MPEG-1 Audio Layer II
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MPEG-1 Audio Layer II (MP2, sometimes Musicam) is an audio codec. The file extension for files containing such audio data is usually .mp2. While it has largely been superseded by MP3 for PC and Internet applications, it remains a dominant standard for audio broadcasting as part of the DAB digital radio and DVB digital television standards. It is also used internally within the radio industry, for example in NPR's PRSS Content Depot programming distribution system.
For details and a short historic introduction to MP2, see MP3.
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How the MP2 Codec works
MP2 is a sub-band audio encoder, which means that compression takes place in the time domain. By comparison, MP3 is a transform audio encoder, which means that compression takes place in the frequency-domain after transformation from the time domain.
Also, the MP2 encoder does not exploit interchannel redundancies. This makes MP2 less efficient than MP3 on low bitrates (lower than 256 kbit/s). For example, a 128 kbit/s MP3 encoded audio usually sounds, to the human ear, truer to the original source than the same audio encoded as 192 kbit/s MP2.
MP2 performs better than MP3 on high bitrates (256 to 384 kbit/s) and is generally more error resilient than MP3, so MP2 is considered optimal by some for broadcast applications.
Like MP3, MP2 is a perceptual codec, which means that it removes information that the human auditory system will not be able to perceive. To choose which information to remove, the audio signal is analyzed according to a psychoacoustic model, which takes into account the parameters of the human auditory system. Research into psychoacoustics has shown that if there is a strong signal on a certain frequency, then weaker signals at frequencies close to the strong signal's frequency cannot be perceived by the human auditory system. This is called frequency masking. Perceptual audio codecs take advantage of this frequency masking by ignoring information at frequencies that are deemed to be imperceptible, thus allowing more data to be allocated to the reproduction of perceptible frequencies.
MP2 splits the input audio signal into 32 sub-bands, and if the audio in a sub-band is deemed to be imperceptible then that sub-band is not transmitted. MP3, on the other hand, transforms the input audio signal to the frequency domain in 576 frequency components. Therefore, MP3 has a far higher frequency resolution than MP2, which allows the psychoacoustic model to be applied more accurately than for MP2. Because the psychoacoustic model can be applied more accurately, MP3 has greater scope to reduce the bit rate, which is why MP3 requires a lower bit rate than MP2 to get an acceptable audio quality.
Musepack is an MP2-based encoder with several improvements which make it perform better on lower bitrates.
Use in DVD-Video
All DVD-Video players in PAL countries contain stereo MP2 decoders, making MP2 a possible competitor to Dolby Digital in these markets. While some DVD recorders record in MP2 and many consumer-authored DVDs use the format, as of 2005 mass produced discs with MP2 soundtracks are very rare.
Use in VCD and SVCD
MPEG layer 2 is the standard audio format used in Video CD and Super Video CD.
Naming confusion
The term MP2 and filename extension .MP2 usually refer to data in the MPEG-1 Audio Layer II format, but can also refer to MPEG-2 BC Audio Layer II, a backwards compatible extension which adds support for multi-channel sound and lower sampling rates. MP2 is sometimes incorrectly believed to be related to the MPEG-2 video and MPEG-2 AAC audio formats.
External links
- MPEG Audio FAQ
- MPEG Audio Resources and Software
- TooLAME - an MP2 encoder
- TwoLAME - a fork of the tooLAME codede:MP2