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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 115

qualityindistinguishablefromtheoriginal,withnoaudiotestitemfallingbelowthe

“perceptible,butnotannoying”thresholdincontrolledlisteningtests.

TheMPEGtestitemsincludethemostdifficultaudioknowntocodecresearchers,sothiswas

dauntingchallenge.Thethinkingwasthatifacodeccouldpassthistest,itwouldsurelybe

transparentfornormalpr

ogrammateriallikevoiceandpopmusic,whicharemucheasierto

encode.AACdesignerschosetouseanewmodularapproachfortheproject,withcomponents

beingplugged‐intoageneralframeworkinordertomatchspecificapplicationrequirements

andthealways‐presentpe

rformance/complexity/delaytradeoffs.

Comparedtothepreviouslayers,AACtakesadvantageofsuchnewtoolsastemporalnoise

shaping,backwardadaptivelinearpredictionandenhancedjointstereocodingtechniques.AAC

supportsawiderangeofsamplingrates(8–96kHz),bitrates(16–576kbps)andfromoneto48

audiochannels.

AACisalotmoresophisticatedtha

nthepreviousMPEGlayers2&3,providingsignificantlymore

codingpower.Becauseitusesamodularapproach,newideascanbedevelopedandpluggedin

tothebasicstructure.Thishadtheadditionaladvantagethatitwaspossibletocombinethe

variouscomponentsfromdifferentdev

elopers,takingthebestpiecesfromeach.AACwasbuilt

onasimilarstructuretoLayer‐3,andthusretainsmostofitsfeatures.Nevertheless,compared

tothepreviousMPEGlayers,AACbenefitsfromsomeimportantnewadditionstothecoding

toolkit:

• Animprovedfilterbankwithafr

equencyresolutionof2048spectral

components,nearlyfourtimesthenumberusedbyLayer‐3.

• TemporalNoiseShaping,anewandpowerfulelementthatminimizesthe

effectoftemporalspread.Thisbenefitsvoicesignals,inparticular.

• APredictionModuleguidesthequantizertoveryeffectivecodingwhen

thereisanoticeablesignalpattern,li

kehightonality.

• PerceptualNoiseShapingallowsafinercontrolofquantizationresolution,

sobitscanbeusedmoreefficiently.



Becauseitusesthismodularapproach,animplementermaypickandchooseamongthe

componenttoolstomakeaproductwithappropriateperformance/complexityratios.Or,ne

w

modulescanbedevelopedlaterand"pluggedin"toitsbasicstructure.Threedefaultprofiles

havebeendefined,usingdifferentcombinationsoftheavailabletools:

• MainProfile.Usesalltoolsexceptthegaincontrolmodule.Providesthe

highestqualityforapplicationswheretheamountofrandomaccessory

memory(RA

M)neededisnotconstrained.

• Low‐complexityProfile. Deletesthepredictiontoolandreducesthe

temporalnoise‐shapingtoolincomplexity.

• Sample‐rateScaleable(SRS)Profile.Addsthegaincontroltooltothelow

complexityprofile.Allowstheleastcomplexdecoder.



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