A '''soft-in soft-out decoder''' ('''SISO) decoder''') is a type of [[soft-decision decoder]] used with [[error correcting code]]s.<ref name="NeubauerFreudenberger2007">{{cite book|author1=Andre Neubauer|author2=Jurgen Freudenberger|author3=Volker Kuhn|title=Coding Theory.: Algorithms, Architectures and Applications|url=https://books.google.com/books?id=j5zNVqjY0uYC&q=Soft-in+soft-out+decoder|date=22 October 2007|publisher=John Wiley & Sons|isbn=978-0-470-51982-0}}</ref> "Soft-in" refers to the fact that the incoming data may take on values other than 0 or 1, in order to indicate reliability. "Soft-out" refers to the fact that each [[bit]] in the decoded output also takes on a value indicating reliability. Typically, the soft output is used as the soft input to an outer decoder in a system using [[concatenated code]]s, or to modify the input to a further decoding iteration such as in the decoding of [[turbo code]]s.
Examples include the [[BCJR algorithm]] and the [[soft output Viterbi algorithm]].
