Select Committee on Culture, Media and Sport Written Evidence


Memorandum submitted by TrinityStar Associates Ltd

INTRODUCTION

  Terrestrial television broadcasting, in a form we recognise today, started almost 60 years ago in the UK. In that time it has undergone one major transition, from black and white 405 lines to colour 625 lines, and is now part way through its second, from analogue to digital. Satellite broadcasting and cable television have also appeared on the scene and become significant. Any change to an existing broadcasting system will always be complex and potentially disruptive because of the large installed base of equipment. In any transition, it is always attractive to go for the very latest technology, however, viewers are used to a reliable rugged system and are not tolerant of their television misbehaving in the way that computers do with new software. For that reason the broadcast tends towards the tried and tested rather than the latest.

  The purpose of this submission is to look at the engineering issues behind analogue switchoff and to explain these in a way that makes the constraints and compromises that go into planning a system clear.

BACKGROUND

  Television broadcasting was restarted immediately post war with a black and white system using 405 lines on VHF Band I with a single programme from the BBC. Near universal coverage of the UK was achieved with around 50 stations. In 1955, ITV was born under the auspices of the Independent Television Authority. ITV also started in 405 black and white but on the much higher Band III channels, pushing transmission technology to the limit. This heralded the first "set top boxes" which were to enable Band I sets to watch Band III.

  Colour transmissions started in the late 60s using new spectrum on UHF bands IV and V. The spectrum use was planned and co-ordinated at an international conference in Stockholm in 1960 (a plan that we use today and is the basis for our digital plan going forward). Two things are important about this:

    —  Due to the propagation characteristics of UHF compared to VHF, over 1,000 stations would now be needed to give universal coverage.

    —  There was spectrum availability to allow full simulcast of the 405 and 625 line services, 405 line finally being closed down in the early 80s.

  At the time the UHF stations were built, provision was made for four UHF channels. This made the building of Channel Four in the early 80s more straightforward as the antennas, structures and combiners were all in place and the building programme was to add the transmitters. Channel 4 launched in November 1982 following the world's fastest ever rollout of a national television network.

  Within bands IV and V, 46 channels are available for transmission. The network, totalling some 1,150 sites, was rolled to provide a service down to communities of 200 people. This added up to 4,600 transmitters, which meant that each available channel was reused 100 times. Analogue transmissions are very susceptible to interference from other transmitters from a considerable distance so this represented quite a feat of planning.

  Research and development work on digital started in the 1980s in the Independent Broadcasting Authority R&D department and was continued post privatisation in 1990. It is interesting to note that the possibility of mutichannel digital terrestrial existed at the time Channel 5 launched and there was press comment at the time about launching with "yesterdays technology".

  The digital terrestrial system was demonstrated in the House of Commons in 1995 and paved the way, under the 1996 Broadcasting Act, for the planning and implementation of the present 80-station DTT system. At the time, the UK was the world leader in this technology.

THE DAWN OF DIGITAL TERRESTRIAL

  No more spectrum was made available for the new DTT system, six multiplexes had to be shoehorned into the existing channels. Furthermore the regulatory constraint was that there must not be any degradation of the analogue service. The best that could be achieved was a network using 80 stations using low power (81 were planned but co-ordination problems meant that the Channel Islands were never built). Furthermore, restrictions were placed on some of the transmitters in some directions to avoid interference, this resulted in some homes receiving a subset of the possible six multiplexes.

  The transmission technology is OFDM (Orthogonal Frequency Division Multiplex), which is, in effect, splitting the digital signal up into "comb" of carriers within the channel. Each element of the comb only carries low speed data and only a small part of the data resulting in a rugged signal. At the time receiver chipsets were available only for the 2K carrier variant (8K has now been adopted in other countries). One of the disadvantages of 2K is greater susceptibility to impulse noise.

  The system went live on 15 November 1998. The transmission network was delivered on time and within budget. Much was learnt in the first years; the interference to analogue was not as sever as feared do the DTT transmitter powers were increased improving coverage. As compression technology improved more channels were squeezed into the platform.

  The use of the platform and the marketing aspects are outside the scope of this submission but there is no doubt that the simple Freeview concept was the key success factor that has lead to DTT being a close second to Sky.

LOOKING TO THE FUTURE, PROJECT GENESIS

  Once the 80 station DTT system was on air a small group started thinking about the "all digital future" and how to get there. This lead to a project commissioned by the ITC called Project Genesis. In the first phase we developed a number of scenarios, looking to beyond analogue switchoff to form a view of the end game. A broad range of views was taken from across the industry and then, in the second phase, some of the scenarios were developed in great detail. It was something of a first bringing together a regulatory, strategic, engineering and planning view to build a picture of the future.

  The co-authors of this report,[102] published in 1999, were Dave Brian, Brian Tait and myself with input from Smith Consulting. The Genesis report forms the blueprint for the future of digital television in the UK and many of its concepts have now found their way into EU thinking. It is a tragedy that Dave was later killed in a road accident, perhaps this submission will stand as a tribute to his fine work.

  The next stage was to look at how to get from where we are now to the all-digital world. In an ideal world we would build out digital to match analogue coverage and then withdraw analogue when digital take-up had reached an acceptable level. The snag was that the existing broadcast spectrum was tightly packed and no extra was available, on the contrary the changeover entailed releasing 14 of the 46 channels.

  The conundrum was that we wanted to build digital in order to be able to close analogue but could not build digital without closing analogue. From early 2003 a colleague, Brian Haseler, and I turned our minds to this. The only solutions being offered in the industry at the time were hugely expensive and technically unworkable, involving multiple changes of transmitter frequency at each site. Between us we developed the concept of closing down one analogue channel at a transmitter site and immediately replacing it with a high power digital multiplex that contained the analogue services being withdrawn. From a viewer perspective, they would lose an analogue channel but would get the service back by getting a digital box (if they had not already done so). After a period of time the remaining analogue services would be withdrawn. From a resource perspective this would break the project down into manageable chunks and phase the activity over time. The plan rapidly became known as "the ntl plan". It had its first public airing at the Digital Action Plan (DAP)Stakeholders Group on 18 February 2004 when Brian and I presented it to the industry. There was widespread support for the principle but no one was going to volunteer to be the first off at that stage.

  The plan was adopted by the DAP and work started on the way in which the changeover should be tackled across the country. A sensible basis was the existing television regions. It rapidly became clear that the order would be determined by engineering constraints as there was only a certain order to do the changeover that avoided one region wiping out existing coverage in an adjacent, yet to be converted, region. The country divided in to two blocks, within those blocks the order was fixed but the order of the two blocks could be swapped. Early work on this by Brian and myself was taken into and developed by the DAP Regional Rollout Group. The North/West block was chosen to go first as it had the lowest risk of being changed in the process of co-ordinating channels with France/Belgium/Holland.

  All this work has now transferred to DigitalUK and forms the basis of very detailed implementation plans.

ENGINEERING ISSUES AND CONSTRAINTS

System choices

  The overarching specification for DTT allows numbers of choices so that the system can be engineered to match the needs of the user. One choice has been mentioned above, that between 2K and 8K carriers. The UK has decided to transition to 8K which will improve performance and avoid being trapped in a 2K system forever. All but the very early DTT receivers cope seamlessly with both standards.

  Other choices involve balancing capacity against ruggedness against transmitter power. There is a choice between modulation schemes referred to as 16QAM and 64QAM. 16QAM represents a more rugged signal against 64QAM having 20% more capacity. The difference in ruggedness can be offset by an increase in transmitter power. Currently both are in use.

  Ofcom[103] consulted on this and other options in February 2005 and concluded that 64QAK would be used. This was in conjunction with the DTT power being at -7dB relative to analogue (one fifth the power) at most sites and at -4dB at some. This would result in a served population of 98.5%.

Coverage and frequency planning

  The actual coverage of a transmitter is determined by three factors:

    —  Distance from the transmitter.

    —  Interference from other transmitters.

    —  Terrain blocking the signal.

  From the above, it is clear why there are few transmitters in East Anglia and lots in South Wales. Although distance is ultimately a limiting factor, in practise the other two are far more significant.

  As things stand today, there is a stable situation with a known population of transmitters in the UK and surrounding countries. Similarly the interference from the UK out and other countries inwards is understood and allowed for. As mentioned above, this is all based on the 1960 Stockholm plan. One possibility would be to start with a clean sheet of paper and totally re-plan for digital, however this would be a mammoth task. What has sensibly been accepted is the concept of "conversions". This was agreed at an international planners conference at Chester and allows an exiting Stockholm 60 analogue allocation to transition to digital. This eases the frequency planning and co-ordination task considerably but it is still complex. The mechanism for co-ordination is explained more fully in the next section.

  The UK frequency plan should deliver 98.5% coverage, however, there is a little more to it than this. Expressed in full the plan will deliver coverage to an acceptable level of all three public service multiplexes to 98.5% population for greater than 99% of the time based on a standard receiving aerial at 10m height. The 99% time reflects the fact that incoming interference changes with weather conditions and very occasionally the weather may give rise to increased interference, (this is the same for analogue). The levels of interference are "worst case" based on known plans of our EU neighbours and the commercial broadcasters rolling out to their full allocation of 200 sites. It may well be that the interference is less, in which case this number served will rise.

  Similarly the remaining 1.5% are not simply "have nots". Some of the 1.5% will get all three multiplexes but at a quality that the planners regard as below standard. In most cases this is recoverable by a better aerial installation. There will be a subset that will get one or two multiplexes and a few that get none.

  It is not possible to say where those few will be other than they will not be concentrated on a particular geographical area and will be scattered house or very small communities.

  As a very general statement, anyone getting a passable analogue signal now should be able to get digital. Using the same planning tool, analogue delivers 98.5%. The note of caution is that the two 98.5%s are not necessarily the same. The differences (which will be very small) arise from inevitable small changes in transmitter antenna pattern when they are re-engineered and different levels of incoming interference from the Continent.

Regional Radio Conference

  The Regional Radio Conference 2006 will put in place a plan to replace the Stockholm 1960 plan. Each country has now submitted its plans and discussions between neighbouring countries have been taking place to mesh their individual plans. For the UK, Ofcom leads this activity with support from the BBC and the two transmission operators, National Grid Wireless and Arqiva.

  The UK, in line with most, of its neighbours, has put in a plan for eight multiplexes (described in the planning as "layers"). Six of these represent the six licences UK multiplexes and the remaining two have been planned in the 14 channels of released spectrum. One of the two new ones is of particular interest to the broadcasters as it is mainly a conversion channel rather than a new allocation and so is eminently suitable for broadcast applications.

  All being well the RRC should be a rubber-stamping exercise but there remains some risk of change to the plan along the South and East coast up to the conference.

Extent of rollout

  In 1993 there was strong feeling in the industry and government that DTT would only roll out to 200 stations and that satellite would be the mechanism to deliver service to 8-10% of the population (5-6 million people). Detailed analysis coupled with developments in small transmitters reversed this sentiment and the Digital Replacement Licences issued by Ofcom in December 1994 contained an obligation to build out to the full 1,154 sites (except the commercial operators who are limited to 200 by spectrum availability).

  From the transmission point of view, the project to build the DTT network will cost circa £500 million of which 20% is for stations 201 to 1,154. These sites vary from quite significant ones covering a large town down to a telephone box sized building covering perhaps 200 people. In total these sites cover 4 million people, which works out to an average of £25/head for conversion (this cost is borne by the transmission company/broadcaster). The tiny sites represent the worst case, if we take the smallest covering 200 people, the conversion cost is around £15,000 which represents £75/head.

  There is a cost should conversion not take place as many of the smaller sites are only used for broadcast and, if this ceases; they will have to be returned to a Greenfield site. The cost to do this is comparable with the conversion cost.

The changeover plan

  The principles behind the changeover plan have already been outlined. This section provides more detail and explains more of the thinking behind the plan.

  The task of changeover is dominated by the work that needs to be done at the main stations; the work at the small relay stations is largely a box swap. It is for this reason that the four-year changeover period has remained even though the number of stations considered has gone from 200 to the full 1,154.

  At the main stations, the existing analogue transmitters and low power DTT transmitters have to be removed and replaced by high power DTT. In two cases the structure needs replacing and in many others strengthening. The antenna at the top of the mast needs upgrading or total replacement. In many cases, electrical switchboards and generators need replacing. Most of the analogue equipment is near end of life and due for replacement.

  Of these tasks the most critical is the structure/antenna work as this can only be done in the summertime.

  A typical project plan for, say, a sports stadium, would have a number of milestones along the way and then a contingency period at the end before it was required for use. This project is unusual in that there are firm milestones representing the changeover date for each region (this would be publicised well in advance and are therefore firm). Contingency at the end of the project is therefore no use. The solution in this case is to build contingency at the beginning by starting the antenna work two years in advance of the time it is needed. The project will stand one or two bad summers before serious slippage occurs.

  The project is divided down to a main transmitter and its relays. Two years before changeover the antenna work will be done. Around six months before changeover the new transmitters will be installed and thoroughly tested. On the changeover night, the BBC2 analogue transmitter will be closed down and a high power multiplex brought up on that channel. Over the next 24 hours all the relays will be visited and converted. One month later the remaining analogue transmitters will close and the other DTT transmitters will go to their new channel and high power and the relay stations completed.

  From a transmission point of view the month is about right, shorter than two weeks would leave insufficient preparation time. Longer than one month would mean diverting teams to another site and then bringing them back.

  This sounds like a great deal to be done from the viewer perspective in a short space of time but it is important to note that the existing sites reach 75% so it is only those on the edge of main station coverage and those in the relay areas who will not have DTT until the first switch.

The Future

  There is no doubt that DTT has become a successful platform, the sales of receivers and the price recently paid for the extra capacity on the platform are proof of that. The platform is now full and there are unlikely to be improvements in the MPEG2 compression technology that will allow much more to be squeezed in.

  The domestic trend towards larger screen sizes will raise awareness and desire for high definition. HD is bandwidth hungry and the present system could not cope with it. There are two (not mutually exclusive choices):

    —  Provide more spectrum.

    —  Move to a more efficient coding system.

  A simple strategy would be to allow one or two more multiplexes for broadcast. Using MPEG2, two HD channels could be put onto a multiplex. A better strategy would be to code the HD signals in MPEG4. Domestic equipment that could cope would receive the HD signals; the remainder would not.

  A better strategy still would be to consider how to migrate the whole platform to MPEG4. At present all consumer equipment for DTT is MPEG2 and suggesting that this should be thrown away is inconceivable. It is probable that boxes will soon be dual-standard so over time the problem will disappear with the natural replacement cycle. The implication of this is to simulcast in MPEG2 and MPEG4 for a period of years. Again this will need more capacity than the current six multiplexes. With seven or eight it would be possible to transmit some HD as well as simulcast some existing channels. Fortunately this has no implications for the transmitter network, which will handle both.

  Ofcom have just launched a consultation into the use of the released spectrum, hopefully it will conclude that some should be used for broadcasting.

CONCLUSION

  This submission has set out to give some background to where we are now, explain some of the issues and give some insight into the engineering and technical aspects of the transmission system. Opinions expressed are those of the author and do not represent any organisation other than TrintyStar.

17 November 2006







102   Genesis Project, Independent Television Commission, February 2000. Back

103   Ofcom, Planning Options for Digital Switchover, February 2005. Back


 
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