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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 124

h

HOT TIP!

Another characteristic of G.722’s statistical framing is that the decoder may remain locked

despite serious corruption of the data, although audio fidelity will degrade. In cases where

you are unable to get a good connection you may find this characteristic desirable, however

you will find that the fidelity will be substantially less under these circumstances.

CASCADING

Thissectionispreliminary,ascodercascadingisanactivefieldofinvestigationamongalgorithm

designers,standardsorganizations,andusers.Telosurgesyoutobewaryandtoletyourown

earsbethefinaljudgeuntilbetterinformationbecomesavailable.

Someofwhatwedoknow:

• SomeCCIRtestshavede

monstratedthatonepassofLayer‐3at56/64kbps

canbecascadedwith2‐5passesofLayer‐2operatingathigh

(112kbps+/channelmono;192kbps+JointStereo)bitrateswithgoodresults.

• InformaltestsattheTeloslabwithtwopassesofZephyrLayer‐3have

provensuccessful,withlistenersnoti

cingnoaudibledegradation,evenon

“difficult”CDs.

• TestswithAPT‐XfollowedbyoneortwopassesofLayer‐3followedbyAPT‐

Xprovedtobequiteacceptable.

• OneuserhasreportedthattwopassesofZephyrLayer‐3,followedbyone

passofSEDAT,isOK

.(Stereoprogrammode.)

• Thegoalistogetasmuch“codingheadroom”aspossibleateachstage.This

isachievedwhenyou:

a) Usethemostpossiblebitsateachstage,withtheleastpossible

compression(forexample,byloweringthesamplerate,andusing

64kbpsratherthan56kbpsconne

ction).Thiswhyweofferthe

mono128modes.and/or

b) Usethemorepowerfulcodingmethodofthoseavailableateach

stageXstreamoffersyoutheAACforthisreason.

c) Reducethenumberofcodedstagesinthechain.



Atthemoment,weofferthefollowingadvice:

• Usecodersonlywher

enecessary.Considerthealternativesateachstage.

Withthecostofharddiskcapacityfalling,isitreallynecessarytocrunchat

thatpoint?

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