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MPEG-1 Audio Layer II (MP2, sometimes Musicam) is an audio codec defined by ISO/IEC 11172-3. An extension exists: MPEG-2 Layer II and is defined in ISO/IEC 13818-3. 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 While it has largely been superseded by MP3 for PC and Internet applications, it remains a dominant standard for audio broadcasting as part of the DAB digital radio and DVB digital television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.
MPEG Layer II characteristics MPEG-1 Layer II is defined in ISO/IEC 11172-3 An extension has been provided in MPEG-2 Layer II and is defined in ISO/IEC 13818-3 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. 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 doesn't need as high a bit rate as 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. DVD-Video players in NTSC countries are not required to decode MP2 audio, though most do. While some DVD recorders record in MP2 and many consumer-authored DVDs use the format, as of 2006 mass produced discs with MP2 soundtracks were very rare. Use in VCD and SVCD MPEG layer 2 is the standard audio format used in the Video CD and Super Video CD formats, as well as the related China Video Disc format. 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. The abbreviation MP2 is also sometimes erroneously used to refer to MPEG-2 video or MPEG-2 AAC audio formats. See also | ||||||||
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