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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 119

codec?Untilrecently,theanswerwasno,butnewdevelopmentsincodecshavechangedthe

picture.Oneofthemainobjectivesinaudiocodingistoprovidethebesttradeoffbetween

qualityandbitrate.Ingeneral,thisgoalcanonlybeachievedatthecostofacertaincoding

delay.Codecsforvoicete

lephoneapplicationshaveuseADPCMandCELPbecausetheyhave

muchlowerdelaythanperceptualcodecs.Theseareoptimizedforvoiceandcanhave

reasonablygoodperformance.

ZephyrusershaveknownforyearsthatLayer‐3offersallthefidelityneededformostbroadcast

situations.However,theyalsoknowth

atthedelayofLayer‐3canbefrustrating,particularlyif

highfidelityisneededinbothdirectionsandpartiesatthetwoendsmustcarryona

conversation.

ThefolksatFraunhoferwereawareofthesefactors,andhavedevelopedanextensiontoAAC

called"AACLowDelay,"or"AAC‐LD"forshort.ACC‐LDoffersquali

tyequivalenttoLayer‐3with

lessthan25%ofthedelay!

AAC‐LDcombinestheadvantagesofperceptualcoders(suchasLayer‐3)withcertainprinciples

ofspeechcoders.Comparedtospeechcoders,AAC‐LDhandlesbothspeechandmusicwith

goodquality

.Unlikespeechcoders,however,audioqualityscalesupwithbitrate,and

transparentqualitycanbeachieved.AAC‐LD’scodingpowerisroughlythesameasLayer3,

meaningthatmonohighfidelity15kHzaudiomaybesentviaoneISDNchannel.WithISDN’s

twochannels,youachievenearCDquali

tystereo.

Delayinperceptualcodecsisdependentonseveralparameters:

• Framelength.Timeisrequiredtocollectallthesamplesforaframe.The

longertheframe,themorethedelay.

• Filterbankdelay.Thiscausesanadditionaldelayequivalentintimetothe

framedelay.

• Look‐aheaddelayforblockswitching.Laye

r‐3andAACusefilterbankswith

highfrequencyresolution.Forsignalswithhightonality,efficiencyishigh.

Butwhentherearetransients,adynamicswitchingprocesschangestoa

filterbankwithlowerfrequencyresolutionandbettertimeresolution.In

ordertocorrectlyde

cidewhentomakethischange,alookaheadprocessis

required,addingdelay.

• Bitreservoir.Thelengthofthisbufferdetermineshowmuchdelaythis

processcontributes.

Theoveralldelayisacombinationofallofthesecomponents,dividedbythesamplingrate.The

delayscaleslinearlyandinverselywiththesamplingf

requency.



HowAAC‐LDGetsitsLowDelay

AAC‐LDisbasedonthecoreAACwork,somuchissimilar,buteachofthecontributorstothe

delayhadtobeaddressedandmodified:

• Theframelengthisreducedto512or480samples,withthesamenu

mber

ofspectralcomponentsatthefilterbankoutput.

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