MPEG-1 Audio Layer II
This article includes a list of general references, but it lacks sufficient corresponding inline citations. (December 2016) |
Filename extension |
.mp2 |
---|---|
Internet media type |
audio/mpeg, ISO/IEC 13818-3[5] |
Open format? | Yes |
Free format? | Yes[6] |
Website | http://mpeg.chiariglione.org/standards/mpeg-1/audio |
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II (MP2, sometimes incorrectly called Musicam or MUSICAM)
History of development from MP2 to MP3
MUSICAM
MPEG-1 Audio Layer 2 encoding was derived from the MUSICAM (Masking pattern adapted Universal Subband Integrated Coding And Multiplexing) audio codec, developed by
It began as the
The Eureka 147 System comprised three main elements: MUSICAM Audio Coding (Masking pattern Universal Sub-band Integrated Coding And Multiplexing), Transmission Coding & Multiplexing and COFDM Modulation.[9]
MUSICAM was one of the few codecs able to achieve high audio quality at bit rates in the range of 64 to 192 kbit/s per monophonic channel. It has been designed to meet the technical requirements of most applications (in the field of broadcasting, telecommunication and recording on digital storage media) — low delay, low complexity, error robustness, short access units, etc.[10][11]
As a predecessor of the MP3 format and technology, the perceptual codec MUSICAM is based on integer arithmetics 32 subbands transform, driven by a psychoacoustic model. It was primarily designed for Digital Audio Broadcasting and digital TV, and disclosed by CCETT(France) and IRT (Germany) in Atlanta during an IEEE-ICASSP conference.
The audio coding algorithm used by the Eureka 147 Digital Audio Broadcasting (DAB) system has been subject to the standardization process within the ISO/Moving Pictures Expert Group (MPEG) in 1989–94.[14][15] MUSICAM audio coding was used as a basis for some coding schemes of MPEG-1 and MPEG-2 Audio.[16] Most key features of MPEG-1 Audio were directly inherited from MUSICAM, including the filter bank, time-domain processing, audio frame sizes, etc. However, improvements were made, and the actual MUSICAM algorithm was not used in the final MPEG-1 Audio Layer II standard.
Since the finalisation of MPEG-1 Audio and MPEG-2 Audio (in 1992 and 1994), the original MUSICAM algorithm is not used anymore.[7][17] The name MUSICAM is often mistakenly used when MPEG-1 Audio Layer II is meant. This can lead to some confusion, because the name MUSICAM is trademarked by different companies in different regions of the world.[7][17][18] (Musicam is the name used for MP2 in some specifications for Astra Digital Radio as well as in the BBC's DAB documents.)
The Eureka Project 147 resulted in the publication of European Standard, ETS 300 401 in 1995, for DAB which now has worldwide acceptance. The DAB standard uses the MPEG-1 Audio Layer II (ISO/IEC 11172-3) for 48 kHz sampling frequency and the MPEG-2 Audio Layer II (ISO/IEC 13818-3) for 24 kHz sampling frequency.[19]
MPEG Audio
In the late 1980s,
The MPEG-1 Audio standard was based on the existing MUSICAM and ASPEC audio formats.[21]
The MPEG-1 Audio standard included the three audio "layers" (encoding techniques) now known as Layer I (MP1), Layer II (MP2) and Layer III (MP3).
All algorithms for MPEG-1 Audio Layer I, II and III were approved in 1991 as the committee draft of ISO-11172
The Layer III (MP3) component uses a lossy compression algorithm that was designed to greatly reduce the amount of data required to represent an audio recording and sound like a decent reproduction of the original uncompressed audio for most listeners.
Emmy Award in Engineering
Technical specifications
MPEG-1 Audio Layer II is defined in ISO/IEC 11172-3 (MPEG-1 Part 3)
- Sampling rates: 32, 44.1 and 48 kHz
- Bit rates: 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 Audio Layer II and is defined in ISO/IEC 13818-3 (MPEG-2 Part 3)[31][32]
- Additional sampling rates: 16, 22.05 and 24 kHz
- Additional bit rates: 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 bit rate
MPEG audio may have
How the MP2 format works
- MP2 is a transform audio encoder with hybrid filter bank, which means that compression takes place in the frequency domainafter 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 format are derived from the basic MP2 algorithm and format.
- The MP2 encoder may exploit inter channel redundancies using optional "joint stereo" intensity encoding.
- Like MP3, MP2 is a perceptual coding format, which means that it removes information that the human auditory system will not be able to easily 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 higher frequency accuracy (due to the larger number of frequency sub-bands used by MP3) explains why MP3 does not 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. This implies less codec time delay — which can make editing audio simpler — as well as "ruggedness" and resistance to errors which may occur during the digital recording process, or during transmission errors.
- The MP2 sub-band filter bank also 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 implies a very good sound quality on audio signals with rapid energy changes, such as percussive sounds. Because both the MP2 and MP3 formats use the same basic sub-band filter bank, both benefit from this characteristic.
Applications of MP2
Part of the
standards.Layer II is commonly used within the broadcast industry for distributing live audio over satellite, ISDN and IP Network connections as well as for storage of audio in digital playout systems. An example is NPR's PRSS Content Depot programming distribution system. The Content Depot distributes MPEG-1 L2 audio in a Broadcast Wave File wrapper. MPEG2 with RIFF headers (used in .wav
) is specified in the RIFF/WAV standards. As a result, Windows Media Player will directly play Content Depot files, however, less intelligent .wav
players often do not. As the encoding and decoding process would have been a significant drain on CPU resources in the first generations of broadcast playout systems, professional broadcast playout systems typically implement the codec in hardware, such as by delegating the task of encoding and decoding to a compatible soundcard rather than the system CPU.
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 Audio Layer II is the standard audio format used in the Video CD and Super Video CD formats (VCD and SVCD also support variable bit rate and MPEG Multichannel as added by MPEG-2).
MPEG-1 Audio Layer II is the standard audio format used in the MHP standard for set-top boxes.
MPEG-1 Audio Layer II is the audio format used in HDV camcorders.
MP2 files are compatible with some Portable audio players.
Naming and extensions
The term MP2 and
See also
- MPEG-1
- MPEG-2
- MP4 (container format)
- Elementary stream
- Musepack originally MP2-based, with numerous improvements
Notes
- ^ Nilsson, Martin (November 2000). "The audio/mpeg Media Type - RFC 3003". IETF. Retrieved 2009-12-07.
- ^ Hoschka, Philipp; Casner, Stephen L. (July 2003). "MIME Type Registration of RTP Payload Formats – RFC 3555". IETF. Retrieved 2009-12-07.
- S2CID 3773268. Reference 3 in the paper is to Committee Draft of Standard ISO/IEC 11172, December 6, 1991.
- ^ a b "ISO/IEC 11172-3:1993 – Information technology — Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s — Part 3: Audio". ISO. 1993. Retrieved 2010-07-14.
- ^ a b "ISO/IEC 13818-3:1995 – Information technology — Generic coding of moving pictures and associated audio information — Part 3: Audio". ISO. 1995. Retrieved 2010-07-14.
- ^ MPEG-1 and MPEG-2 Layer II Audio Encoding (Partial draft). Sustainability of Digital Formats. Washington, D.C.: Library of Congress. 17 May 2012. Retrieved 1 December 2021.
- ^ a b c "MPEG Audio FAQ Version 9". 1998. Retrieved 2010-08-22.
- ^ Techopedia (16 February 2015). "MPEG-1 Audio Layer II (MP2)". techopedia.com. Retrieved 2022-03-17.
- ^ A J Bower (1998). "Digital Radio — The Eureka 147 DAB System". UK: BBC. Retrieved 2010-08-22.
- S2CID 62615197.
- ^ a b "AES E-Library – Musicam Source Coding". 1991. Retrieved 2010-08-22.
- ISBN 0-7803-0003-3.
- ^ Box, Alan (1991-04-15). "A DAB commentary from Alan Box, EZ communication and chairman NAB DAB task force" (PDF). Broadcasting. Vol. 150, no. 15. p. 26.
- ^ Digital Audio Broadcasting (DAB); DAB to mobile, portable and fixed receivers – Details of 'DE/JTC-DAB' Work Item – ETS 300 401, 1995-02-15, retrieved 2010-08-23
- ^ DAB – Service planning for terrestrial Digital Audio Broadcasting (PDF), 1992, archived from the original (PDF) on 2005-05-04, retrieved 2010-08-22
- ^ a b "Status report of ISO MPEG – September 1990". 1990. Archived from the original on 2010-02-14. Retrieved 2010-08-22.
- ^ a b Telos Systems. "Facts about MPEG compression". Archived from the original on 2001-05-08. Retrieved 2010-08-22.
- ^ MUSICAM USA. "MUSICAM USA Frequently Asked Questions – Isn't MUSICAM simply your implementation of ISO/MPEG Layer 2?". Retrieved 2010-08-23.
- ^ Radio Broadcasting Systems; Digital Audio Broadcasting (DAB) to mobile, portable and fixed receivers – Details of 'REN/JTC-DAB-36' Work Item – EN 300 401, 2006-06-15, retrieved 2010-08-23
- ^ Chiariglione, Leonardo (October 1989). "MPEG Press Release". Kurihama: Moving Picture Experts Group. Archived from the original on 2010-08-05..
- ISBN 3-540-76357-0p. 144: "In the year 1988, the MASCAM method was developed at the Institut für Rundfunktechnik (IRT) in Munich in preparation for the digital audio broadcasting (DAB) system. From MASCAM, the MUSICAM (masking pattern universal subband integrated coding and multiplexing) method was developed in 1989 in cooperation with CCETT, Philips and Matsushita."
- ^ Chiariglione, Leonardo (November 1991). "MPEG Press Release". Kurihama: Moving Picture Experts Group. Archived from the original on 2011-05-03. Retrieved 2010-07-17.
- ^ ISO (November 1991). "CD 11172-3 - Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to About 1.5 MBit/s Part 3 Audio". neuron2.net. Archived from the original (DOC) on June 11, 2010. Retrieved 2010-07-17.
- ^ Patel, Ketan; Smith, Brian C.; Rowe, Lawrence A. Performance of a Software MPEG Video Decoder (PDF). ACM Multimedia 1993 Conference. Article's reference 3 is: 'ISO/IEC JTC/SC29, "Coded Representation of Picture, Audio and Multimedia/Hypermedia Information", Committee Draft of Standard ISO/IEC 11172, December 6, 1991'.
- ^ ISO (1992-11-06). "MPEG Press Release, London, 6 November 1992". Chiariglione.org. Archived from the original on 12 August 2010. Retrieved 2010-07-17.
- ^ a b "Press Release - Adopted at 22nd WG11 meeting" (Press release). International Organization for Standardization. 1993-04-02. Archived from the original on 2010-08-06. Retrieved 2010-07-18.
- ^ ISO (October 1998). "MPEG Audio FAQ Version 9 - MPEG-1 and MPEG-2 BC". ISO. Retrieved 2009-10-28.
- ^ Brandenburg, Karlheinz; Bosi, Marina (February 1997). "Overview of MPEG Audio: Current and Future Standards for Low-Bit-Rate Audio Coding". Journal of the Audio Engineering Society. 45 (1/2): 4–21. Retrieved 30 June 2008.
- ^ National Academey of Television Arts and Sciences, Outstanding Achievement in Technical/Engineering Development Awards (PDF), archived from the original (PDF) on 2010-04-14, retrieved 2010-08-01
- ^ "CCETT - DAB : Digital Audio Broadcasting (archived website)". 2001-02-11. Archived from the original on 2001-02-11. Retrieved 2010-08-01.
- ^ Werner Oomen, Leon van de Kerkhof. "MPEG-2 Audio Layer I/II". chiariglione.org. Retrieved 2009-12-29.
- ^ a b Predrag Supurovic (September 1998). "MPEG Audio Frame Header". Retrieved 2009-07-11.
- ^ ISO MPEG Audio Subgroup, MPEG Audio FAQ Version 9, MPEG-1 and MPEG-2 BC, retrieved on 2009-07-11.
- ^ TwoLAME: MPEG Audio Layer II VBR, retrieved on 2009-07-11.
References
- Genesis of the MP3 Audio Coding Standard by Hans Georg Musmann in IEEE Transactions on Consumer Electronics, Vol. 52, Nr. 3, pp. 1043–1049, August 2006
- MUSICAM Source Coding by Yves-François Dehery, AES 10th International Conference: Kensington, London, England, (7-9 Sept 1991), pp 71–79.
External links
- The history of MP3 from Fraunhofer IIS
- MPEG Audio Resources and Software
- TooLAME – An MP2 encoder
- TwoLAME – A fork of the tooLAME code
- RFC 3003 – The document defining MIME type for MPEG-1 Audio Layer II
- A MPEG Audio Layer II decoder in 4k – Source code for small open source decoder.
- Official MPEG web site
- Patent Status of MPEG-1, H.261 and MPEG-2 – Some information about patents