Hybrid codec

A lossless/lossy 'hybrid' codec provides encoding to a lossless and a lossy stream simultaneously within the same format: it stores a lossy audio stream that can be played back as such, and then a correction stream such that decoding is lossless when both are present. The correction stream can be in the same file as the lossy stream, or in a separate file.

Hybrid encoding can have a variety of purposes, at least including the following:

• For end-users to keep lossy versions (say, for portable use) to be copied without the time or computing effort needed to transcode lossy to lossless every time; rather than keeping two sets of files, one can maintain one single copy in one single library, such that e.g. tag updates need only be done once. WavPack and OptimFROG provide for this option through their hybrid modes.
• A similar purpose, but where the output stream is automatically selected by a hardware or software application, for example defaulting to a lossy stream if the transmission cannot support the lossless - or possibly for DRM purposes. In Blu-Ray, support for DTS (also often found in DVDs) is mandatory, and DTS-HD will serve the DTS stream to players which do not support the lossless stream - or those players will transmit the DTS stream to e.g. AV receivers without DTS-HD support. The rarer MPEG-4 SLS format has an AAC core file and a correction file.
• To retro-fit a lossless option into a lossy format without breaking compatibility. This can be done by storing it the same way as a tag chunk, to be ignored by applications that don't support them. ATRAC's lossless mode and the short-lived Mp3HD were such retro-fits.