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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 113

Thestepsinvolvedintheperceptualcodingprocessareshownbelow:

Thecomponentsworkasfollows:

• Theanalysisfilterbankdividestheaudiointospectralcomponents.

Sufficientfrequencyresolutionmustbeusedinordertoexceedthewidthof

theear'scriticalbands,whichis100Hzbelow500Hzand20%ofthecenter

frequencyathigherfrequencies.

• Theestimatio

nofmaskedthresholdsectioniswherethehumanear/brain

systemismodeled.Thisdeterminesthemaskingcurve,underwhichnoise

mustfall.

• Theaudioisreducedtoalowerbitrateinthequantizationandcoding

section.Onetheonehand,thequantizationmustbesufficientlycoarsein

ordernottoexce

edthetargetbitrate.Ontheotherhand,theerrormust

beshapedtobeunderthelimitssetbythemaskingcurve.

• Thequantizedvaluesarejoinedinthebitstreammultiplex,alongwithany

sideinformation.



Doingaudiocodingeffectivelymeansmanagingseveraltradeoffs.Mo

stimportantisthe

numberofsamplescodedtogetherinoneframe.Longframeshavehighdelay,butaremore

efficientbecausetheheaderandsideinformationistransmittedlessfrequently.Longerframes

offerthepossibilitytousefilterbankswithbetterfrequencyresolution.Afundamental

principleinsignalprocessingisthatspectralspli

ttingfiltersmayhaveeithergoodtime

resolution,orgoodfrequencyresolution,butnotboth.Thismakessensewhenyouconsider

thatalongertimewindowmeansthattheanalyzerhasmorecompleteinformation,morefull

audiocycles,toworkwith

1

.

Inthecaseofrapidlychanginginputsignals(transients)longframesarepoorerthanshortones

becausethetimespreadwillleadtosocalledpre‐echoes.Forsuchsignals,thesizeoftheframe

shouldcorrespondtothetemporalresolutionofthehumanear.Thiscanbeachievedbyus

ing

shortframesorbychangingtheframelengthaccordingtotheimmediatecharacteristicsofthe

signal.

1

Perhaps this is the DSP designer’s equivalent to the economist’s TANSTAAFL: There

ain’t no such thing as a free lunch.

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