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by Brian Aird


The birth of digital television in the UK last Autumn, has started a process that will bring multi-channel television within the reach of virtually every household. Indeed, for many who already have terrestrial reception, adding the ONdigital terrestrial service will need nothing more than an appropriate set top box. ONdigital is the major digital terrestrial broadcaster in the UK whose shareholders, Carlton Communications and Granada Group both have a 50% interest. The service roll-out in November last year was initially set back by a lack of set top boxes, but in all other respects the launch preparations have been excellent and the launch itself remarkable by being on schedule and without serious technical problems.

The new terrestrial services have to operate in a UHF spectrum that is already crowded with existing analogue television services. The fact that it has done so with remarkably few problems is a tribute to the robustness of the COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation system, used for DAB and DVB-T. COFDM, which uses relatively low power transmitters, is able to resist both interference from and to analogue transmissions and is effectively immune to the problems of multipath reception.

ONdigital was successful in obtaining three of the four multiplexes advertised by the ITC, known as multiplexes B, C, and D. Some eighteen channels are available by subscription, and nine are currently free to air from the BBC and ITVchannels 4 and 5. The remaining advertised multiplex, A, went to SDN, and the two ‘gifted’ multiplexes, referred to as 1 and 2 by the ITC, went to the BBC and Digital 3&4 respectively.

In order to provide the wide range of channels that are required to appeal to all types of viewers, ONdigital depends heavily on the 20% increase in efficiency brought by statistical multiplexing. In fact if statistical multiplexing was not used the current program structure would be greatly reduced. Another consequence is that only a joint venture with another operator, where bandwidth is shared, would allow pay-per-view in addition to the current bouquet. This is exactly what ONdigital have done in a joint venture with SDN, where ten major sporting events and recent film releases will be available on PPV each night on top of existing sport and movie channels.

Up to 70% of UK households can already receive the digital terrestrial service, and this will rise to at least 90% by the end of 1999 as more transmitters come on-line. The ITC are working on ways to extend coverage to match the 99.4% of the population provided by analogue services. S4C who broadcast on analogue terrestrial (and now digital terrestrial via SDN), have had to make their service available on Astra to achieve adequate coverage in the hills and valleys of Wales, despite numerous analogue relay stations. However, the ITC recently published research into a number of innovative ways to get DTT to the areas it does not yet reach and the prospects for implementing these methods look promising.

The Digital Television Group are operating an innovative dealer web based resource which includes a predicted reception tool that requires only a post code to reveal if a potential customer should be able to receive ONdigital. Subsidised set top boxes are only sold to those who qualify for good reception, as ONdigital are obliged to make the service work for their subscribers.

At present, some areas do not receive the service or only some of the multiplexes are available, this situation will improve as transmitter roll-out completes. However, as predicted by frequency planners, terrestrial transmission in the UK is characterised by interference limited coverage. In cases where aerials are installed properly, most marginal reception occurs where signal strength is only a few dBs above the digital ‘cliff effect’, and impulsive interference from sources such as badly suppressed vehicles cause the clever DVB error correction strategy to tip over and fail. This tends to cause blocky artefacts, freezing, or simply black screens. Heavy rain has been known to cause similar problems with digital satellite reception.

Until analogue transmission is switched off it will be very difficult to achieve coverage comparable to the 99.4% of analogue. One reason is that in many areas the digital transmitters have to be ‘tuned’ to avoid interference with analogue transmitters. This can create ‘nulls’ where digital terrestrial cannot be received. This explains the reports on the internet of people being refused a subsidised receiver, when someone a only few blocks away can get good reception.

The DTG encourage dealers to ensure installations have adequate headroom to avoid the ‘cliff effect’ and ONdigital engineers will provide assistance to subscribers who experience problems, and may even supply new aerials or help to align existing aerials.

The ONdigital Project

The work that led up to the faultless November launch from the ONdigital MPEG multiplex centre at Marco Polo House in London was the result of the combined effort of a number of specialist technical teams. Canal + provided conditional access, the set top box operating system, integration and Centralised System Information (CSI). DiviCom supplied the compression and multiplexing systems, whilst Castle Transmission and British Telecom provided distribution and transmission.

The ONdigital technical team included Operations Director, John Egan; Chief Engineer, Chris Hibbert and Senior Engineer, Meirion Hughes, who had between them the task of system specification. Bob Gentry, Operations Manager, joined the company later with responsibility for operational staff recruitment and training.

Other specialists contracted to build the required infrastructure included, integrators Electronic Media Systems who designed and built most of the broadcast facilities, Mike Tooms Associates and Cooper Systems who assisted with the design and specification of the end-to-end MPEG encoding and re-multiplexing system and the specialist companies responsible for plant installation and the design of building services. Most installation work was supervised by building management company MACE, who took responsibility for overall project management.

The suppliers for most items of equipment employed in the system were selected as a result of a competitive tendering process managed by EMS. This helped ensure that the installed equipment not only met a series of technical specifications drawn up jointly by ONdigital and EMS as cost effectively as possible, but that projected delivery times accorded with ONdigital’s tight time scales for completion. EMS selected the base band equipment in consultation with EMS, whilst ONdigital had it's own direct contracts for all remaining equipment.

The task of providing a distribution system was awarded to Castle Transmission International, who in turn subcontracted the task of establishing an optical fibre SDH network between the 81 transmitting, stations and Marco Polo house to British Telecom. CTI also handle transmission for the BBC, whilst NTL were awarded transmission contracts for the digital networks of SDN and Digital 3&4. CTI and NTL operate a site sharing agreement at most of the 81 sites. BT also supplied fibre routes between the program suppliers and ONdigital.

Fault tolerance and redundancy are important in any transmission environment and they feature strongly throughout the ONdigital project. For example there are two diversity feeds of ONdigital’s output to BT in addition to a dedicated tertiary feed to Crystal Palace for the estimated 10 million potential audience in the London area.

The power for the complex is supported by external generators, a UPS system with some 25 tonnes of batteries on the roof and even a socket on the side of the building to allow a mobile generator to be hooked up. ONdigital is certainly not taking any chances with shareholders investment!

The air conditioning is particularly well designed with pairs of equipment racks provided with main and backup driver units, and glass doors ensure that an optimum air-flow can be maintained without restricting access. There are even water run-off trays above each row of racks to prevent flood water from leaking roofs or broken plumbing from affecting transmission. This may seem over cautious, but it does stem from real life water damage that Chris and Meirion have experienced at other stations.

All program material entering ONdigital do so as SDI signals with embedded audio on fibre, and although direct MPEG contribution is a theoretical possibility, it is not practised. As is well known, compression works best with original material that is as free from noise as possible and the onus is always on suppliers to provide the highest quality SDI copies they can. ONdigital work closely with all program suppliers to ensure that compression driven quality guide lines are met. There is a playout facility capable of driving one channel which is currently used for the India/Pakistan cricket on First ONdigital.

Synchronised program inputs are switched through Philips routers to the encoders under the control of the Canal+ designed Digital Broadcast Manager. A small presentation suite can be routed into this path to provide transitions and a means of creating the appropriate interface for some sports and planned pay-per-view events. Snell&Wilcox Prefix pre-processors can also be inserted prior to encoding where program sources require advanced noise reduction.

The heart of the installation is the DiviCom encoding and re-multiplexing system, this uses the MV40 encoder and the MN20 statistical re-multiplexers in an n+1 redundancy configuration for the MV40’s and 1+1 for the MN20’s for all three multiplexes under the control of a main and backup System Controller from DiviCom. Canal + provide a Digital Broadcast Controller, which is linked to the Subscriber Authorisation System (SAS) and controls Conditional Access (CA) for all multiplexes.

DiviCom, a wholly owned subsidiary of C-Cube Microsystems, won the ‘Editors Pick of the Show’ award at NAB `98 for their dual pass or ‘lookahead’ encoder technology. The MV40 encoder has three codec ‘engines’ two of which are used at ONdigital to implement statistical multiplexing together with the MN20 re-multiplexer. During the first pass, the first MV40 codec analyses the source material. This information is then used in the second pass under the control of the MV20 statistical re-multiplexer via a feedback path, to produce the best result possible encoding operation having regard to the needs of the total multiplex. This technique allows more program material to be processed in a multiplex by varying the bit rates dynamically at ONdigital, between 1 and 8 Mbit/s. The total data rate on any one multiplex is 26.1818 Mbit/s including Reed Solomon error correction.

The DiviCom encoders and statistical re-multiplexers bring together, encoded and encrypted MPEG-2 transport streams with other packetised streams such as DVB subtitles for onward delivery to BT termination equipment. BT receives seven dual diversity feeds of each mulitplex for the regions plus additional, redundant feeds of each multiplex for Crystal Palace making 45 ASI feeds in total.

The chosen output format, ASI, has a similar format to serial digital but required special handling from both Snell&Wilcox, who supplied distribution and interfacing equipment, and EMS who selected appropriate cable. It was found that the ASI format does not have the spectral shaping and polarity insensitivity of serial digital, and suffers greater losses in long cable runs. EMS were obliged to go back to a good quality analogue cable to avoid signal degradation for all ASI runs.

The Subscriber Authorisation System inserts Entitlement Management Messages into the bit stream to update STBs according to client payments. However, the SAS only keeps records of payments against customer ID numbers, but no other client details are kept on these servers. Full client details are kept only at the Customer Management Systems at the call centres. This strategy allows for multiple call centres, but avoids the possibility of call centres touting for business to individuals.

The SECA (Société Européenne de Contrôl d’Accès) conditional access system known as MediaGuard supplied by Canal+ Technologies is used instead of the NDS CA system used by SKY digital. However, DiviCom are due to update their MPEG Media Toolkit at ONdigital to handle simulcrypting, (that is both MediaGuard and NDS CA systems) as this is an ITC requirement. However, it is considered unlikely that subsidised STBs will be capable of handling multiple CA systems. Integrated Digital TVs are more likely to have the required Common Interface slots to take CA modules for simulcrypt operation.

In order to simplify the handling of SI data, ONdigital offered to act as a central SI manager for all multiplex operators. This has removed the need to insert SI information at individual transmitter sites, which would have been a formidable administrative task. The SI data is collated at Marco Polo House by the CSI system, and data is fed to the twenty nine Service Insertion Points, (SIPs) which feed the 81 sites. There is a separate feed of SI data for SDN which uses satellite for distribution. The centralised SI system was designed and implemented by Canal+ Technologies.

Canal+ Technologies also supplied the MediaHighway interactive software which was revised to incorporate the MHEG-5 graphic interpreter adopted by the Digital Terrestrial Group (DTG) as the UK’s standard for digital terrestrial TV.

The main network monitoring mirrors the basic signal flow, with 24 quad split VGA Barco monitors arranged either side of a 40 inch true widescreen plasma display. The first six quad split monitors at the left of the monitor stack are dedicated to incoming programs, the next six act as pre-encoder monitors whilst on the right of the widescreen plasma a further six monitors can be switched between the output of the ASI multiplexes, via professional IRDs, or the output of domestic STBs fed via an RF feed from Crystal Palace the main London Transmitter site.

Each quad split monitor can carry descriptive text under each video tile, however it was felt that the amount of text required would have cluttered the display. The solution which EMS provided was to add UMD displays above and below each monitor. This has had the added benefit that audio level/presence indicators were easily incorporated into each UMD.

The remainder of the monitors are used mainly for dedicated off-air monitoring of London transmissions of all digital multiplexes. This is done so that ONdigital will be alerted to any problem that might result in a call to their customer management centre in Plymouth. It was felt that the ability to respond to callers reporting a problem with any of the digital terrestrial services would only serve to enhance the performance of the call centres in the eyes of the public. ONdigital rely on CTI for monitoring the remaining regions.

A dedicated Quality Control area monitors selected programs from the mux and can be used to make quality decisions about the material transmitted. However, the bit rate for each particular program within any multiplex is determined automatically by the DiviCom statistical re-multiplexers.

There is also a small presentation desk with a Philips mixer and Collage CG to provide facilities for ONdigital programming such as First ONdigital. There are also plans to build graphics and edit facilities in the future to support planned interactive programming.

Many of the technologies used by the new venture are very new, and ONdigital have had no choice but to build the infrastructure and most operational procedures from scratch, although working parties did visit Canal+ whose consultants worked along side the ONdigital project team. The multiplex centre has been likened to a ‘warehouse’ for video programming and much of the infrastructure has more similarity with a complex data warehouse than with a conventional broadcaster. For this reason most staff have been recruited from the IT world, since it was expected that IT people would learn broadcasting more easily than many broadcast engineers might learn adequate IT skills.

One need only look at the multitude of MPEG and Transport Stream analysis equipment to gauge the engineering complexity at the heart of the new industry that ONdigital represents. Although every effort has been made to simplify the error reports that originate from the DiviCom management systems and the Transport Stream analysers. The ideal of a simple icon managed system that only provides operators with need to know information has been a concern of ONdigital senior engineer, Meirion Hughes right at the outset. Meirion who was involved in setting up the infrastructure at Carlton, envisioned an icon driven integrated alarm and status reporting system that could provide summary information for all critical paths and major items of equipment within the entire installation.

It may come as no surprise that whilst there is a centralised building management system, the total integration of all electronic sub-systems is yet to be realised. However, the DiviCom system does have an integrated management system that allows operators to assess the health of the entire encoding and re-multiplexing system from user friendly GUI driven software. The top level screen shows a simplified schematic of the system and by clicking on icons it is possible to drill down to view the status of individual components. Any fault conditions or alarms change the colours of the icons alerting operators to search for details so that appropriate action can be taken.

Divicom Open View interface


Work is also in progress at Snell&Wilcox to design a computer system for all the interface and distribution equipment at ONdigital, that will allow operators to view entire signal paths using an icon based interface. This will be an extension of the Snell&Wilcox RollCall system that allows modules to communicate status information to a central computer display. The new tool will allow engineers to see at a glance if a module is reporting problems and it will provide a visual location and even give the module locations by equipment rack and bay number, allowing almost instant remedial action. The facilities are also supported by a well equipped laboratory with state of the art MPEG analysis equipment from a range of well know suppliers such as Tektronix, Adherent and Snell&Wilcox. The lab is split into two parts, one area is dedicated to the testing of set top boxes where STB suppliers are invited to bring their new products to test compatibility with ONdigital transmissions. A second area contains the DiviCom management computers and a data logging system that might look more at home in a bank.

The EMC Symmetrix data logger uses DLT linear tape transports to provide ONdigital with 10 days worth of transmission recording for three multiplexes complete with error correction and the CSI data. The architecture makes use of ‘data movers’ that take RAID buffered ASI data in chunks into the tape database so that a totally self contained 10 day archive is created. There is no demand for operators to change tapes, unless copies are required. The system allows any service within any multiplex to be re-played for later analysis, provided it lies within the 10 day window.

The data logger allows ONdigital to analyse most of the days transmission either as part of routine checking or in response to any received complaints. It should be realised that complaints may arise not just from picture or audio artefacts as a result of excessive compression or poor program quality, but the service tables that are transmitted along with the data may have been corrupted so that some services could not be selected from the ONdigital set top box menu. The data logger will allow ONdigital to verify such complaints and perhaps discover the source of the problem.


There is no doubt that the ONdigital project team have led from the front, with both new technology and a new business model. ONdigital will be most interesting to watch over the next year or so as subscriptions and transmitter roll-out to see if actual results match the forecasts that have been made. Telecommunications and Broadcasting Services have published a report summarising their findings on the UK digital television service including results of tests carried out in the London and South East area. Initial measurements indicate that the DTT system performs up to 3-4dB above its theoretical performance. The general conclusion was that the ‘economically accessible coverage’ achievable by UK DTT is now likely to be up to 95% by the Millennium. The report went on to claim that DTT coverage could not be cost effectively matched by other systems. ONdigital released its first quarter subscription figures at the beginning of April, revealing 110,000 new customers, over and above the expectations of the City. The additional news that a number of manufacturers will be bringing integrated digital televisions onto the market in June, with prices starting as low as £500, may help demonstrate that the business was off to the right start. This is clearly only the beginning for ONdigital.


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