RK Audio (RKAU) is a legacy lossless (and lossy) audio codec for the Windows platform. It was developed by Malcolm Taylor together with RK (a general purpose data compressor) and RKim (a lossless image compressor).
Development apparently ceased around year 2000; the author offered it for download until circa 2005 at his new company MSoftware, which then for a while revived the compression suite (including an audio compressor) as the WinRK archiver, now also discontinued.
Testing by ReallyRareWares and Squeezechart indicated excellent compression level by year 2000 standards, at the cost of high CPU usage. Features were few.
ffmpeg has a decoder obtained by reverse engineering the lossless RKAU codec. Apparently it also decodes one of the two known lossy profiles (the "simple" lossless profile), but the user should be cautioned that the other lossy profile ("VRQ") is decoded wrong and produces noise without giving any warning (as of ffmpeg 6.0).
|Audio codecs: type/usage||for end-users to encode||on physical media etc.||legacy formats|
|Lossless||(Uncompr. linear PCM) • ALAC • FLAC • Monkey's Audio • MPEG-4 ALS • OptimFROG • TAK • TTA • WavPack||MLP/Dolby TrueHD||La • LPAC/LTAC • RealAudio Lossless • RK Audio • Shorten • WMA Lossless • (others)|
|Lossy||AAC • AC-3 • ADPCM • MP2 • MP3 • Musepack • Opus • Vorbis||AC-3 • DTS||ATRAC • QDesign • RK Audio (lossy mode) • Speex • VQF • WMA|
|Hybrid lossless&lossy||OptimFROG DualStream • WavPack||DTS-HD MA • MPEG-4 SLS||ATRAC Advanced Lossless • mp3HD|