DAB+ the additional audio codec in DAB

DAB+

The additional audio codec in DAB

2007 (Updated March 2008)

Contents

Why DAB+

Features of DAB+

Possible scenarios with DAB+

Comparison of DAB+ and DMB for radio services

Performance of DAB+

DAB+ roll out

Markus Prosch,WorldDMB Technical Committee Audio Codec Task Force Chairman. Markus Prosch is a research engineer at Fraunhofer IIS, Erlangen, Germany.

WorldDMB Forum Project Office30LeicesterSquare,London,WC2H7LA,UnitedKingdomTel:+442072884640Fax:+442072884643www.worlddab.org

Why DAB+?Inthedigitalage,broadcastingstandardsneedtobalancethebenefits

ofstabilityandinnovation.Stabilitygivesconfidencetobroadcasters,

manufacturersandconsumers.Andyet,enhancingastandardtotake

advantageoftechnologicalinnovationcanoffernewbenefitsandprotect

astandard’scompetitivenessinarapidlychangingmarketplace.The

internationalorganisationresponsiblefortheEureka147standards,

WorldDMB(formerlyknownastheWorldDABForum)hascarefully

monitoreddevelopmentsinaudiocoding.AlthoughWorldDMBremains

astrongadvocateofstability,itsSteeringBoardtookthedecisionthat

thebenefitsofaddinganadditionalhighlyefficientcodectoEureka147’s

optionsweresosignificantthatthisinnovationshouldbeincorporated.

The benefits of DAB+ include:• LatestMPEG-4audiocodecdeliversexceptionalperformanceefficiency

• Morestationscanbebroadcastonamultiplex

• Greaterstationchoiceforconsumers

• Moreefficientuseofradiospectrum

• Lowertransmissioncostsfordigitalstations

• NewreceiversbackwardscompatiblewithexistingMPEGAudio

LayerIIbroadcasts

• CurrentMPEGAudioLayerIIservicesandconsumersunaffected

• Compatiblewithexistingscrollingtextandmultimediaservices

• Robustaudiodelivery

• Optimisedforlivebroadcastradio

• Broadcasters/regulatorscanselecteitherstandardMPEGAudio

LayerII,ortheadditionalaudiocoding,orboth,tosuittheircountry

• Fastre-tuningresponsetime(lowzappingdelay)

• MPEGSurroundispossible

Untilnow,DABdigitalradiohasalwaysbeenbroadcastusingMPEGAudio

LayerIIcoding.IntheyearssincetheDABdigitalradiostandardwas

firstdefined,moreefficientcodingschemesandalgorithmshavebeen

devised.Theseallowaudiowithequivalentorbettersubjectivequalitytobe

broadcastatlowerbitrates.OtherbroadcasttechnologiessuchasDVB-H

(digitalvideobroadcastingforhandheld),DRM(DigitalRadioMondiale;i.e.

digitallong,mediumandshortwave)orMediaFLOusetheaudiocoding

MPEG-4HE-AACv2andareabletocarrymultipleaudioservicesinthe

digitalcapacityneededforasingleradioserviceusingMPEGAudioLayerII.

WorldDMBcreatedaTaskForceofitsTechnicalCommitteetodevelopthe

additionalstandard.Afterexaminingtheoptions,DAB+usingMPEG-4HE-

AACv2wasadopted.DAB+waspublishedinFebruary2007asETSITS

102563“DigitalAudioBroadcasting(DAB);TransportofAdvancedAudio

Coding(AAC)audio”.

Thesignificantlyincreasedefficiency,whichisdiscussedinmoredetaillater

inthisdocument,offersbenefitsforGovernmentsandRegulators(even

betterspectrumefficiency),broadcasters(lowercostsperstation)and

consumers(abiggerchoiceofstations).Itisdesignedtoprovidethesame

functionalityasthecurrentMPEGAudioLayerIIradioservicesincluding

servicefollowing(e.g.tothesameserviceonanotherDABensembleorits

FMsimulcast),trafficannouncements,PADmultimediadata(e.g.dynamic

labelssuchastitleartistinformationornewsheadlines;complementary

graphicsandimagesetc.).

InsomecountrieswhereDABdigitalradiohasalreadybeenlaunched,

broadcastersarecommittedtocontinuingtouseMPEGAudioLayerII.

However,incountriesplanningtolaunchdigitalradiotheargumentsin

favouroflaunchingwithDAB+arecompelling.

ToemphasisethatDAB+isintendedforradioservices(i.e.,withstreamed

audiobeingtheessentialcomponent),thefollowingtextusestheterm

“radioservice”insteadoftheDABterm“audioservice”.

Features of DAB+

DAB+usesMPEG-4HighEfficiencyAACv2profile(HE-AACv2).Thisaudio

codecisthemostefficientaudiocompressionschemeavailableworldwide.

Itcombinesthreetechnologies:

• ThecoreaudiocodecAAC(AdvancedAudioCoding).

• AbandwidthextensiontoolSBR(SpectralBandReplication),which

enhancesefficiencybyusingmostoftheavailablebitrateforthelower

frequencies(lowband)oftheaudiosignal.Thedecodergeneratesthe

higherfrequencies(highband)byusingthelowbandandsideinformation

providedbytheencoder.Thissideinformationneedsconsiderablylessbit

ratethanwouldberequiredtoencodethehighbandwiththecoreaudio

codec.

• Parametricstereo(PS):amonodown-mixandsideinformationis

encodedasopposedtoaconventionalstereosignal.Thedecoder

reconstructsthestereosignalfromthemonosignalusingthesideinformation.

HE-AACv2isasupersetoftheAACcorecodec.Thissupersetstructure

permitstouseplainAACforhighbitrates,AACandSBR(HE-AAC)for

mediumbitratesorAAC,SBRandPS(HE-AACv2)forlowbitrates.

ThereforeHE-AACv2providesthehighestlevelofflexibilityforthebroadcaster.

AdetaileddescriptionofHE-AACv2isavailableontheEBUwebsite1.An

introductiontoMPEG-4isavailableontheMPEGIndustryForumwebsite2.

HE-AACv2providesthesameperceivedaudioqualityataboutonethird

ofthesub-channelbitrateneededbyMPEGAudioLayerII.Thesame

audiocodingisalsousedinDRMandDMB.Devices,whichalsoinclude

DMBorDRMcanbenefitfromthefactthattheaudiocodingforallthese

technologiesisessentiallythesame.

AllthefunctionalityavailableforMPEGAudioLayerIIservicesisalso

availableforDAB+:servicefollowing(e.g.toFMorotherDABensembles),

trafficannouncements,PADmultimedia(dynamiclabelssuchastitleartist

informationornewsheadlines;stillimagessuchasweathercharts,images

andothermultimediacontent.),service

languageandprogrammetypeinformation

(e.g.ClassicalMusic,RockMusic,Sport)etc.

MPEGAudioLayerIIandHE-AACv2radio

servicescancoexist inoneensemble.

However, legacy receiversmight list

HE-AAC v2 radio serviceseven though

they will not be able todecode them.

Thegeographicalcoverageareaofradio

servicesusingHE-AACv2isslightlylarger

thanthatforradioservicesusingMPEG

AudioLayerII.Themultimediainformation

carriedinPADofanHE-AACv2radio

Other systems using AAC:

• iPOD

• DRM(DigitalRadio

Mondiale)

• 3GPP/3GPP2

• T-DMB

• S-DMB(Korea)

• MediaFLO

• ISDB(Integrated

ServicesDigital

Broadcasting;Japan)

• DVB-H

• XMSatelliteRadio

(USA)

• HDRadio(USA)

serviceismuchbetterprotectedagainsttransmissionerrorsthanPAD

dataofaradioserviceusingMPEGAudioLayerII.

AnimportantdesigncriterionforDAB+wasashort“zapping”delay.Both

thetimeittakestoswitchfromoneradioservicetoanotherstationonthe

sameDABensembleaswellasthetimeittakestotunetoaradioservice

onanotherDABensemblewasminimized.

CurrentlyallDABradioservicesaremonoorstereo.However,DAB+also

providesthemeanstobroadcastsurroundsoundinabackwards

compatibleway.UsingMPEGSurrounditispossibletobroadcasta

stereosignal togetherwith surroundside information (e.g.5kbpsside

information).Standardstereoradioswillignorethissideinformation

anddecode the stereosignal.MPEGSurround receiverswill evaluate

thesideinformationandreproducesurroundsound.Soatacomparatively

lowadditionalbitrate,thebroadcastercanincreasetheaudioexperience

onsurroundsoundreceivers,andstillprovidehighqualitysoundtoall

otherradios.

1 EBUTechreview:MPEG-4HE-AACv2–audiocodingfortoday’sdigitalmediaworld(2006) http://www.ebu.ch/en/technical/trev/trev_305-moser.pdf2AnMPEGIFWhitePaper:UnderstandingMPEG-4:Technologies,Advantages,andMarkets http://www.m4if.org/public/documents/vault/MPEG4WhitePaperV2a.zip

Possible scenarios with DAB+

ThefollowingfiguresshowhowthebitrateofaDABensemblemaybe

assignedto:

A40kbpssubchannelwithHE-AACv2providesasimilaraudioquality

(evenslightlybetterinmostcases)toMPEGAudioLayerIIat128kbps.

Comparison of DAB+ and DMB for radio servicesDMBisanaudio/videostandard,asopposedtoaradioplusmultimedia

standard,andsoisoptimisedforthatpurpose.Somefeaturesthatareimportant

forradioservicesarenotpossibleforDMBservices.DMBisdesignedand

optimisedformobiletelevision,anditisnotrecommendedforradioservices.

Multiplex with HE-AAC v2 (DAB+)

28 radio services using HE-AAC v2 at 40 kbps and 1 audio service using HE-AAC v2 at 32 kbps.

Multiplex with MPEG Audio Layer II (DAB)

9 radio services using MPEG Audio Layer II at 128 kbps

Multiplex with MPEG Audio Layer II and HE-AAC v2 (DAB and DAB+)

5radioservicesusingMPEGAudioLayerIIat128kbpsand12radioservicesusingHE-AACv2at40kbpsand1radioserviceusingHE-AACv2at32kbps

OneconsiderationisthatDMBismissingsomefeatureswhichareexpected

forradioservicese.g.servicefollowing,dynamictextlabels,etc.

DMBusesMPEGTransportStreamtocarrytheaudio,video(andpossibly

alsomultimedia)objectsandcontainsmechanismstosynchronizethemto

ensurethingslikelip-syncarecorrect.Thesemechanismsrequireadditional

overheadwhichcanbesignificantattherelativelylowbitratesusedfor

audioservices.Theadditionalbit rate required for thevideoobjectmust

alsobefactoredin.

Incontrast,DAB+isoptimisedforradioservices(includingPADinformation)

andthushasamuchloweroverhead(seefigure).

Anotherconsiderationisthetimetakenforareceivertoplaytheaudioafter

serviceselectionbytheuser.TheerrorprotectionschemeofDMBrequiresa

fixedamountofdatatobereceivedbeforedecodingmaybegin,andsothe

delayisinverselyproportionaltothebitrateoftheservice.Incontrast,the

errorprotectionschemeforDAB+isdesignedtohaveafixedtimedelay.At

therelativelylowbitratesusedforaudioservices,thetune-intimeforDAB+

ismuchquickerthanforDMB,althoughaDMBservicewithnormalframe

ratevideowillhaveahigherbitrateitwillalsohaveasimilartuningtimeto

DAB+.

AllthefunctionalityavailableforDABservicesisalsoavailableforDAB+,but

itisnotavailableforDMB(itisnotneededforamobiletelevisionservice):

servicefollowing(e.g.,toFM,AM,DRMoranotherDABradioservice),

trafficannouncements,PADmultimedia(dynamiclabelssuchastitleartist

informationornewsheadlines;stillimagessuchasweatherchartsorCD

covers,etc.),servicelanguageandprogrammetypeinformationetc.

Theresistancetotransmissionerrorsofbothsystemsissimilar,sincethey

arebasedoncomparableerrorcorrectionschemes,butthedetailsofthe

twoschemeshavebeenoptimisedforthetwofunctions.

Performance of DAB+

Duringthestandardisationprocess,fieldtestswereconductedintheUK

andAustralia.Theyshowedthatthegeographicalcoverageareaofradio

servicesusingHE-AACv2isslightlylargerthanthatforradioservicesusing

MPEGAudioLayerII.

AudioservicesusingHE-AACv2performedabout2-3dBbetteratthethreshold

ofaudibility.Thismeansthatinsomeareasclosetothecoverageareawhere

This figure shows a comparison of the relative overhead (used for error correction and transport protocol) of additional audio coding (assuming no PAD information) and DMB (PAD information not possible) in case of a radio service.

100

Subchannel bit rate in kbps

DMB:at least 25% overhead

additional audio coding about 10% overhead

90

80

70

60

50

40

30

20

10

0

8 24 40 56 72 88 104 120 136 152 168 184

additional audio coding

DMB

Relative overhead of additional audio coding and DMB

MPEGAudioLayerIIservicesalreadyshowedaudibleartefacts,HE-AACv2

radioservicesshowednoaudibleartefacts.TheerrorbehaviourofMPEGAudio

LayerIIisdifferenttothatofHE-AACv2.WithMPEGAudioLayerII,theweaker

theDABsignalgets,themoreaudibleartefactscanbeheard.

HE-AACv2producesnoaudibleartefacts,butwhenthesignalgetstooweak,

anincreasednumberofaudioframeswillbelostandthiscausesshortperiods

ofsilence(fade-outandfade-in).Testlistenerspreferredthiserrorbehaviour.

ComparedtoradioservicesusingMPEGAudioLayerII,radioservicesusing

HE-AACv2willfaillater(theycancopewithaslightlylowerDABsignalquality),

butthemarginfromerrorfreereceptiontolossofreceptionissmaller.

Todeterminetheaudioqualityatlowbitrates,listeningtestswereperformedby

theEBU(EuropeanBroadcastingUnion)in2003.Forstereophonicaudio,the

listeningtestsshowthatatanaudiobitrateof48kbps,HE-AACoffersgoodto

excellentquality,atanaudiobitrateof64kbpsitoffersexcellentquality.

Atthetimeofthesetests,HE-AACv2wasnotyetavailable.ThePS(parametric

stereo)tool,whichwasaddedaftertheEBUtestswerecompleted,significantly

increasestheperceivedaudioqualityatlowerbitrates.

Itshouldbenotedthatthebitratescitedfromtheselisteningtestsarepure

audiobitratesandnotDABsub-channelbitrates.Inordertocarryaudioina

DABmultiplexusingthenewspecificationa10%overheadshouldbetaken

intoaccount.

AudiocomparisontestsperformedinAustraliain2005confirmedthat

HE-AACv2providessimilarperceivedaudioqualityataboutonethirdof

thesub-channelbitrateneededbyMPEGAudioLayerII.

Broadcasterswillwanttomakefurthertestsbeforetakingoperational

decisionsabouttheappropriatebitrateforaparticularservice.

DAB+ Roll OutManycountriesaroundtheworldhaveacknowledgedthebenefitsofDAB+

fortheirindividualmarkets.TheAustraliangovernmenthasannounced

DAB+astheofficialdigitalradiostandardinthecountry,withacommercial

rolloutplannedforJanuary2009.OthercountriessuchasItaly,Germany,

Switzerland,theCzechRepublic,theNetherlands,Malta,Malaysiaand

ChinahavealsoexpressedtheirinterestinrollingoutcommercialDAB+

services.Testsandtrialsarebeingcarriedoutaroundtheworld(seethe

mapbelow).ManufacturersarealsoensuringDAB+’squickrolloutwithan

increasingnumberofDAB+receiversinthemarket.InmatureDABmarkets

manyreceiverswillbeDAB+upgradeable.

Countries with DAB and DAB+

DAB Commercial Launch DAB Commercial Launch andDAB+ Trial /TestDAB Trial /Test DAB+ Trial /Test

R U S S I A N F E D E R A T I O N

ME

XI

CO

I T A L Y

SOUTH AFR ICA

I N D I A

C H I N A

A U S T R A L I A

P E R U

S. KOREA

S P A I N

F R A N C E

GERMANY

UN I TEDK INGDOM

PORTUGAL

GREECE

SWEDEN

NORWAY

SWITZ.

REP. OFIRELAND

BELGIUM

LUX.

NETHER-LANDS

DENMARK

HUNGARY

CZECH REP .

KUWAIT

C A N A D A

U N I T E D S T A T E S

O F

A M E R I C A

NEWZEALAND

A L A S K A( U S ) ICELAND

MALTA

SINGAPORE

MALAYSIA

HONG KONG

CHINESETAIPEI

C A L I F O R N I A

I T A L Y

S P A I N

F R A N C E

GERMANY

UN I TE DK INGDO M

PORTUGAL

GREECE

SWEDEN

NORWAY

SWITZ.

REP. OFIRELAND

BELGIUM

LUX.

NETHER-LANDS

DENMARK

HUNGARY

CZECH REP .

ICELAND

MALTA