Digidesign's predecessor to the Sound Designer II format. Unlike the second generation format, it does not use resources to store header information. It has a large header, although most of it is used internally by their software. It can only contain mono data.
Most files are usually 16-bit, 44.1 kHz. SoundApp plays this fileformat. SD is also the standard working format for most professional harddisk recording software, such as Deck II.