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CBCTECHNOLOGYREVIEWJanuary2007 1/12TheDos&DontsofTransportStreamLevelNetwork

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THEDOS&DONTSOFTRANSPORTSTREAMLEVELNETWORKINGGuyBouchardSeniorManager,NewBroadcastTechnologies

ABSTRACTMPEG transport streams are making their way intobroadcast and cable plants. A transport

stream isadifferentvehiclethatcancarrymultipleprograms inasinglecableorcarrier.This

constitutesanewvehicle forprogrammingmaterial,and thisnewvehicle isopeningupnew

possibilities.However,ithasitsownsetofrules,itsownenforcementtools,anditsownissues.

Thispaperwillprovidethereaderwithenoughinformationtobringhimorheruptospeedon

theMPEGtransportvehicle.

INTRODUCTIONBroadcastershaveusedseveraldifferentvehiclestocarrytheircontents.Theveryfirstonewas

composite video. Component analog video was introduced in the 80s, with limited success.

Component digital (SDI) was widely adopted by the broadcast community. All of these

uncompressedvideo transportvehicleshad theirownsetofrules (e.g.,onevideoperstream,

video equalization, maximum distance, minimum return loss requirement, and embedded

audiosupport).

TheMPEG transportstream is thevehicle for compressed television signals. It isopening up

new opportunities: reducedbandwidth, multiple services, drop & insert, and others. It also

comeswithnewrules,includingMPEGcomplianceandtwodifferentsyntaxes.

This article reviews a few MPEG transport concepts, then answers some frequently asked

questionsaboutthesubject.

I Januar 2007 www.cbc.radiocanada.ca

CBCTECHNOLOGYREVIEW


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THEABCsOFMPEGTRANSPORT1. SystemLayers

LiketheOSImodelusedinthecomputernetworking industry,theMPEGcompressed

signaliscarriedoverseverallayers:

Thephysical

layer

(cable,

modulation

and

coding).

Thetransportlayer.

Theapplicationlayer(DVB,ATSC)

Mosttransportstreamsarecarriedonthefollowingphysicallayer:

SMPTE310

o SMPTE310 relies on a coax transmission systembased on a synchronous

feedat19.39Mb/s.

DVBASI

o DVBASI relies on a coax transmission based on a subset of the SDI

specifications. The signal is always transmitted at 270 Mb/s. However, a

cleverstuffingprotocolpermitstransmissionfrom1to214Mb/s.

IP

o IPreliesmostlyonunshielded twistedpair (UTPorCat5)cable.Transport

streamsareencapsulatedin1388bytepacketscarriedmostlyasUDPtraffic.

2. TransportStream

Atransportstream(TSorTP)isaformatspecifiedinMPEG2Part1,Systems(ISO/IEC

standard138181).Itsdesigngoalistoallowmultiplexingofdigitalvideoandaudioand

tosynchronize

the

output.

SERIALFORMATMPEGdatamustbeserialized inorder tobe transmissibleovercableorRF.Theserialization

processiscarriedoutaccordingtoastrictprotocolbasedonpacketbasedmultiplexing.

Theencodermustfitalltheprogramelementsintothetransportstream:

Audio

Video

Data


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UnlikeIP,theMPEGtransportstreamreliesonfixedlengthpackets.The lengthofastandard

MPEGpacketis188bytes.

Thebesthumanscalemodelforadatapacketisatraincar.

It carries a certain payload. In terms of data, it can be

expressed as the number ofbytes it carries. In the specific

case

of

MPEG,

the

payload

is

188

bytes.

All

bytes

look

the

same,soaheaderhastobeaddedtothepacketsothatthe

demultiplexercanknowwhatthedatapacketscontentsare

orwhatitsdestinationis.

Payload

He

ader

188 Bytes

Figure1 Transport

3. ProgramElementaryStream

Apacketizedelementarystream(PES)isthuscreated

=

Figure2 Packetizedelementarystream(PES)

Theprogramcomponentsare:

Video,includingclosedcaptioning

Audio

Data

Theelementscanbedifferentiatedinthestream,evenifallpacketslookthesame,viaahighly

flexiblesetofindextablescalledMPEGtables.

MPEGTABLESTheMPEGtablesareasfollows:

Theprogramassociationtable(PAT),which isthemotherofalltables.Itgivestheequipment

thatreadstransportstreamsthefollowinginformation:

ThetransportstreamID(anumberuniquetoeachtransportstreamthat

identifiesitsorigin).

Howmanyservicesarepresentinthestream.

Thelocation(PIDnumber)ofeachserviceindextable(PMTs).


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Theprogrammaptable(PMT),whichisanindextablethattellstheequipmentthatreadsthe

transportstreamthelocation(PIDnumbers)ofeachservicecomponent:

Video

Audio(s)

Data

DEFINITIONOFAPIDAPID,orprogramidentifier,isanumber(13bitinteger)locatedinthetransportpacketheader.

The PID is used to index MPEG packets. Like IP routers, the MPEG TS handling equipment

doesnothavetoreadthe188bytepacketsintheirentirety.ItreliessolelyonthePIDnumberto

decideifagivenpacketisrequiredandwhereitistoberoutedto.

ThePIDnumberislocatedinthetransportstreamheader.

Payload (Variable)Header32 bits

AdaptationField

(Variable)

Sync Byte 8Bits 0x47

Flags(3 bits)

PID (13 bits)Flags(4 bits)

ContinuityCounter (4

bits)

32 bits

188 bytes

Figure3 TypicalMPEGpacket

HANDLINGOFMPEGTABLES

Assoon

as

MPEG

handling

equipment

is

connected

to

aTS,

the

acquiring

equipment

looks

for

theprogramassociationtable(PAT).Ithasnoproblemfindingit,sincethePAThasafixedPID

assignment(PID#0).

ThePATwillprovidethelocationsofalltheprogramservicesavailableonthisprogramstream

bycallingtheirprogrammaptables(PMTs).


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Figure4 TypicalMPEGtable

Inthisexample,thePATpointstothreePMTs,butonlyoneisdisplayed.

PAT (0)

PMT(30)

Audio (34) VBI (37) Data(38)Video(31)CC

PCR(31)

MPEG Structure

PMT(40)


PCR(41)

PMT(50)


PCR(51)

Service 3

Service 4

Service 5

Figure5 TypicalMPEGtransportstreamtreediagram

Figure 5 shows a typical tree diagram for a multiprogram transport stream. Once the MPEG

handling equipment has acquired all of these tables, it is ready to route the packets to the

respectivehardwarethatdecodesthesignal.Dependingonwhichservicestheuserhasselected,

differentpacketswillberoutedtotheaudioandvideodecoders.Mostoftheotherpacketswill

bediscarded.


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AllOtherPID's

41

48

VideoBuffer

VideoDecoder

VideoAssembler

AudioBuffer

AudioDecoder

44

1 FFF +Indirect EPG

Engine

Junk Yard

Service 4 selected:

Figure6 DemultiplexedMPEGtransportstream

MPEGDESCRIPTORSServiceDescriptors

Once the PES reaches the audio or video decoder, additional information is needed for

instructingthedecoderastotheschemethatwasoriginallyusedtoencodethesignal.

Typicalcodingstandards:

o Video(MPEG1,MPEG2,AVC,etc.)

o Audio(MPEGlayer1,DolbyAC3,etc.)

Thisinformationiscarriedonan8bitnumbercalledaservicedescriptor.Notethatthevalues

areexpressedinhexadecimalformat.

Service Descriptor

MPEG2video 01h

MPEGlayer2audio 03h

DolbyAC3 81h

Figure7 MPEGservicedescriptors


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LANGUAGEDESCRIPTORSFurthermore, as several audio services in several languages maybe present on the PES, the

decoder needs tobe instructed as to the language carried on each video service. Language

descriptorswereestablishedforindexingaudioservices.Thelanguageselectionismadeusing

a

worldwide

standard,

ISO

639,

that

defines

most

current

languages.

It

assigns

a

2

letter

code

to

mostcurrentlanguages.ExamplesoflanguagedescriptorsareprovidedinFigure8.

Language ISO639

Descriptor

English en

French Fr

German de

Finnish fi

Polish Pl

Portuguese Pt

Russian ru

Figure8MPEGLanguagedescriptors

STREAMCAPACITYEvery MPEG transport stream has a given data rate allocation that is normally based on

hardwarelimits.Forinstance,anATSCtransmitterislimitedto19.392658Mb/s.Mostterrestrial

networksaresoldinincrementsof20Mb/s,e.g.,20,40or60Mb/s.However,ifweaddupall

thestreamscomponents(audio+video+data),itsnominaldataratewillnotquitebereached.

Theleftover

payload

will

be

filled

with

packets

packed

with

zeros,

or

null

packets.

In

order

to

berecognizedassuchbyanyequipmentinthechain,allnullpacketsmustbecarriedonPID#

1FFFh.

The difference between the TS nominal rate and its useful payload is referred to as the

headroom.Forexample,a50Mb/sstreamonwhich45Mb/sisoccupiedwithaudio+video+

dataisreferredtoasaTSwith5Mb/sor10%headroom.

STREAMSYNTAXIntheapplicationlayer,thesetofrulesthatappliestoatransportstreamanditscomponentsis

calledasyntax.Therearethreemainsyntaxesinuse:

MPEG basiclevel(PAT,PMT,etc.).

ATSC,forterrestrialtransmissiongNorthAmerica(allMPEGsyntax+

navigationalinformationbasedatPID1ffB[PSIP]).

DVB,forsatelliteandterrestrialnetworking(allMPEGsyntax+navigational

informationbasedatPID16h[DVBSI]).


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FAQsCanunwantedprogramsbefilteredoutfromamultiprogramtransportstream?Tofilteroutunwantedprograms,thestreamgroomingequipmentmust:

ModifythePATsothattheunwantedelementsarenotreferredtointhetable.

Discardthe

unwanted

PIDs

(all

service

elements

+PMT).

PAT (0)

PMT(30)


PCR(31)

MPEG Structure

PMT(40)


PCR(41)

PMT(50)


PCR(51)

Service 3

Service 4

Service 5

Program Association Table (PAT)

Table_id 0x00

Transport_stream_id 0x001234

PMT PIDProgram number

40h4

30h3

50h5

Figure9ModifiedMPEGstream(basedontheexampleprovidedinFigure5)

Can incoming services from twodifferentTSsbemerged together into a single transportstream?It is possible to merge operations. An MPEG multiplexer is required for this. The following

tasks

must

be

performed:

EnsurethatnotwoserviceorPIDnumbersareidentical(e.g.,twoservice#2sor

twoPID#44s).Iftwoofthemareidentical,themultiplexerwillrenameoneof

them.

Checkthatthetotalpayloadofthenewexpandedstreamdoesnotexceedthe

maximumallowableoutputrate.

ModifythePATsothatalltheservicesarerepresented.

PassalltherequiredPIDs.


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PMT(50)


PCR(51)

Service 5

PAT

Figure10 Singleprogramtransportstreamtobeadded

PAT (0)

PMT(30)

Audio (34) VBI (37) Data(38)Video(31)

CC

PCR(31)

MPEG Structure

PMT(40)


CC

PCR(41)

PMT(50)


CC

PCR(51)

Service 3

Service 4

Service 5

PMT(60)

Audio(64)

VBI (67) Data(68)Video(61

)CC

PCR(61)

Service 6

PMT PIDProgramnumber

40h4

30h3

50h5

Program Association TablePAT

Table _id 0x00

Transport _ stream_id 0x 001234

60h6

Figure11 Augmentedmultiprogramtransportstream

Canservice1berenamedbygivingitanotherservicenumber(forexample,5)?Service remapping (in MPEG jargon) is a common feature of most MPEG multiplexers. Themultiplexer simply reformats the PAT and PMTs accordingly.


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Furthermore, can a PID number be renamed?

Yes.anyPIDcanberenamed,aslongasnoconflictiscreated.However,somePIDnumbersare

tobe avoidedbecause they mightbe used in the next service layer (ATSC, DVB). The PID

numbers1FFBhand16hare reserved fornavigational information.1FFFh isreserved fornull

packetsandwillbediscardedbyanyequipment,regardlessofwhatthesignaliscarrying.

CanMPEGdescriptorsbealtered?Changing service or language descriptors is not advisable. For example, an audio service

running with Dolby Ac3 compression is normally carried on a PID holding the service

descriptor 81h. If the descriptor is changed to 3h, which means MPEG layer 2 audio, it will

becomenondecodable.

CanPIDsthatarenotdefinedinanyoftheMPEGtablesbetransportedinanMPEGTS?The

carriage

of

ghost

PIDs

(another

piece

of

MPEG

jargon;

it

is

used

to

describe

orphan

PIDs)

is

adelicateoperation.AmultiplexercanpassanyghostPIDaslongas:

ItisdeclaredintheMUXghostPIDwindows.

ItdoesnotcreateaconflictwithanyofthePIDSpresentintheTS.

Caution: When a ghost PID is allowed to pass, the MUX loses control over its size. The user

mustbe absolutely certain that an excessive amount of data will not flow through the PID,

unlesssufficientheadroompresentintheTS.

GhostPIDscanbereincarnateddownstream in thebroadcastchain foruse in theapplication

layer.

CanaTSbecarriedoverdifferentphysicallayers?Transportstreamtranslationispossible,aslongasyoustaywithintheintrinsicphysicallayer

bandwidthlimits.

The SMPTE310 physical layer is limited to 19.392658 Mb/s. Obviously, a 30Mb/s DVBASI

stream cannotbe translated into SMPTE310, unless services are dropped until the limit is

reached.

ASIhasabandwidthlimitofabout214Mb/s.

TheEthernetphysicallayerhaslimitsofitsown.ItisbasedonMPEGtolerancetopacketjitter.

Thislimitvariesaccordingtothelinkdesign.


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Can DVB-ASI be distributed in a broadcast plant using the existing SDI routing

infrastructure?

TheDVBASIandSDIsharethesameelectrical layer(270Mb/scoax,NRZcoding).However,

theircodinglayersaredifferent.Thus,DVBASIispolaritysensitive,whereasSDIisnot.Some

videodistributionamplifiersareknown to feature 4noninvertingoutputs+4 invertingones.

NoninvertingoutputsshouldbechosentocarryDVBASI.

CanaTSbecarriedoveranEthernetelectricallayerbecarriedalongwithotherITtraffic?Yes,but appropriate care mustbe taken, since the MPEG transport stream carried on the

Ethernetdoesnothaveamanagementlayer,andnoqualityofservice(QOS)isappliedtoit.

SuccessfulimplementationhasusedsomeoftheexistingITmanagementtools,suchasVLAN.

AlthoughMPEGhasworkedreliablyoverundersubscribedlinks,itsperformanceisverylikely

todegradeinthepresenceofthepacketjittertypicalofabusyITtransportinfrastructure.

Howcanthetransportstreamcomplianceofanygivensyntaxbeassessed?A stream analyzer will give you stream conformity information based on two reference

documents:

ETR290,forDVBbasedstreams

ATSCA78,forATSCbasedstreams

Bothdocumentswill indicate thealarm thatyouneed todealwith immediately, theoneyou

willneedtohandleshortly,andtheonethatdoesnotreallymatterinthenearterm.

CanaTSbetransferredfromonegivensyntaxtoanother?Syntax transfers arepossible,but theyareadelicateoperationbecauseeachsyntax (ATSCor

DVB) uses fixed PID numbers for carrying its navigational information index table.

Navigational information can get lost in the syntax translation process. Ghost PIDs will

inevitably be created. A special MPEG setup may have to be created to preserve this

information if the signal has tobe reincarnated into the original syntax. This is ajob for an

MPEGexpert.

Canbroadcasters

interconnect

with

a

BDU

(cablecasters

and

satellite

operators)

via

a

transportstream?Yes.Infact,thisisthepreferredformatformostBDUs.However,afewoperationalrulesneed

tobefollowed:

Changesinprogramnumbers,PIDnumbers,etc.requireadvancednoticewith

theBDU.

Streamcompliancehastobemaintainedatalltimes.

Thenominalrateofthetransportstreammustremainconstantatalltimes.


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CBCTECHNOLOGYREVIEWJanuary2007 12/12Th D & D t f T t St L l N t k

GuyBouchardistheseniormanagerofNewBroadcastTechnologies

(NBT) at CBC/RadioCanada. He is responsible for digital delivery

technology studies and projects within CBC Technology.During his

22 years with the CBC, Guy has worked in analog and digital

television

transmission

and

production

systems,

as

well

as

in

satellite

andterrestrialmicrowavecommunicationsystems.Heholdsadegree

intelecommunicationsfromtheUniversitduQubecandisaboardmemberoftheInstituteof

ElectricalandElectronicsEngineers(IEEE)BroadcastTechnologySociety.Guyhasbeenserving

thebroadcastindustrysince1979withaspecialinterestindigitalcommunicationssystems.He

haswritten and delivered papers on DTV for the IEEE, Canadian Digital Television (CDTV),

andtheSocietyofMotionPictureandTelevisionEngineers(SMPTE).

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