User`s manual

8 Dolby
®
DMA8Plus Digital Media Adapter Installation and User’s Manual
Installation and Maintenance
Professionaldigitalaudioequipment typicallyusestheAES/EBUmethodbecause
balancedoperationyieldssuperiornoiseimmunity,asitdoeswithanalogaudiosignals,
andbecauseXLRconnectorsarethestandardonanalogprofessionalaudioequipment.
ProfessionalvideoequipmenttypicallyusestheAES‐3idvariationofthisinterface,with
BNCconnectors.As
withXLRconnectorsonprofessionalaudioequipment,theadoption
ofBNCconnectorsfortheaudioonprofessionalvideoequipmentstemsfromtheexisting
useforthevideosignal.Also,theunbalancedAES‐3idsignalcanconnecttomorethanone
pieceofequipmentbyusingtheloop‐throughconnectorsavailableon
somedevices.The
signalisrobust forlongcableruns.
2.2.1 Consumer Interface Standards for Digital Audio
TheconsumerinterfacestandardfordigitalaudioisS/PDIF(IEC61937).S/PDIFuses
coaxialunbalancedconnections(onesignalconductorplusshield)with acharacteristic
inputimpedanceof75ΩwithRCA(phono)connectors,orafiber‐opticcablewithToslink
TM

connectors.Theunbalancedcoaxialconnectionhasapeak‐to‐peaksignallevelof0.5V. The
typicalmaximumtransmissiondistanceis10meters(33feet).AlthoughS/PDIF‐specific
cableswithsuitableconnectorscanbepurchased,youcanalsoobtaingoodresultsusing
high‐quality75Ωvideocablewiththeappropriate
connectorsand/oradapters.
2.2.2 Cable Issues
Evenindigitalaudio,noise‐freesignalsareveryimportant.Thecableusedfordigital
signalsisspecifical lydesignedforsuchuse,althoughitlooksthesameasthecableused
foranalogaudioorvideosignals.Anyprofessionalaudioequipmentorbroadcastsupply
companycanprovide110Ωcablewith
connectors(orwithout,ifyou’dliketoterminate
themyourself)forAES/EBUconnections,andhigh‐quality75ΩvideocableswithBNC
connectorsforAES‐3idconnections.Useofcablesorconnectorsnotdesignedfordigital
transmissionorwithincorrectimpedancecompromisestheintegrityofthebitstream.This
canresultin
unreliablehardwareinterconnections,especiallywithlongcableruns.
2.2.3 Multiple Sources: Conversion Between Interface Standards
AlthoughsomedetailsofthebitstreamsusedintheAESandS/P DIFstandardsare
different,theaudioinformationisexactlythesame.Asaresult,mostaudioequipment
acceptseitherstandardwithnoneedtoconvertthebitstream itself;thisisthecasewith the
DMA8Plus.However,ifyouintendto
connectsourcesacrossdifferenttypesofdigital
audioinputs,donotattempttoconvertadigitalinterfacetypebydirectlywiringanXLR
connectortoaBNCorRCAplug.This causesanimpedancemismatchandsignal
reflections,resultingindigitalwaveformdegradation.Thismayappeartowork,butthe
resultsareunreliableanddropoutsoccur.
ForconversionbetweentheAES‐3idandS/PDIFformats,youcanusehigh‐quality
RCA‐to‐BNCadaptersbecausethecableandimpedanceareboththesame(75Ω).
ForconversionbetweentheAES/EBUandAES‐3idorAES/EBUandS/PDIFformats, a
simpleandeconomical
methodusesinlinetransformers.Thesedevicesperformthe
necessaryimpedanceandbalanced/unbalancedconversion.Table 2‐1showssome
examplesofsuitableadapters.TheunbalancedconnectorintheseexamplesisBNC.