAC?

High-Efficiency AAC (HE-AAC) is an extension to the AAC codec, which further optimizes the codec for low-bitrate streaming (less than 128 kbps). Low bitrate streams are more reliable on mobile devices as mobile network bandwidth can vary significantly depending on cellular coverage. HE-AAC v1 provides improved sound quality at lower bitrates by removing high frequencies during encoding, and then resynthesizing these during playback using a technique called Spectral Band Replication (SBR).

HE-AAC v2 (AAC+) adds another optimization for low bitrates called "Parametric Stereo", which encodes the audio as a single mono channel, which is used to synthesize a stereo signal. The result is that convincing stereo audio is achieved at much lower bitrates than were previously possible, although with lower reproduction accuracy. This tradeoff makes AAC+ an excellent choice for streaming radio.