Perceptual Audio Coding

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■ Preface

■ Physiology of thehuman ear� Critical bands

� Threshold of hearing

� Amplitude masking

� Temporal masking

■ Rationale forperceptual coding

■ Coding techniques� Subband coding

� Transform coding

■ MPEG Audio standards� MP1

� MP2

� MP3

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Preface

■ Traditionally, audio recording systems haveused objective parameters as their designgoals - flat response, minimal noise, and soon.

■ Perceptual coders recognize that the finalreceiver is the human auditory system andmake use it to code audio signals.

Physiology of the human ear

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Critical bands

■ The ear contains roughly 30,000 hair cellsarranged in multiple rows along the basilarmembrane.

■ Hair cells respond to the strongeststimulation in their local regions calledcritical bands.

■ Critical bands are not fixed and any audibletone will create a critical band centered onit.

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■ Critical bands are much narrower at lowfrequencies than at high frequencies.

■ Critical bandwidth (Hz) =24.7(4.37fc+1),where fc is the center frequency in kHz.

■ The critical band concept is an empiricalphenomenon.

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■ The bark is a unit to measure the criticalband rate.

■ A critical band has a width of 1 bark.

■ Critical bands are important in perceptualcoding because they show that the eardiscriminates between energy in the band,and the energy outside the band; inparticular, this promotes masking.

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Threshold of hearing

■ Two fundamental phenomena that governhuman hearing are the minimum hearingthreshold and masking.

■ The threshold of hearing curve describes theminimum level at which the ear can detect atone at a given frequency.

■ The ear is most sensitive around 1 to 5 kHz.

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Amplitude masking

■ Amplitude masking occurs when a toneshifts the threshold curve upward in afrequency region surrounding the tone.

■ The masking threshold describes the levelwhere a tone is barely audible.

■ The strong sound is called the masker andthe softer sound is called the maskee.

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■ Masking theory argues that the softer tone isjust detectable when its energy equals theenergy of the part of the louder maskingsignal in the critical band.

■ Masking can overlap adjacent critical bandswhen a signal is loud or contains harmonics.

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■ Louder or more complex tones providegreater masking with masking curves thatare broadened and with greater high-frequency extension.

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■ Simultaneous masking curves areasymmetrical in a way that the slope of theshifted curve is less steep on the high-frequency side.

■ As sound level of the masker increases, thethreshold curve broadens, and in particularits upper slope decreases while lower sloperemains relatively unaffected.

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Temporal masking

■ Temporal masking occurs when tones aresounded close in time, but notsimultaneously.

■ A louder tone appearing just after(premasking), or before (postmasking) asofter tone overcomes the softer tone.

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Before maskerexists

After maskerexists

Maskerexists

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■ Amplitude and temporal masking form acontour that can be mapped in the time-frequency domain.

■ Perceptual coders identify this contour forchanging signal conditions, and code thesignal appropriately.

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■ Using diverse and dynamically changingpsychoacoustical cues and signal analysis,inaudible components can be removed withacceptable degradation.

Rational for Perceptual Coding

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■ Perceptual coding systems analyze thefrequency and amplitude content of theinput signal, compare it to a model ofhuman auditory perception, and code itaccordingly.

■ Tests show that ratios of 4:1 or 6:1 can betransparent.

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■ The coding performance of perceptualcoding relies on the following factors:� Only audible information is coded.

� Bits are assigned according to audibility.

� Quantization error is confined in a critical band.

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■ Perceptual coding is tolerant of errors.� With PCM, an error introduces a broadband

noise.

� With most perceptual coders, the error islimited to a narrow band corresponding to thebandwidth of the coded critical band, thuslimiting its loudness.

Coding Techniques

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■ There are two types of frequency domaincoders: subband and transform coders.

■ Both coders operate over a block ofsamples.

■ This block must be kept short to stay withinthe temporal resolution of the ear.

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■ In practical applications, many coders arehybrid coders which combine techniquesfrom both subband and transform coding.

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Subband coding:

■ Blocks of consecutive time-domain samplesrepresenting the boardband signal arecollected over a short period and applied toa digital filter bank.

■ The filter bank divides the signal intomultiple bandlimited channels toapproximate the critical band response ofthe human ear.

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■ The samples in each subband are analyzedand compared to a psychoacoustic model.

■ The coder adaptively quantizes the samplesin each subband based on the maskingthreshold in that subband.

■ Each subband is coded independently withmore or fewer bits allocated to the samplesin the subband.

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Average level

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■ Bit allocation is determined by apsychoacoustic model and analysis of thesignal itself.

■ Samples are dynamically quantizedaccording to audibility of signals.

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Average energy

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■ The signal-to-mask ratio (SMR) of aparticular subband is the difference betweenthe maximum signal and the maskingthreshold in that subband and is used todetermine the number of bits assigned to asubband.

■ The signals below the minimum or maskingcurve are not coded.

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■ The number of bits given to any subbandmust be sufficient to yield a requantizationnoise level that is below the masking level.

■ The quantization noise in a subband islimited to that subband and can be maskedby the audio signal in that subband.

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■ An inverse synthesis filter bank sums thesubband signals to reconstruct the outputbroadband signal.

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Transform coding:

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MPEG-1 Audio standard

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■ The audio portion of MPEG1 standard(11172-3) has found many applications suchas VCD, CD-ROM, and digital audiobroadcasting.

■ It supports coding of 32, 44.1 and 48 kHzPCM data at bit rates of 32 to 192kbps/channel.

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■ The standard describes three layers ofcoding.� Layer I describes the least sophisticated method

and operates at 192 kbps/channel.

� Layer II is based on layer I and operates at 96-128 kbps/channel.

� Layer III is conceptually different from I and II,and operates at 64 kbps/channel.

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■ Layers I and II are based on MUSICAM(Masking-pattern Universal SubbandIntegrated Coding And Multiplexing)coding algorithm.

■ Layer III is based on both MUSICAM andASPEC (Adaptive Spectral PerceptualEntropy Coding).

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MPEG-1 Layer I:

■ Layer I is a simplified version of theoriginal MUSICAM standard.

■ A polyphase filter bank is used to split thewideband signal into 32 subbands of equalwidth.

■ Adjacent subbands overlap, and the filterbank and its inverse are not lossless.

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■ The filter outputs 32 samples, one sampleper band, for every 32 input samples.

■ Each subband group of 12 samples is givena bit allocation.

■ A fixed scale factor exponent is computedfor each subband, which is based on thelargest sample value in the subband.

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■ SMR determines the minimum signal-to-noise ratio that has to be met by thequantization of the subband samples.

■ When available, additional bits are added tocodewords to increase the S/N ratio abovethe minimum.

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MPEG-1 layer II

■ Layer II is essentially identical to theoriginal MUSICAM standard.

■ Layer II is similar to layer I, but moresophisticated in design.

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■ Data is coded in three groups of 12 samplesfor each subband (Layer I uses one group).

■ Accordingly, the FFT analysis block size isincreased to 1024, which improves thefrequency resolution.

■ Tonal and nontonal components aredistinguished to better determine their effecton the masking threshold.

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MPEG-1 Layer III

■ Layer III combines elements fromMUSICAM and ASPEC, and is morecomplex than Layers I and II.

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