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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 111

ear’ssensitivitytosignalsneartheconstanttoneisgreatlyreduced.Tonesthatwerepreviously

audiblebecome“masked”inthepresenceof“maskingtones,”inthiscase,theoneat300Hz.

Allsignalsbelowtheupper“thresholdofaudibility”curve,orMaskingThresholdarenot

audible,sowecandropth

emoutorquantizethemcrudelywiththeleastnumberofbits.Any

noisewhichresultsfromcrudequantizationwillnotbeaudibleifitoccursbelowthethreshold

ofmasking.Themaskingdependsuponthefrequency,thelevel,andthespectraldistributionof

boththemaskerandthemaskedsounds.

Masking effects in the frequency domain. A masking signal inhibits audibility of

signals adjacent in frequency and below the threshold.

Tobenefitfromthemaskingeffects,perceptualcodersuseafilterbanktodividetheinputaudio

intomultiplebandsforanalysisandprocessing.Themaximummaskednoiseleveliscalculated

dependinguponthespectralcontent,andtheavailablebitsareallocatedsoastokeepthe

quantizationnoisebelowthemaski

ngthresholdateverypointinthespectrum.

Whilecodingefficiencyincreaseswithmorebandsandbetterfrequencyresolution,thetime

domainresolutiondecreasessimultaneouslyowingtoaninevitableside‐effectoftheband

filteringprocess.Higherfrequencyresolutionrequiresalongertimewindow–whichlimitsthe

timeresolution.Happily,maskingwor

ksalsointhetimedomain.Ashorttimebeforeanda

longertimeafteratoneisswitchedonandoff,othersignalsbelowathresholdamplitudelevel

arenotnoticeable.Filterbankswithhigherfrequencyresolutionnaturallyexploittheear’s

time‐maskingproperties.

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