Audio CompressionMotivationCompression GoalsCompression TechniquesMPEGMPEG Audio CompressionMPEG Audio FeaturesMPEG Audio Feautures cont.OverviewSlide 10The Polyphase Filter BankPsychoacousticsNoise masking thresholdThe Psychoacoustic ModelBasic StepsMPEG Audio Layer IMPEG Audio Layer IIMPEG Audio Layer IIISlide 19Layer III enhancementsMPEG and the Future?ReferencesSlide 23Audio CompressionUsha SreeCMSC 691M10/12/04MotivationEfficient StorageStreamingInteractive Multimedia ApplicationsCompression GoalsReduced bandwidthMake decoded signal sound as close as possible to original signalLowest Implementation ComplexityRobustScalableCompression TechniquesVoc File CompressionLinear Predictive CodingMu-law compressionDifferential Pulse Code ModulationMPEGMPEGMoving Picture Experts GroupPart of a multiple standard forVideo compressionAudio compressionAudio, Video and Data synchronization to an aggregate bit rate of1.5 Mbit/secMPEG Audio CompressionPhysically Lossy compression algorithmPerceptually lossless, transparent algorithmExploits perceptual properties of human earPsychoacoustic modelingMPEG Audio Standard ensures inter-operability, defines coded bit stream syntax, defines decoding process and guarantees decoder’s accuracy.MPEG Audio FeaturesNo assumptions about the nature of the audio sourceExploitation of human auditory system perceptual limitationsRemoval of perceptually irrelevant parts of audio signalIt offers a sampling rate of 32, 44.1 and 48 kHz.Offers a choice of three independent layersMPEG Audio Feautures cont.All three layers allow single chip real-time decoder implementationOptional Cyclic Redundancy Check (CRC) error detectionAncillary data may be included in the bit streamAlso features such as random access, audio fast forwarding and audio reverse are possible.OverviewQuantization, the key to MPEG audio compressionTransparent, perceptually lossless compressionNo distinction between original and 6-to-1 compressed audio clipsThe Polyphase Filter BankKey component common to all layersDivides the audio signal into 32 equal-width frequency subbandsThe filters provide good time and reasonable frequency resolutionCritical bands associated with psychoacoustic modelsPsychoacousticsThe aim is to remove irrelevant parts of the audio signalThe human auditory system is unable to hear quantization noise under conditions of auditory masking Masking occurs whenever a strong signal makes a neighborhood of weaker audio signals imperceptibleNoise masking thresholdHuman ear resolving power is frequency dependentNoise masking threshold, at any frequency, depends only on the signal energy within a limited bandwidth neighborhood that frequencyThe Psychoacoustic ModelAnalyzes the audio signal and computes the amount of noise masking as a function of frequencyThe encoder decides how best to represent the input signal with a minimum number of bitsBasic StepsTime align audio dataConvert audio to frequency domain representationProcess spectral values into tonal and non-tonal componentsApply a spreading functionSet a lower bound for threshold valuesFind the threshold values for each subbandCalculate the signal to mask ratioMPEG Audio Layer ISimplest codingSuitable for bit rates above 128 kbits/sec per channelEach frame contains header, an optional CRC error check word and possibly ancillary data.Eg. Philips Digital Compact CassetteMPEG Audio Layer IIIntermediate complexityBit rates around 128 kbits/sec per channelDigital Audio Broadcasting (DAB)Synchronized Video and Audio on CD-ROMForms frames of 1152 samples per audio channel.MPEG Audio Layer IIIBased on Layer I&II filter banksMost complex codingBest audio qualityBit rates around 64 kbits/sec per channelSuitable for audio transmission over ISDNCompensates filter deficiencies by processing outputs with a two different MDCT blocks.Layer III enhancementsAlias reductionNon uniform quantizationScalefactor bandsEntropy coding of data valuesUse of a “bit reservoir”MPEG and the Future?MPEG-1: Video CD and MP3.MPEG-2: Digital Television set top boxes and DVD MPEG-4: Fixed and mobile web MPEG-7: description and search of audio and visual content MPEG-21: Multimedia FrameworkReferencesDigital Audio Compression -http://das.iocon.com/res/docs/pdf/Digital_Audio_Compression_01oct1993DTJA03P8.pdfMPEG Audio Standard-www.cs.columbia.edu/~coms6181/slides/6R/mpegaud.pdfThank
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