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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 116

TheblockdiagramoftheAACencoderisshownbelow.Itisconsiderablymoresophisticated

thanthepreviousMPEGLayer‐2andLayer‐3systems,andthereforeoffersmorecodingpower.

BecauseAACwasbuiltonasimilarstructuretoLayer‐3,itthereforeretainssomeofits

powerfulfeatures:

• RedundancyReduction.AHuffmanencodingprocesscausesvaluesthat

appearmorefrequentlytobecodedwithshorterwords,whilevaluesthat

appearonlyrarelyarecodedwithlongerwords.Thisresultsinanoverall

increaseincodingefficiency–withnodegradation,sinceitisaco

mpletely

losslessprocess.

• BitReservoirbuffering.Often,therearesomecriticalpartsinapieceof

musicthatcannotbeencodedatagivendataratewithoutaudiblenoise.

Thesesequencesrequireahigherdataratetoavoidartifacts.Ontheother

hand,somes

ignalsareeasytocode.Ifaframeiseasy,thentheunusedbits

areputintoareservoirbuffer.Whenaframecomesalongthatneedsmore

thantheaverageamountofbits,thereservoiristappedforextracapacity.

• AncillaryData.Thebi

treservoirbufferoffersaneffectivesolutionforthe

inclusionofsuchancillarydataastextorcontrolsignaling.Thedataisheld

inaseparatebufferandgatedontotheoutputbitstreamusingsomeofthe

bitsallocatedforthereservoirbufferwhentheyarenotrequiredforaudio.

• TheJointStereomodetak

esadvantageofthe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material.



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