The High Efficiency Advanced Audio Coding (HE-AAC) format, also known as AAC+, is a lossy audio compression format designed for low bit rates and low bandwidths. It is an extension of the popular Advanced Audio Coding (AAC) format and is commonly used in online streaming and digital radio broadcasting.

HE-AAC uses a technique called Spectral Band Replication (SBR), which separates the audio frequencies into different bands and replicates the high frequencies, while reducing the lower ones. This allows for a more efficient use of data, reducing the file size of the audio while still maintaining a high level of audio quality.

Another important feature of HE-AAC is its ability to switch between different bit rates and sample rates on the fly. This is useful for streaming audio content, especially for mobile devices with varying network speeds and bandwidths.

In addition, HE-AAC includes a feature called Parametric Stereo (PS), which reduces the file size even further by encoding one channel in stereo and then deriving the other channel from it through mathematical calculations. This results in a smaller file size with no loss of audio quality.

HE-AAC is supported by a wide range of devices and platforms, including iOS and Android operating systems, web browsers, and multimedia players. It is also the standard format for Digital Audio Broadcasting (DAB) and Digital Radio Mondiale (DRM) systems.

In conclusion, HE-AAC is a highly efficient audio compression format that offers high audio quality at low bit rates and bandwidths. Its ability to switch between different bit rates and sample rates on the fly, as well as its support for various devices and platforms, makes it a popular choice for online streaming and digital radio broadcasting.