Article - Issue 37, December 2008
The demand for HD TV
Dr David Harrison
More than one-third of all television sets in the UK now use the free-to-view digital terrestrial television system. Freeview currently delivers over 40 conventional standard definition services not only to conventional TVs but also to a rapidly growing number of sets that can show even sharper high definition pictures, or HD TV. The demand for HD signals is increasing rapidly. Dr David Harrison, Head of Broadcast and New Media Technology at Ofcom, outlines how the UK is planning to meet this demand without disrupting existing services. He also describes how Ofcom’s engineers are creating space for HD channels by using compression technologies and how they streamline transmission services.
The really big challenge of bringing high-definition television to most of the UK via the digital terrestrial system is to squeeze more TV signals into the finite TV part of the broadcasting spectrum. This spectrum simply cannot be increased in size, so for a long time we have been looking for
ways to squeeze more material into the existing space.
Now that ‘HD Ready’ TV sets are selling strongly there’s a driver to power this work forward. People buy big-screen TVs and put them in their living rooms and see for the first time the comparative ‘graininess’ of existing 625-line terrestrial digital images – the two HD formats use 720 lines and 1080 display lines. These viewers want HD TV signals via their Freeview terrestrial digital system.
The problem is that these HD signals convey much more information than standard definition TV in each new television picture frame – and both require typically up to five times more transmission capacity. This massive increase in capacity has proved the main barrier to delivering these services on today’s digital terrestrial platform, Freeview.
Digital terrestrial tv today
UK engineers played a central role in the early development of digital television. Ten years ago they created the world’s first digital terrestrial platform, which has acted as the springboard for similar developments worldwide. The Freeview platform handles digital terrestrial services broadcast in the ultra-high frequency (UHF) band, or 470 to 850 MHz, using signals from more than 1,000 elevated high-power and medium-power transmission sites.
These transmissions can span large distances and cause interference to other television services operating in other parts of the UK and in neighbouring countries. To manage this interference, the broadcast frequencies are integrated into an internationally co-ordinated plan. Working within the constraints of this plan are six digital terrestrial channels in operation in the UK. Each of these digital channels delivers a combination of digital television programmes, radio services and interactive service and electronic programme data, all bunched together into what is known as a digital multiplex.
These six digital channel multiplexes are broadcast using a standard developed in the 1990s by the European Digital Video Broadcast group. This standard allows a trade-off of capacity against coverage by changing the amount of information broadcast on a multiplex. If more information is broadcast the receiver needs to have a stronger signal to better differentiate between the information bits that are now placed closer together, generally requiring the receiver to be closer to the transmitter.
To prevent interference with analogue services, the digital multiplexes operate at differing levels of reduced power before digital switchover. To optimise the level of digital coverage and capacity, four of these multiplexes currently operate with a lower information rate, that has 16 individual modulation states and delivers 18 Mb/s of capacity, while two operate with 64modulation states and deliver 24 Mb/s of capacity. Switching off the analogue transmissions will allow the multiplexes to switch to higher capacity 64 QAM and provide an increase in platform capacity of 24 Mb/s.
To maximise the number of digital television services that can be delivered in each multiplex, the TV pictures are processed to reduce their data requirements. Typically, one frame in a TV picture is very similar to the previous frame, broadcast one twenty-fifth of a second earlier. So rather than sending all the details of each frame, we can make savings by sending only the differences between frames. Other techniques including filtering out the least noticeable picture details and can also be used to further reduce data requirements.
Together these techniques are known as ‘digital compression’ and are widely used in audio, still pictures and moving pictures. It is this compression that allows digital broadcasting to carry more channels than analogue broadcasting – around eight times as many.
The compression currently in use for digital terrestrial is known as the MPEG-2 standard, developed by the international Motion Pictures Expert Group in the 1990s. HD, however, requires around five times as much capacity as SD, potentially negating many of the advantages gained by moving to digital. So the big challenge to delivering HD on terrestrial was to find ways both to reduce the data requirements for HD and to increase the capacity of the terrestrial channel to accommodate enough HD channels to make viewing attractive – without reducing the number of existing
SD channels available.
Maintaining SD is a key requirement. HD TV will be available to those people who want it. It will not be foisted on everyone who owns a TV. We are committed to ensuring that standard definition terrestrial digital services – with which many viewers are perfectly happy – are unaffected by our technical advances.
Step one: increasing the capacity
Our first step was to revisit the way in which multiple different TV channels were combined together to form the multiplex. The major achievement here has been to recommend that broadcasters adopt a standard which includes an improved process to correct errors that occur between transmission from the mast and reception at the viewer’s TV, generally caused by noise and interference from other services. Error correction is widely used in most digital systems, including CDs and computer data transfer, and many different error correction schemes are known. They tend to trade computational requirements against the overhead they impose. But in the years since terrestrial TV was first standardised (the DVB-T format) more powerful processing chips now allow better error-correction schemes to be deployed. They can remove the same amount of errors for a lower overhead.
This enabled the DVB-T format to be upgraded to DVB-T2 with new error correction systems that provide excellent performance in the presence of high noise levels and interference. A new technique called ‘rotated constellations’ is also used to provide additional signal robustness in difficult propagation channels. DVB-T2 is compatible with the existing digital terrestrial transmitter infrastructure and only requires a simple change of coding and modulator equipment at the main transmitter sites. However, it is not compatible with existing digital receivers in consumer equipment – a point we will return to later.
Moving from DVB-T to DVB-T2 provides up to 45% more capacity. But this alone is not enough to generate sufficient capacity for HD broadcasting.
Step two: reducing data requirements
In the same way that increased processing power can improve error correction, it can also improve compression by enabling transmission of video of the same quality at a lower data rate. For example, by recognising that a sequence of frames are nearly identical except for one or two objects moving from frame to frame, only the first frame, the location, and movement vector of the objects need be sent. The receiver can then reassemble the image. Improvements to this process requires more computing power both at the transmitter and receiver, but since the standardisation of MPEG-2 this has steadily become available, resulting in the MPEG-4 standard.
MPEG-4 improves on MPEG-2 by modifying the size of the blocks into which it divides the picture for processing purposes according to the content and encodes the movement vectors more efficiently. The result is a reduction in the bit rate required to deliver standard definition and high definition services by up to 50% compared with MPEG-2. MPEG-4 is four times more computationally demanding than MPEG-2 but can now be easily supported in the latest generation of high-performance household TV receivers.
Taken together, the use of DVB-T2 and MPEG-4 can provide an increase in effective platform capacity of more than 150% – enough to deliver high definition and other new services. The main obstacle to using these new technologies is that they are not compatible with existing receivers.
Step 3: ensuring compatibility
The only way to retain compatibility is to keep most of the multiplexes as they are so that they can continue to be received by existing TV sets. By a careful reshuffling of programmes, we have managed to relocate the services carried on one of the digital multiplexes to the other remaining five DVB-T MPEG-2 multiplexes. This means we can convert the cleared multiplex to DVB-T2 with MPEG 4 compression. Consumers with existing (DVB-T MPEG-2) receivers are then able to continue to receive the services on the five multiplexes, and those purchasing new multi-standard DVB-T/T2 MPEG-2/4 high definition receivers are able to receive both existing services and new high definition services delivered via the re-engineered multiplex using DVB-T2 and MPEG-4.
This approach will only work, however, if sufficient capacity exists on the five remaining DVB-T MPEG-2 multiplexes to accommodate the displaced services from the sixth multiplex. Digital switchover holds the key to achieving this because when analogue transmissions are turned off, so too does the need for the digital terrestrial transmissions to run at low power to avoid interference. This means that their power levels can be increased. This in turn will enable the higher information rate option to be adopted, providing an additional 24 Mb/s of multiplex capacity – enough capacity to accommodate the displaced services.
Digital switchover sets hd timetable
As described in Ingenia 31 the digital switchover is an enormous engineering challenge, involving the upgrade of antenna masts and transmission equipment at more than 1,000 UK sites. Many are situated in remote locations where engineering work cannot easily proceed during the winter months. So digital switchover is being delivered on a region-by-region basis across a five-year span. This process will start in earnest later this year in the Border television region, following a successful pilot conversion to all-digital reception of 25,000 homes in Copeland in 2007.
The process of clearing and converting a digital multiplex to DVB-T2 MPEG-4 operation, as digital switchover begins across the country, has now been agreed in consultation with key stakeholders. Broadcasters, transmission infrastructure providers, consumer groups and manufacturers are all on board. Our approach will be implemented first in the Granada television region in late 2009, when the first DVB-T2 MPEG-4 high definition receivers are expected to become available in the market. It will then be implemented throughout the whole of the UK, based on the regional digital switchover timetable.
Creating a virtuous circle
We recognise that this initiative will only succeed if we create a virtuous circle in which attractive services are made available using DVB-T2 and MPEG-4, which drives sales of compatible receivers, which in turn brings an increase in the number of services broadcast using these standards and which itself then increases demand for new HD receivers.
To kick-start this process, the capacity available on the cleared DVB-T2 MPEG-4 multiplex will be initially divided into three slots, each of which can deliver one HD channel. One of these slots has been pre-assigned to the BBC, the operator of the cleared multiplex. The two remaining slots will be allocated by Ofcom to broadcasters proposing services judged to provide a compelling and attractive consumer proposition. With ongoing improvements in MPEG-4 compression efficiency, which will be compatible with the first generation of MPEG-4 receivers, we anticipate a fourth slot will be made available before 2012, and potentially as early as 2010, capable of delivering a further high definition service.
Pilot DVB-T2 MPEG-4 transmissions will begin later this year for two main reasons: to confirm the performance improvements provided by these standards and to provide an early test platform for prototype transmission and receiver equipment. The trial will be also promoted internationally to encourage the wider take up of DVB-T2 and create greater economies of scale for DVB-T2 and MPEG-4 equipment.
Normal service continues
Broadcasters are keen to offer HD on their Freeview services. Understandably they are anxious not to lose-out in the marketplace to satellite and cable TV provides, who already offer HD. If Ofcom’s proposals for the future of Freeview HD television go ahead, which seems likely, people will still have the option of switching to satellite or cable TV but we expect most viewers to continue using their Freeview units (which are built-in to many modern TV receivers) rather than changing the way we watch and pay for television.
Interlaced Scan: a complete image frame is encoded in two fields – one containing all the odd lines, and the next containing all the even lines. When displayed, the image is drawn with all the odd lines being scanned from top to bottom, followed by all the even lines top to bottom. Fifty separate fields are shown every second.
A powerful driver of demand for HD terrestrial digital TV capability has emerged in the shape of large flat-panel TV sets that have stimulated enormous customer interest. Estimates suggest more than 10million UK households now own one of these ‘HD Ready’ displays. Another factor sharpening demand for HD content is the ending of the HD DVD standards ‘war’ in favour of the Sony Blu-ray system, to which broadcasters have responded by producing ever more programmes in high definition.
Behind these crisp images on the HD screen lies an array of engineering challenges. Television pictures comprise small dots, or screen pixels, arranged into a series of 625 horizontal display lines.
The information conveyed by each of these pixels is updated at a fast enough rate to create the illusion of picture motion. The odd and even lines are refreshed alternately at a rate of 25 Hz using an approach known as ‘interlaced scanning’. This reduces the amount of transmission capability required but it can compromise the ability to replicate screen motion.
High definition has more picture lines than standard definition, or SD. Two formats are available at present, 720p and 1080i, which are compatible with TV sets bearing the ‘HD Ready’ logo. The first uses 720 display lines, all of which are refreshed at a rate of 50Hz. The second uses 1080 display lines and the same interlaced scanning approach as SD TV. Both formats convey much more information than SD TV in each new television picture frame – and both require typically up to five times more transmission capacity. This massive increase in capacity has proved the main barrier to delivering these services on today’s digital terrestrial platform, Freeview.