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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 118

AAC-LD (AAC Low Delay)

Whenannouncersusecodecsforbroadcastremoteapplications,theyoftenneedtohave

naturaltwo‐wayinteractionwithotherprogramparticipantslocatedbackatthestudio,or

callers.BecauseitisahottopicforengineersworkinginthefieldofInternettelephony,a

numberofstudieshavebeenconductedtodeter

mineuser’sreactionstodelaysintelephone

conversations.Thedataapplydirectlytotheapplicationofprofessionalcodecstoremotes,soit

isinterestingtotakeapeakovertheshoulderofthetelecomguystoseewhattheyhave

learned.

Forbroadcastremotes,wetrytoarrangeoursystemsotha

tthereisnopathforthefield

announcer’svoicetoreturntohis/herheadphones.Nevertheless,sometimesechois

unavoidable.Forexample,thiscanhappenwhenatelephonehybridhasleakageorwhena

studioannouncerhasopen‐airheadphonesturned‐uploudandtheaudiomakesitswayin

tothe

studiomicrophone.

Whenthereisnoecho,ithasbeendiscoveredthatanythinglessthan100msone‐waydelay

permitsnormalinteractionbetweenparticipants.Between100and250msisconsidered

“acceptable.”ITU‐TstandardG.114recommends150msasthemaximumfor“good”

interactivity.Echointroducesadifferentcase.Asyo

umightexpect,echoismoreorless

annoyingdependinguponboththelengthoftimeitisdelayedanditslevel.Telephone

researchershavemeasuredandquantifiedreactions,andITU‐TG.131reportsthefindingsand

makesrecommendations.

Summary of ITU-T G.13, with recommendations for designers of telephone systems that must cope

with echo. This shows Talker Echo Loudness Rating vs delay.



Therearecodecsusingotherthanperceptualtechnologiesthathavelowerdelay,buttheyare

notaspowerful.Thatis,foragivenbitrate,theydonotachievefidelityasgoodastheMPEG

systemswehavebeenexamining.ThecommonG.722isanexample.ItusesADPCM(Adaptive

DeltaPulseCo

deModulation),whichcanhavedelayaslowas10ms,butwithmuchpoorer

quality.Sothequestionarises:Isitpossibletohavehighqualityandlowdelayinthesame

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