The BBC™s ﬁ Digital Television...

In the last few months we�ve seen what canonly be described as a revolution in tele-vision broadcasting in the UK, and the BBCis proud to be a part of that change. What Iwant to do in the next twenty minutes is todescribe the new services that the BBC hascreated for Digital, some of the technicalarchitecture and installations that supportthem, and some of the enhancements thatare made possible by the new technology.

© BBC Research & Development

The BBC�sThe BBC�sDigital Television ServiceDigital Television Service

Where are we now?Where are we now?

Richard Marsden

BBC Research & Development

.Introduction

This is the script of a presentation

given by Richard Marsden

at the IEE on Tuesday 16 March 1999, as

part of a colloquium

�Digital Television:

where is it and where is it going?�

The BBC�s services will be present on allthree delivery platforms, satellite, terrestrialand cable. Our services have been availableon satellite since October last year and onterrestrial since last November. We areexpecting Digital Cable to launch later thisyear. Our services will look broadly similaron all three platforms, although there aretechnical and practical differences betweenthe platforms that we intend to exploit.

So what�s on?


BBC Digital Public Services

DSAT

DTT

DCABLE

October 1998

November 1998

1999

.The digital platforms

On BBC Digital, you�ll find BBC ONE andTWO, simulcast with analogue but inwidescreen, then two new channels createdspecially for digital, BBC CHOICE andNEWS24. There will soon be an enhancedinformation service � currently called BBCTEXT, also a new learning channel later thisyear, and BBC PARLIAMENT, whichlaunched just before Christmas. I�ll say alittle more about the new services later.

For DSAT we�ve taken two transponders onAstra 2A, each one giving us a capacity ofjust under 34 Mb/s. This lets us transmit thefour national regional variations ofBBC ONE � that�s England, NorthernIreland, Scotland and Wales, plus the samefor BBC CHOICE. We�ve squeezed PARLIAMENT in by taking a small slot onanother Astra transponder.


BBC Digital public services

BBC ONE

Simulcastin widescreen

BBC TWO

Simulcastinwidescreen

BBC CHOICE

in widescreen

BBC NEWS 24

in widescreen

BBC TEXT

(informationservices)

BBCLEARNING

BBCPARLIAMENT

.BBC Digital services

The satellite footprint covers the whole ofthe UK, but the signal can also be receivedin the nearer parts of mainland Europe. Thisspillover is unfortunate � we don�t haveprogramme rights to transmit outside theUK. So we have to encrypt the signal, withsmartcards issued only to UK addresses.


DSAT coverage

BBC National regionalvariations only

.DSAT coverage

For BBC ONE we have to go further � forsome programmes we only have the rightsfor one National region � so your smartcardwill only let you watch your local version ofBBC ONE.


DSAT coverage

BBC ONE region variants:

Encryption ensures that�out-of-region� viewing isnot possible

.DSAT national regional coverage

On DTT we have a single multiplex of about24 Mb/s. That doesn�t sound a lot comparedto satellite, until you realise that each trans-mitter could potentially radiate a differentmultiplex. So on DTT we can carry the10 English regions plus London, and thethree national regions. We�ve still to installthe equipment in the English regions � it�ll bein service by the end of the year, so for themoment England has just one service. We domake one economy to save bit rate � BBCParliament on DTT is an audio service only.

The fact that the BBC is both a regional anda national broadcaster has had a profoundinfluence on our technical architecture. Earlyon, there were two key decisions � compres-sion coders for regional contributions wouldbe installed in the regions, and commoncoding would be used wherever possible forall platforms.


DTT coverage

Full regionality (BBC)

lThree National RegionsrScotlandrWalesrNorthern Ireland

l11 English Regions

.DTT English regional coverage

These are the main signal flows betweenLondon, a national regional studio centre, anEnglish regional studio centre, the satelliteup-link site and the terrestrial transmitters.The underlying concept is that regional vari-ants are produced using sustaining feedsfrom London as a basis. The sustainingfeeds also carry the UK-wide services thatthe regions need to complete their DTTpackages, such as NEWS24 � because asyou can see, the feeds to the terrestrialtransmitters go straight out from the localstudio centres � they don�t go back toLondon. All the links between the studiocentres are MPEG-2 main profile at mainlevel, carried on ATM-based circuits.


The architecture -regional connections

London

English region

Nationalregion

sustaining feeds

sustaining feeds

DSAT contribution

.The architecture � national and regional interconnections

Looking at London in more detail � Londoncodes the sustaining feeds and the UK-wideservices, assembles its own service and theDSAT feeds. The video bit rate on eachservice is just over 5Mb/s, but the sustainingfeeds of BBC ONE and BBC CHOICE arecoded at a higher rate of about 8.7Mb/s.This is because inserting the regional varia-tions involves a decode/recode operation,and the extra bit rate allows us a qualitymargin in which to do this. Note that theregional contributions for DSAT need simplyto be multiplexed with the London materialto form the two transponder feeds. There�sno other processing of these contributions inLondon other than conditional accessencryption.


The architecture - London

routeingplayoutsuites

ATM adaptation

routeingand

multiplex

ConditionalAccess

encryptionMPEG2coding

andmultiplex

to regions

sustaining feeds 3 Nations DSAT

DSAT1

DSAT2

LondonRegionDTTUK-wide,

local

. The architecture � London

So, in a national regional centre, and theseare in Belfast, Cardiff and Glasgow, the highbit rate sustaining feeds of BBC ONE andCHOICE are decoded, the regional contentswitched in in a presentation suite, and thesignals recoded at the final emission rate. Ifyou�re wondering why we bother to code thesustaining feeds at all, the answer�s simple� money. It costs a lot less to get a 9 Mb/scircuit to Scotland than one at 270 Mb/s.The final process is to multiplex in theremaining services, which are already codedat emission rate, and that completes theDTT feed. And as I said, the regional contri-butions for DSAT are back-hauled to London.


The architecture - Nations

localsources

decodepresentation

suitecode

ATM adaptation

UK-wideDTT

services

Nationalcontributionfor DSAT

regionally optedservices

DTTmux

.The architecture � Nations

The English region is similar, but doesn�tcontribute to DSAT so there�s no return feedto London. It�s an even simpler arrangementat the moment because there aren�t anyEnglish regional variations on DTT. So wesend a feed of English DTT from Londonand pass it straight through. Well not quite.If we did that it would still carry the Londonidentification, and we�d need to geteveryone to rescan their set top boxes andIRDs when the regional service startedoperating. So to keep the service seamlesswe�ve already put the transmultiplexer ineach region to put in the right identifier.There�s another difference from the Nations.Even after the opting out facility is installed,and the high-bit rate sustaining feeds set up,the complete DTT sustaining feed will still besent. This seems inefficient � why send BBCONE at the high rate and as part of thecomplete multiplex? The reason is to protectthe service � if there is a fault in the Region,and English regions are not staffed 24hrs aday, the transmultiplexer can immediatelydrop the local output and revert to theLondon feed � no regional opt-outs until thefault is repaired, but at least a service.


decode

The architecture -English regions

localsources

ATMadaptation

presentationsuite

coderegionally-

optedservices

LondonDTT

multiplex

DTTmux

.The architecture � English regions

With digital comes the opportunity to developand enhance the services themselves.


BBC Public Service DTT

Enhancements:

l Better programme schedule information

l Digital Text

l Better subtitles

l Disability services

.Enhancements

Both the DTT and DSAT platforms carryprogramme schedule information. Thisallows the viewer to see schedules invarious forms on the screen and forexample to select a service directly withoutrecourse to channel numbers. Both systemscontain cross-carried information, giving theviewer access to schedules for all services,and indeed all service providers in hisregion, not just the one he is watching.



Enhancements:


l Digital Text

l Better subtitles


.Programme schedule information

On DTT, cross-carriage is achieved througha central schedule collator operated byONdigital. Schedules for all BBC servicesare created in London, and this informationis sent to the collator via TCP/IP links. Theregional nature of DTT means that SI (thatactually stands for Service Information,although schedule information is the bulk ofit) will vary across the country. This situationis further complicated because the serviceproviders don�t follow a common set ofregional boundaries, and some only operatenationally. The UK DTT transmitter networkhas therefore been divided into 29 SIPs � SIinsertion points. Within a SIP, all the trans-mitters carry the same combination ofregional services, both BBC and inde-pendent, or more strictly, need the samecombination of SI.


BBC Digital - schedule information

BSkyBschedule collator

programmes and schedules

schedules forinsertion

programmes

schedulesand

triggers

BBC national regionalschedule information

BBC scheduleand automation

systemLondon

BBC digitalplayout system

�SIPSI�at main

transmitters

Ondigitalcentral schedule

collator

collatedschedules

.Schedule information signal paths

continued ...

Each multiplex operator has chosen aslightly different arrangement for injecting SI,so this part of the diagram applies specifi-cally to the BBC. Within each SIP onetransmitter acts as the master, and receivesthe feed of collated SI in the form of a lowbit-rate transport stream. This is alreadypartly filtered for regionality at the output ofONdigital�s collator � there are eleven sepa-rate circuits feeding different parts of thecountry � and the final filtering and injectionof the relevant schedules then takes placeat the transmitter, in the box labelled �SIPSI�


� SI insertion point SI injector. An inde-pendent path also exists for the BBC�s ownSI, in that it can be injected in the BBCtransport stream at the originating studiocentre, although at the moment we�re notusing that facility.

Other transmitters in the same SIP can thenreceive the BBC transport stream from themaster site. There is no further transportstream processing, because the daughtersites can be detailed in the alternative


frequency list in the Network InformationTable.

Currently both systems display the BBC�sschedules using the published times as thereference. However real programme startand finish times often vary by a minute ortwo from the published times, and a lotmore if a live event over-runs. So we alsosend real-time triggers, so that �now andnext� displays can tally accurately with theprogramme on the screen.

... continued

DTT SI is assigned up to 500 kb/s in theBBC multiplex, and currently shows �nowand next� information only, using publishedtimes as the trigger � and this is a typicaldisplay from a domestic television fed froman ONdigital set top box.


.Schedules on DTT � screen shot

The DSAT schedule information system isoperated by BSkyB. As with DTT, the BBCschedules are sent via TCP/IP to a centraldatabase. However, the arrangements foradding the data to the BBC multiplexes aremuch simpler, because there are only twotransponder feeds and a single uplink site.The schedule data is sent back to TelevisionCentre and injected into the transponderfeeds before they leave for the uplink site.


BBC Digital - schedule information

BSkyBschedule collator

programmes and schedules

schedules forinsertion

programmes

schedulesand

triggers

BBC national regionalschedule information

BBC scheduleand automation

systemLondon

BBC digitalplayout system

�SIPSI�at main

transmitters

Ondigitalcentral schedule

collator

collatedschedules


A bit rate of 1Mb/s is reserved in each BBCDSAT multiplex for SI. The DSAT systemcurrently shows schedules up to 60 hours inadvance, and the �Now and next� displayuses the triggers from the BBC automationsystems. Again a typical display on adomestic television driven from a set top box.


.Schedules on DSAT � screen shot

I should add that the BBC�s services arealso represented in the Sky Guide.


.Sky Guide � screen shot

Digital text � first a few words about teletext� the DSAT system does support teletext.The BBC�s teletext service is carried in stan-dard DVB WST form in the transport streamfor BBC ONE, TWO, NEWS 24 andPARLIAMENT. It�s reconstituted by the set-top box and put back in the vertical blankinginterval of its UHF and Scart PAL outputs.This allows domestic television receivers todecode and display WST in the usual manner.

An enhanced text service is currently beingdeveloped by BSkyB, based on OpenTV.The BBC will also provide a text serviceusing this API.

However, UK DTT doesn�t support WST.What it does support is the open standardMHEG 5, and the BBC is developing anenhanced Digital Text service based on theUK-profile as defined in the D-book.



Enhancements:


l Digital Text

l Better subtitles


.Enhancements � Digital Text

We are already transmitting a limitedcarousel of pages, to assist manufacturersin developing decoders. If you want to findout more, we have a live demonstration, incollaboration with Sony, in the Faradayroom, here today. We call it digital text, but itwill also handle stills and graphics.


.Digital Text � screen shot

And on a related theme, subtitles. Workcommissioned by the RNIB, in consultationwith the ITC and the broadcasters, hasresulted in new highly readable font forsubtitles � Tiresias. The traditional coarsecharacters of teletext page 888 subtitles arereplaced by proportionally-spaced anti-aliased letterforms, transmitted asrun-length-encoded pixel data. This meansthey�re rendered at the coding end, not inthe set top box.



Enhancements:


l Digital Text

l Better subtitles


.DVB Subtitles

DVB Subtitles are now carried by many BBCDigital programmes, and are already avail-able to DTT viewers � and again, in theFaraday room, we have a demonstration.


.DVB Subtitles � screen shot

Disability services � the BBC is working withother organisations through the DigitalTechnology Group (DTG) to support theseservices, specifically an audio descriptioncomponent for visually-impaired viewers,and signing for hearing-impaired viewers.


Disability Services

l Audio description - an optional audio �side-channel� containing a commentary on the picture action

l Signing - British Sign Language - an on-screen animated figure?

Both may be implemented as CommonInterface Modules

.Disability services

continued ...

It�s early days, particularly for signing,there�s still some invention to do, althoughpossible mechanisms have been identified,initially involving the use of the commoninterface.


Disability Services

l Audio description - an optional audio �side-channel� containing a commentary on the picture action

l Signing - British Sign Language - an on-screen animated figure?

Both may be implemented as CommonInterface Modules

.Disability services

...continued

Digital also means a move to widescreendelivery, 16 × 9. But, not all material iswidescreen, and even if it�s been shot in1 by 9, the action may have been kepttowards the centre of the image to protectthe 4 by 3 viewer. And, after all, most peopleare still watching 4 by 3 televisions, even ifthey have digital set top boxes. This couldtake all day to explain, so I�ll go straight tothe message.

Whichever aspect ratio receiver is used bythe viewer, the BBC is keen to see that thedisplayed image is the most appropriate forthe material being transmitted, without theviewer necessarily needing to intervene. Ihope you�d agree this shot of an operawouldn�t look quite right cropped 4 by 3.There�s an aspect ratio flag in the MPEGheader, which we do implement. But what


.Widescreen programmes

continued ...

we really recommend is the Active FormatDescriptor, or AFD. This is additional infor-mation transmitted in the stream, telling thereceiver more about the way the frame wasfilled � is it unrestricted 16 by 9, were 14 × 9shoot and protect limits respected, and soon. This information, coupled with theviewer�s preferences should make itpossible always for the receiver to select thebest presentation.

And there�s another advantage. AFD�s canbe changed at the start of any frame. Aspectratio signalling can be changed only at whatare called GOP boundaries � every12 frames or nearly half a second in ourcurrent configuration. So eleven times out oftwelve, you�ll get a visible double-jumpunless you fade the picture to black throughthe change, and we think that�s presenta-tionally unacceptable.


.Widescreen programmes

... continued

Last topic � I mentioned earlier that we�relaunching a learning channel this year. It willeventually contain a substantial text andinteractive component. An immediate chal-lenge is the lack of spare capacity on ourexisting multiplexes, and the expense oftaking more.

We don�t currently use statistical multi-plexing � quality issues apart, there are twophysical restrictions. Unless you�re preparedto put in a very high speed link, the MPEGcoders in a stat mux ensemble must beco-located. Second, if a service is to beincluded in two multiplexes, it needs twocoders and the economy of common codingis lost. It�s not therefore easily compatible


BBC Digital public services

BBC ONE

Simulcastin widescreen

BBC TWO

Simulcastinwidescreen

BBC CHOICE

in widescreen

BBC NEWS 24

in widescreen

BBC TEXT

(informationservices)

BBCLEARNING

BBCPARLIAMENT

.The BBC Learning Channel

continued ...

with a broadcasting infrastructure like ours,where there is a mixture of local and centralcoding. However, on DSAT, it just sohappens that one of the transponderscarries only one service that isn�t coded inLondon � BBC ONE Northern Ireland as ithappens. So, we�re going to break awayfrom common coding. A second set ofcoding equipment is being acquired, to allowstatistical multiplexing of the London-codedservices on this transponder, and to get fiveservices where we currently have four. Forthe other platforms, I can only say at thistime that carriage arrangements are undernegotiation.


.The BBC Learning Channel

... continued

Finally can I remind you of our demonstra-tions in the Faraday room, DVB subtitlesand the digital text service. Both of theseare being received live off-air, from ourpublic digital transmissions from the CrystalPalace transmitter. I�d also like to thank theBBC for permission to give this presentation.


Demonstration -Live off-air

l DTT DVB subtitles

l DTT Digital Text service - in conjunction with Sony

The Faraday Room

12.30 - 14.00 (lunchtime)

15.00 - 15.20 (tea time)

16.45 - 17.30

.BBC demonstration

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