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The WAVEFORMATEXTENSIBLE data structure defines common properties of an audio stream and occurs in a variety of multimedia file formats, including:

Since the format originated on Windows/Intel computers, the defined, multi-byte numbers are stored in little endian format, even if the data structure appears in QuickTime files.

bytes 0-1    wFormat
bytes 2-3    nChannels
bytes 4-7    nSamplesPerSec
bytes 8-11   nAvgBytesPerSec
bytes 12-13  nBlockAlign
bytes 14-15  wBitsPerSample
bytes 16-17  cbSize
bytes 18-19  wValidBitsPerSample OR wSamplesPerBlock OR wReserved
bytes 20-23  dwChannelMask
bytes 24-39  subFormat [given as a 16 byte GUID]
bytes 40..   extradata [actual number of bytes is determined by the value of the cbSize field minus 22]

It is an extension of the WAVEFORMATEX structure, which is typically used for data with at most two channels. A WAVEFORMATEXTENSIBLE structure can be recognized by wFormat being equal to 0xfffe.