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
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
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
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
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
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
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