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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 125

• Usethemaximumbitrateyoucanaffordateachstage.

Harddiskrecordersandotherstudiosystemsoftenhaveanoptiontoadjust

this.Forverycriticalwork,rememberthatZephyrXstreamhasseveral

modeswhereamonoprogramissplitovertwodigitalnetworkchannels,

therebycuttingthecompressionratioby50%.

• Getth

eMPEG2AACandMPEG4AAC‐LDadvantageonlowbitratechannels.

• Whenusingdedicatedlines,considerusingV.35Singlemode(seeAppendix

6‐SpecialOperationalModes)at256or384kbpstoallow"headroom"for

othercodinginthechain.



Mixed MPEG Signal Chains

Whataboutthecasewher

eyouwillbeusingLayers2,3andAACinasignalchain?Itturnsout

thatthetwomethodsarenicelycomplimentary.

Atlowbit‐rates,AACorLayer‐3getsmoresignal‐to‐maskmarginthanLayer‐2.Thisiswhythey

performbetterinthelowbit‐ratetests.Itaccomplis

hesthisbyusingafilterbankwithmore

bands,2048vs576vs32.Oneeffectofthisis“timespread.”(Morefrequencyresolution

requiresalongertimewindow.It’salawofphysicsthing...)Forasmallnumberofpasses(one

ortwo),thisisgood,asth

eearhasmaskinginthetimedomainaswellasthefrequencydomain,

andLayer‐3naturallyexploitsthisadditionaldimension.Thedown‐sideisthatwhenmany

stagesofLayer‐3areusedatlowbit‐rates,thetimespreadcanbecomeaudible(softeningof

transientsandpre‐echoes,mostly),andthisisabadthing.W

hileLayer‐2doesnothavethis

problem,ithasanother.Sinceitisclosertotheedgeforsignal‐to‐noise,multiplegenerations

resultinunmasking(noiseandgrit,mostly).

ButtheISO/MPEGpeopledonotproposethatabu

nchofpassesofLayer‐3beused.Theideais

thatAACorLayer‐3beusedatISDN/SW56bit‐ratesforfieldpick‐upandthatLayer‐2beusedat

higherbit‐ratesinotherpartsofthesignalpath.

ThisiswhytheISOgroupdecidedtorecommendtheLayersasthe

ydid:AACfor64kbps/channel

andLayer‐2forequaltoorgreaterthan128kbps/monochannel.

Ourownexperimentswithcodeccascadingconfirmthatthisistherightapproach:thetwo

codingmethodsseemtocomplementeachother.TwopassesofLayer‐3(stereo@128kbp

s)

soundnoticeablybetterthantwoofLayer‐2;apassofLayer‐3followedbyapassofLayer‐2also

soundsbetterthantwoofLayer‐2.Moreover,we’vehadcustomerswhohaveusedapass,or

two,ofLayer‐3followedbySEDATwithoutevidentproblems.

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