PsychoacousticsandtheMP3Codec

PresentationBy:RamiBotrosSupervisor:RobOpdam

Purpose:GiveanintroductionaboutPsychoacoustics,thenshowapplicationsofitintheMP3codec

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Introduction

FromtheofficialhomepageoftheMovingPictureExpertsGroup

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Outline

Psychoacoustics Perceptualcodecs Masking

TemporalMasking SimultaneousMasking

MP3codec Encodingscheme

OutlookandConclusionPsychoacousticsandtheMP3Codec 4

Psychoacoustics

Therelationbetweenphysicalaspectsofacousticeventsandtheresultinghumanauditoryperceptions[2].

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Psychoacoustics

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Psychoacoustics

Examples: Sensitivitytorangesofamplitudesorfrequenciesloudness

Indisguinshablity betweencertainwaveforms Soundlocalization

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Psychoacoustics

Psychoacousticsdealswithsubjectiveperception

Sometimesreliesonsurveyinghumansubjects

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PerceptualCodecs

Codec:Algorithmtoconvertinformationtocodeandviceversa.

Humansdonotprocessalldatatheysense Perceptualcodecneglectsdatathatcannotbeperceived

Perceptualcodecsarelossy. Upto90%compression

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MaskingEffect

Afainter,butdistinctlyaudiblesignal(themaskee)becomesinaudiblewhenacorrespondinglyloudersignaloccurs.

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TemporalMasking

Similarfrequencyconsecutivesoundscanmaskeachother

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[2]

TemporalMaskingDemonstration

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Forwardmasking 100msdelay 10msdelay

BackwardMasking 100msdelay 10msdelay

SimultaneousMasking

Forsignalswithsimilarfrequencies Total:24frequencybands[4]

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[2]

FrequencyMaskingDemonstration

Singlereferencetonefollowedbytwotones Thehigherfrequencytoneisreducedinamplitudebystepsof5dB.

Sequencerepeatedwithlargerfrequencyseparation

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FrequencyMaskingDemonstration2

Largeramplitudemasksbroaderrangeoffrequencies

Puretonesmaskhigherfrequenciesbetterthanlowerfrequencies.

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MP3Encoding

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MP3Encoding

Signalbrokendownintotimeframes Conversiontofrequencydomain Spectralenergydistributiondeterminedandbitratedistributedaccordingly

Frequencydistributioncomparedwithpsychoacousticmodelandbitratesloweredaccordingly

Huffmanencoding(lossless)

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MP3Decoding

Straightforwardreconstructionofbitstream Nofreemodels

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Successor

AACisthedefaultorstandardaudioformatfor YouTube, iPhone, iPod, iPad, NintendoDSi, iTunes, DivXPlusWebPlayerandPlayStation3.

Features: Morechannels Variablebitrateandframelength Moreflexibility Highercodingefficiencyandaccuracy

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Conclusion

Humanperceptionofsoundnotstraightforward

Maskingeffect:Onesignalcomponenthidestheother

MP3useslossy compressionbasedonpsychoacousticmodelofimperceptiblesignals.

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References

1. M.Kahrs,K.Brandenburg,ApplicationsofDigitalSignalProcessingtoAudioandAcoustics,KluwerAcademicPublishers,2000

2. S.Hacker,MP3:TheDefinitiveGuide,OREILLY,2000

3. http://mpeg.chiariglione.org/standards/mpeg2/audio,Retrievedon07Mar13

4. R.Raissi,TheTheoryBehindMp3,2002

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Questions

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