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Fraunhofer's new HD-AAC codec provides lossless compression of the 24-bit, 96KHz audio now commonly used for commercial recordings. By contrast CDs store 16-bit, 44.1KHz audio. But HD-AAC can also store a compressed, AAC version of the same audio track, providing backwards-compatibility with devices and software capable of playing the standard AAC format, which is the default for iPods and iTunes.

"Consumers will be able to buy content in online music stores that sounds better than CDs, and preserve their existing CD collection for the future by encoding it in HD-AAC", said Harald Popp, head of the Multimedia Realtime Systems

The HD-AAC encoding process preserves every bit of information contained in the uncompressed original music track with lossless compression rates that Fraunhofer claims are comparable or superior to other lossless formats. Due to its AAC-LC core layer, an HD-AAC file can be directly played on many existing music players and mobile phones. For decoding of the fully lossless signal, future devices will need with an HD-AAC decoder.

Songs stored on media servers in the HD-AAC format can be streamed to multiple devices at varying bitrates. This is designed to maximise the sound quality for network conditions by matching the bitrate to the available bandwidth.

HD-AAC comprises two subparts of the MPEG-4 Audio standard: MPEG-4 Advanced Audio Coding (AAC) and MPEG-4 Scalable Lossless Coding (SLS). For more information see HD-AAC: MPEG-4 SLS - High Definition Scalable Lossless Audio Coding.