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MP3

MP3 (or, more precisely, MPEG-1/2 Audio Layer 3)an audio compression algorithm capablegreatly reducingamountdata requiredreproduce audio, while sounding likefaithful reproduction oforiginal uncompressed audio tolistener.

Tablecontents
1 History
2 QualityMP3 audio
3 Bit Rate
4 Design bugsMP3
5 EncodingMP3 audio
6 AlternativesMP3
7 Licensingpatent issues
8 External links

History

The MPEG-1/2 Layer 2 encoding started asDigital Audio Broadcast (DAB) project initiated by Fraunhofer IIS-A. This project was financed byEuropean Union aspart ofEUREKA research program wherewas commonly known as EU-147.

RuntimeEU-147 was from 19871994. In 1991 there were two proposals available: Musicam (known as Layer II)ASPEC (Adaptive Spectral Perceptual Entropy Coding) (with similaritiesMP3). Musicam was chosen dueits simplicityerror resistance.

A working group around Karlheinz BrandenburgJürgen Herre took ideas from Musicam, from ASPECown ideascreated MP3, which was designedachievesame quality at 128 kbit/s as MP2 at 192 kbit/s.

Both algorithms were finalized1992 as partMPEG-1,first phasework by MPEG, which resulted ininternational standard ISO/IEC 11172-3, published1993. Further work on MPEG Audio was finalized1994 as part ofsecond phase, MPEG-2, which resulted ininternational standard ISO/IEC 13818-3, originally published1995.

Compression efficencylossy encoderstypically defined bybitrate, because compression rate depends on bit depthsampling rate ofinput signal. Nevertheless thereoften published compression rates, which are usingCD parameters as reference (44.1 kHz, 2x16 bit). Sometimes alsoDAT SP parametersused (48 kHz, 2x16 bit). Compression ratiothis referencehigher, which demonstratesproblem ofterm compression ratiolossy encoders.

Karlheinz Brandenburg used Suzanne Vega's CD Tom's Diner as his model formp3 compression algorithm. This CD was chosen becauseits softnesssimplicity, makingeasierhear imperfections incompression format during playbacks.

FhG official webpage publishfollowing compression ratiosdata ratesMPEG-1 Layer 1, 23:

These valuesmore or less public relation values, because

More realistic bitrates are:

Comparingnew file format typicallydone by comparingmedium quality encoder ofold format andhighly tuned encoder encoder ofnew format.

The algorithm ofMP3 format uses, at its heart,hybrid transformtransformtime domain signal intofrequency domain signal:

In terms ofMPEG specifications, AAC from MPEG-2 isbesuccessor ofMP3 format. In practice, however, duenumerous patentinglicensing issuesvarious parts ofMPEG specifications, Ogg Vorbis seems positionedbemostly likely successorMP3 aspopular formataudio interchange. Ogg Vorbisalsoformat usedsounds inWikipedia.

MP2MP3 hitInternet

In October 1993, MP2 (MPEG-1 Audio Layer 2) files appeared onInternetwere often played back using Xing MPEG Audio Player,later inprogramUNIX by Tobias Bading called MAPlay initially released on Feb 22 1994. (MAPlay was also portedMicrosoft Windows.) Initiallyonly encoder availableMP2 production wasXing Encoder, accompanied byprogram CDDA2WAV,CD ripper that copied CD audiohard disks.

Beginning infirst half1995, MP3 files, file representationsMPEG-1 Audio Layer III data, began flourishing onInternet. Its popularity begat such companiessoftware packages as Nullsoft's Winamp, mpg123 andnow Bertelsmann-owned Napster.

QualityMP3 audio

Many listeners acceptMP3 bitrate128 kilobits per second (kbit/s) as near enoughCD quality; this providescompression ratioapproximately 11:1, although listening tests show that withbitpractice, most listeners can reliably distinguish 128 kbit/s MP3s from CD originals. To many other listeners, 128 kbit/sunacceptably low quality, whichunfortunate since many commonly-available encoders set this as their default bitrate.

Possible encoders:

QualityMP3 depends on qualityencoder anddifficulty ofsignal which must be encoded. Good encoders gave acceptable quality at 128...160 kbit/s, nearly transparenceachieved at 160...192 kbit/s. Low quality encoders never reach nearly transparence mode, even not at 320 kbit/s. Sois pointlessspeak from 128 kbit/s or 192 kbit/s quality. A 128 kbit/s MP3 encoded withgood encoder typically sounds better than192 kbit/s MP3 encoed file withbad encoder.

An important featureMP3that itlossy -- meaning thatremoves information frominputordersave space. Asmost modern lossy encoders, MP3 algorithms work hardensure thatsoundsremoves cannot be detected by human listeners, by modelling chacteristicshuman hearing such as noise masking.

However, experienced listeners can telldifference fromoriginal at 192 kbit/s,even at 256 kbit/s on some ofless powerful (and obsolete) encoders. If your aim isarchive sound filesno lossquality, you may be more interestedlossless audio compression such as FLAC, SHN, or LPAC -- these will generally compress16-bit PCM audio streamapproximately 50-75% oforiginal size (depending uponcharacteristics ofaudio itself).

Bit Rate

The bit rates, i.e. numberbinary digits streamed per second,variableMP3 files. The general rulethathigherbitrate,more informationincluded fromoriginal sound file,thushigher isqualityplayed back audio. Inearly daysMP3 encoding,fixed bit rate was used forentire file.

Bit rates availableMPEG-1 layer 332, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256320 kbit/s ( bits per second), andavailable sample frequencies32, 44.148 kHz. 44.1 kHzalmost always used as this isaudio CD frequency,128 Kbitsome sortde facto "good enough" standard. MPEG-2(non-official) MPEG-2.5 adds more bitrates: 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160 kbit/s.

However, audioMP3 filesdivided into chunks called frames, which all havebitrate marker, so itpossiblechangebitrate dynamically asfileplayed. This technique makespossibleuse more bitsparts ofsoundhigh dynamics (much "sound movement")less bitspartslow dynamics. Some encoders utilize this possibilitygreater or lesser extent.

Design bugsMP3

Thereseveral flaws inMP3 file format, which can't be fixed bygood encoder. This flawsinherent properties ofMP3 file format.

In parentheses arefile formats where this bugfixed.

EncodingMP3 audio

The MPEG-1 standard does not includeprecise specificationan MP3 encoder. The decoding algorithmfile format, ascontrast,well defined. Implementors ofstandard were supposeddevise by themselves algorithms suitableremoving parts ofinformation inraw audio (or rather its MDCT representation infrequency domain). This processtypically based on psycho-acoustic coding, i.e., you remove things thathuman listener will not notice anyway by modeling our audio perception system (bothour earsin our brain).

Asresult, theremany different MP3 encoders available, each producing filesdiffering quality; asSeptember 30, 2001,best encoder at high bitrates (128 kbit/sup)LAME, andbest at low bitratesFraunhofer's own encoder. MP3 decoding, however,carefully defined instandard. Most decoders"bitstream compliant", meaning thatwill each produce exactlysame uncompressed output fromgiven MP3 file.

AlternativesMP3

Many other lossy audio codecs exist, including:

mp3PRO, MP3, AAC,MP2all members ofsame technological familydepend on roughly similar psychoacoustic models. The Fraunhofer Gesellschaft owns many ofbasic patents underlying these codecs,Dolby Labs, Sony, Thomson Consumer Electronics,AT&T holding other key patents.

Therealso some non-lossy (lossless) audio compression methods used oninternet. While theynot similarMP3, theygood examplesother compression schemes available. These include:

MP3, which was designedtuneduse alongside MPEG-1/2 Video, generally performs poorly on monaural data at less than 48 kbit/s orstereo at less than 80 kbit/s.

Though proponentsnewer codecs such as WMARealAudio have asserted that their respective algorithms can achieve CD quality at 64 kbit/s, listening tests have shown otherwise; however,qualitythese codecs at 64 kbit/sdefinitely superiorMP3 atsame bandwidth.

Thomson claims that its mp3PRO codec achieves CD quality at 64 kbit/s, but listeners have reported that64 kbit/s mp3PRO file comparesquality to112 kbit/s MP3 filedoes not come reasonably closeCD quality until about 80 kbit/s.

The Xiph.org Foundation,developers ofVorbis algorithm used innew Ogg format, claims that Vorbis somewhat exceeds MP3WMA sound quality while infringing no patents,providesweb pagelistening testsdemonstrate this.

Licensingpatent issues

Thomson Consumer Electronics controls licensing ofMPEG-1/2 Layer 3 patentscountries such asUnited StatesAmericaJapan that recognize software patents. Thomson Consumer Electronics has decidedcashonpatents. In fact Microsoft,makers ofWindows operating system, chosemove away from MP3their own proprietary Windows Media formatsavoid any patent implications.

For information aboutcosts see [1][1].

In spitethese threats,perpetuation ofMP3 format continues;reasonsthis appearbenetwork effects caused by:

External links


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