MPEG-1 Audio Layer II

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Not to be confused with MPEG-2.

MPEG-1 Audio Layer 2
File extension:	`.mp2`
MIME type:	`audio/mpeg`
Type of format:	Audio

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 While it has largely been superseded by MP3 for PC and Internet applications, it remains a dominant standard for audio broadcasting as part of the DAB digital radio and DVB digital television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.

1 History of Musicam development
2 MPEG Layer II characteristics
3 How the MP2 Codec works
4 Use in DVD-Video
5 Use in VCD and SVCD
6 Naming confusion
7 See also
8 External links

[edit] History of Musicam development

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A lot of the early history of Musicam technological development has been forgotten, however one incident stands out.

Musicam (as a psychoacoustical compression algorithm) was nearly perfectly developed until samples from glockenspiels (related to the xylophone) were introduced into the audio sample set. It took some 9 months (and one extra layer of codec complexity) to get rid of the audible artifacts in the Musicam codec algorithm after this instrument was added.

MP2 was proposed by the Advanced Television Research Consortium as a contender for the ATSC standard. It was subsequently replaced with MPEG-2 Layer II for the DTV "Grand Alliance" shootout, but lost out to Dolby AC-3.

[edit] MPEG Layer II characteristics

MPEG-1 Layer II is defined in ISO/IEC 11172-3

Sampling rates: 32, 44.1 and 48 kHz
Bitrates: 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320 and 384 kbit/s

An extension has been provided in MPEG-2 Layer II and is defined in ISO/IEC 13818-3

Additional sampling rates: 16, 22.05 and 24 kHz
Additional bitrates: 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144 and 160 kbit/s

[edit] 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, and is the de facto standard, for broadcast applications. Typically, private broadcasters worldwide compress their material at 256kb/s while their counterparts in public broadcasting (including the Canadian Broadcasting Corporation, Channel Africa, Deutsche Welle, Radio France Internationale, Radio Canada International, Radio Netherlands, the SABC, the Singapore Broadcasting Corporation, and VOA, to cite a few) use 384kb/s.

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.

[edit] 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.

[edit] 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.

[edit] 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.