MPEG-1 Audio Layer I

MP1 (formally MPEG-1 Audio Layer I or MPEG-2 Audio Layer I) is a lossy audio codec and one of three audio formats included in the MPEG-1 standard. For files only containing MP1 audio, the file extension .mp1 is used.

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 due to wider acceptance of the more complex MPEG-1 Audio Layer II (MP2) and MPEG-1 Audio Layer III (MP3) codecs.

A limited version of MPEG-1 Audio Layer I was used by the Digital Compact Cassette format, in the form of the PASC (Precision Adaptive Subband Coding) audio compression codec. PASC and MPEG-1 Audio Layer I were essentially identical, except:

• The bit rate of PASC was fixed at 384 kilobits per second.
• The only channel configurations supported by PASC were Stereo, Joint Stereo, and Dual Mono (on prerecorded cassettes only). Mono was not supported because of the fixed bit rate.
• PASC regards the padding "slots" that are needed for encoding at 44.1 kHz stereo as "dummy" and sets them to zero, whereas the ISO/IEC 11172-3 standard uses the extra 32 bits in each padded frame to store data.