MPEG-1 Audio Layer I

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MPEG-1 Audio Layer 1
Filename extension .mp1
Internet media type audio/mpeg,[1] audio/MPA[2]
Type of format Audio compression format, audio file format
Standard(s) ISO/IEC 11172-3,[3]
ISO/IEC 13818-3[4]

MPEG-1 Audio Layer I, commonly abbreviated to MP1, is one of three audio formats included in the MPEG-1 standard. While supported by most media players, the codec is considered largely outdated, and replaced by MP2 or MP3.

For files only containing MP1 audio, the file extension .mp1 is used.

MPEG-1 Layer I is defined in ISO/IEC 11172-3, which first version was published in 1993.

  • Sampling rates: 32, 44.1 and 48 kHz
  • Bitrates: 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416 and 448 kbit/s[5]

An extension has been provided in MPEG-2 Layer I and is defined in ISO/IEC 13818-3, which first version was published in 1995.

  • Additional sampling rates: 16, 22.05 and 24 kHz
  • Additional bitrates: 48, 56, 80, 112, 144 and 176 kbit/s[5]

MP1 uses a comparatively simple sub-band coding, using 32 sub-bands.[6]

MPEG-1 layer I was also used by the Digital Compact Cassette format, in the form of the PASC audio compression codec. Because of the need of a steady stream of frames per second on a tape-based medium, PASC uses the rarely used (and under-documented) padding bit in the MPEG header to indicate that a frame was padded with 32 extra 0-bits (four 0-bytes) to change a short 416-byte frame into 420 bytes. The varying frame size only occurs when a 44.1 kHz 16 bits stereo audio signal is encoded at 384 kilobits per second, because the bitrate of the uncompressed signal is not an exact multiple of the bitrate of the compressed bit stream.

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