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MPEG-1 Audio Layer II

History of development from MP2 to MP3

MP2 began in the late 1980s as part of the ISO's Moving Picture Experts Group (MPEG) effort to standardize digital audio and video encoding for digital radio and TV broadcasting (DAB, DMB, DVB), and use on Video CD. The MPEG-1 audio standard was based on the existing MUSICAM and ASPEC audio formats. The MPEG-1 standard, including the three audio "layers" (encoding techniques) now known as MP1, MP2 and MP3, was finalized by December 1991 in the committee draft of ISO-11172. MPEG audio was further advanced through its Layer III (MP3) component, which allowed for even lower bitrates.

Technical Specifications

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, 40 and 144 kbit/s

Multichannel support - up to 5 full range audio channels and an LFE-channel (Low Frequency Enhancement channel)

The format is based on successive digital frames of 1152 sampling intervals with four possible formats:

mono format

stereo format

intensity encoded joint stereo format (stereo irrelevance)

dual channel (uncorrelated) format

Variable bitrate

MPEG audio may have variable bitrate (VBR), but it is not widely supported. Layer II can use method called bitrate switching. Each frame may be created with different bitrate.
According to ISO/IEC 11172-3:1993, Section 2.4.2.3: In order to provide the smallest possible delay and complexity, the (MPEG audio) decoder is not required to support a continuously variable bitrate when in layer I or II.

How the MP2 Codec works

MP2 is a sub-band audio encoder, which means that compression takes place in the time domain with a low-delay filter bank producing 32 frequency domain components. By comparison, MP3 is a transform audio encoder with hybrid filter bank, which means that compression takes place in the frequency domain after a hybrid (double) transformation from the time domain.

MPEG Audio Layer II is the core algorithm of the MP3 standards. All psychoacoustical characteristics and frame format structures of the MP3 codec are derived from the basic MP2 algorithm and format.

The MP2 encoder may exploit inter channel redundancies using optional "joint stereo" intensity encoding.

MP2 performs similarly to MP3 on high bitrates (192 to 384 kbit/s) and is considered more error resilient than MP3, so MP2 is often still used for broadcast applications. Typically, private broadcasters worldwide compress their material at 256kbit/s (stereo) 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 384 kbit/s (although the BBC in the United Kingdom uses 128kbit/s for most of its digital radio broadcasts, the exceptions being Radio 3 and Radio 7 being broadcast at 192kbit/s and 80kbit/s respectively). Many today use MP3 (L3), AAC or aacplus instead of MP2, because of their improved low-bitrate performance.

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 higher frequency resolution than MP2, which allows the psychoacoustic model to be applied more selectively than for MP2. So MP3 has greater scope to reduce the bit rate.

The use of an additional entropy coding tool and this higher frequency accuracy explains why MP3 doesn't need as high a bit rate as MP2 to get an acceptable audio quality. Conversely MP2 shows a better behavior than MP3, in the time domain, due to its lower frequency resolution which implies less codec time delay (simpler editing) and native ruggedness to the digital recording and digital transmission errors.

Moreover, the MP2 sub-band filter bank provides an inherent transient concealment feature due to the specific temporal masking effect of its mother filter. This unique characteristic of the MPEG-1 Audio family codecs implies a very good sound quality on audio signals with rapid energy changes such as percussive sounds both on the MP2 and the MP3 codecs which use the same basic sub-band filter bank.

Applications of MP2

Part of the DAB digital radio and DVB digital television standards.

Used internally within the radio industry, for example in NPR's PRSS programming distribution system.

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, although most do. While some DVD recorders store audio in MP2 and many consumer-authored DVDs use the format, commercial DVDs with MP2 soundtracks are rare.

MPEG-1 layer 2 is the standard audio format used in the Video CD and Super Video CD formats (VCD and SVCD also support variable bitrate and MPEG Multichannel as added by MPEG-2).

MPEG 1 layer 2 is the standard audio format used in the MHP standard for set-top boxes.

MPEG 1 layer 2 is the audio format used in HDV camcorders.

Naming and Extensions

The term MP2 and filename extension .mp2 usually refer MPEG-1 Audio Layer II data, but can also refer to 'MPEG-2 Audio Layer II, a mostly backwards compatible extension which adds support for multichannel audio variable bitrate encoding, and additional sampling rates, defined in ISO/IEC 13818-3. The abbreviation MP2 is also sometimes erroneously applied to MPEG-2 video or MPEG-2 AAC audio.