SDII (Sound Designer II, sometimes seen abbreviated as SD2) is a monophonic/stereophonic audio file format, originally developed by Digidesign for their Macintosh-based recording/editing products.
It is the successor to the original monophonic Sound Designer I audio file format.
An SDII file can be monophonic or stereophonic. When stereo is used, the tracks are interleaved (sample-001-left, sample-001-right, sample-002-left, sample-002-right, etc.) Files also store sample rate and bit depth information.
The SDII file has become a widely accepted standard for transferring audio files between editing applications. Most Mac CD-ROM writer software, for example, specifies SDII or Audio Interchange File Format as the file format needed when making audio CDs.
The SDII file has also become accepted among personal computer audio application developers. This makes transferring audio from Mac to PC platforms much easier. When used on a PC, the file must use the extension of ".sd2".