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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 121

standardizedinISO/MPEGIS‐11172.PriortotheavailabilityofAAC,Layer‐3wasspecifically

recommendedfor56and64kbpschannel.

Layer-3 Features

PsychoacousticMasking

TheaudioinLayer‐3isdividedinto576frequencybands.First,apolyphasefilterbankperforms

adivisionintothe32“main”bandswhichcorrespondinfrequencytothoseus

edbytheless

complexLayer‐2.Filtersarethenusedtofurthersubdivideeachofthemainbandsinto18

more.Ata32kHzsamplingrate,theresultingbandwidthis27.78Hz–allowingveryaccurate

calculationofthemaskingthresholdvalues.Sufficientfrequencyresolutionisavailableto

exce

edthewidthoftheear’scriticalbands(100Hzbelow500Hz;20%ofthecenterfrequency

athigherfrequencies)acrosstheaudiblespectrum,resultinginbetterhidingofnoisethan

wouldotherwisebepossible.

RedundancyReduction

RedundancyreductionisaccomplishedbyaHuffmancodingprocesstotakeadv

antageofthe

statisticalpropertiesofthesimplifiedsignaloutputfromthepsychoacousticstage.Valuesthat

appearmorefrequentlyarecodedwithshorterwords,whereasvaluesthatappearonlyrarely

arecodedwithlongerwords.Thisresultsinanoveralldecreaseinthedatarate,withno

degradation,sinceitisalosslessreducti

onscheme.

Noticethatthisredundancyreductionprocessistheidealsupplementtopsychoacoustic

masking.Ingeneral,maskerswithhightonalityhavemoreredundancybutallowlessmasking,

whilenoise‐likesignalshavelowredundancyandhighmaskingeffect.

BitReservoirBuffering

Often,therearesomecriticalpartsinapieceofmu

sicthatcannotbeencodedatagivendata

ratewithoutaudiblenoise.Thesesequencesrequireahigherdataratetoavoidartifacts.Layer‐

3usesashorttime“bitreservoir”buffertoaddressthatneed.Similartoasavingsaccount,this

bufferisfilledin“easytim

es”withdatabitsthatarenotrequiredfortheactualframe.Ifa

criticalpartoccurs,theencodercanusethesavedbitstocodethispartwithahigherdatarate.

AncillaryData

Thebitreservoirbufferoffersaninterestingcapability:aneffectivesolutionfortheinclusionof

suchancillarydataastex

torcontrolsignaling.Thedataisheldinaseparatebufferandgated

ontotheoutputbitstreamusingthebitsallocatedforthereservoirbufferwhentheyarenot

requiredforaudio.

Layer‐3J‐Stereo

Ajointstereomodepermitsadvantagetobetakenfromth

eredundancyinstereoprogram

material.TheencoderswitchesfromdiscreteL/RtoamatrixedL+R/L‐Rmodedynamically,

dependingupontheprogramcontent.Thematrixedmodeofoperationtakesofadvantageof

theusualredundancyofthe“center”channelinformationandthereforesignificantlyimproves

overallfidelity.

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