MPEG-1 Audio Layer I, commonly abbreviated to MP1, is one of three audio formats included in the MPEG-1 standard. It is a deliberately simplified version of MPEG-1 Audio Layer II, created for applications where lower compression efficiency could be tolerated in return for a less complex algorithm that could be executed with simpler hardware requirements. While supported by most media players, the codec is considered largely obsolete, and replaced by MP2 or MP3.
For files only containing MP1 audio, the file extension .mp1 is used.
A limited version of MPEG-1 layer I was also used by the Digital Compact Cassette format, in the form of the PASC audio compression codec. The bit rate of PASC was fixed at 384 kilobits per second.