Exploring the Status of Transmission
July 21, 2010
Once upon a time there were only a few TV channels, but as the new digital terrestrial standards developed and were implemented, the markets began to change and the number of channels began to grow. Now there are hundreds of channels available on satellite and cable, but not as many on Digital Terrestrial TV (DTT).
Terrestrial TV is, in fact, being squeezed as its spectrum is re-assigned to other services. This started with the re-allocation of Band I and Band III analogue TV frequencies in many countries and now the EU is continuing to “twist the screw” by enforcing the re-assignment of the top part of Band V, nicknamed the “digital dividend.”
Where and when these pressures were not so great, countries such as Australia went ahead with HDTV transmission on DTT, but this limited the number of channels that could be provided. Fortunately improvements in MPEG compression standards and the use of statistical multiplexing have increased the number of SD services that can be transmitted in a single frequency channel and this has also led to more countries starting HDTV services using the original DVB-T standard.
In the United States the ATSC 8-VSB standard is still used. In Japan, as well as a number of other countries, the choice has been ISDB-T or variations of it; and in China they use their own hybrid standard.
Unfortunately all these first generation DTT standards have a problem, which is their limited data capacity. The usable bit rate is limited and the number of services available can only be increased by better compression coding or by reducing service quality.
HDTV AND 3DTV
In the last few years we have seen the growth of HDTV services from satellite and cable and the United Kingdom’s BskyB recently announced it will soon have 50 HD channels available. BskyB has also begun broadcasting experimental 3DTV programmes and many other countries are undertaking trials.
While DTT can never compete with this number of channels, especially as the allocations are shrinking, a new standard for terrestrial delivery has been developed, which has more than 67 percent more data capacity than DVB-T. This standard together with the use of the MPEG-4, H.264, audio visual codec for compression is transforming digital terrestrial TV delivery. The standard is called DVB-T2.
The DVB-T2 standard, now being deployed in the United Kingdom, not only provides 67 percent more data capacity than DVB-T, but has the added benefit that multiple platforms such as SDTV, HDTV and mobile TV can be transmitted at the same time, in the same multiplex. In addition, each of these services can be individually optimised in terms of the best modulation and coding scheme for each of them and then transmitted together in the same multiplex and from the same transmitter or network, potentially earning additional revenue for the provider.
FIXED RECEPTION OF DVB-T2
The United Kingdom launched its DVB-T2 service in December 2009, and Finland and Italy will follow soon. Trials are also taking place in the Czech Republic, Denmark and Germany.
In the United Kingdom the existing DVB-T service delivers 24.1 Mbps using 64 QAM in a single 8 MHz channel, while the DVB-T2 service delivers 40.21 Mbps using 256 QAM, also in an 8 MHz channel. This is a 67 percent improvement in data capacity over DVB-T for similar transmission robustness and will allow up to four MPEG-4, H.264-coded HDTV services in a single multiplex.
DVB-T2 transmitters can also be further apart in an single frequency network (SFN) than was possible with DVB-T. This is enabled by FFT options up to 32K in DVB-T2, which result in the data being spread across a greater number of carriers transmitted in the OFDM signal. Because of this the data symbols are longer and the guard interval, which makes SFNs possible, does not need to be as great as in DVB-T. This can either be used to increase data capacity when it is small or to increase the distance between transmitters in an SFN when a bigger guard interval is selected.
MOBILE DVB-T2 RECEPTION
One early criticism of DVB-T was that it was not suitable for mobile reception. This was disproved by many public demonstrations of mobile TV at both the IBC Conference in Amsterdam and the NAB Show in Las Vegas. DIBCOM also produced a special chipset, which optimised its performance even further increasing the speed at which it could be received. However battery power was a key problem and in those days DVB-T viewing time on a mobile phone was limited. This led to the development of the DVB-H standard specifically for the delivery of TV to mobile handsets. One of its features was that it used a technique called time slicing, which allowed the receiver to be switched off while data, which wasn’t being watched, was transmitted. Hence battery life was improved. DVB-T2 also has structures, which allow battery consumption to be minimised, and other features specifically added to improve reception by mobile terminals.
With all complicated specifications it is important to ensure that they are being implemented correctly in equipment, and also that they do not contain unforeseen ambiguities, which can result in incompatibility problems.
In June 2010 a Multi-PLP (Multiple Physical Layer Pipe) plugfest was held in Berlin with the aim of ensuring that equipment was compatible and also that the Multiple PLP function within DVB-T2 worked. This Multiple PLP function is the one that allows different transport streams using different modulation/coding schemes, such as those required for side-by-side fixed and mobile services, to be carried in the same transport stream.
During the plug fest a 2.3 kW transmitter in Berlin’s Alexanderplatz site was used to transmit a DVB-T2 signal on channel 51 for the tests. The results were successful and organisers Media Broadcast Gmbh will go on to test a 3D transmission in the T2 multiplex before the end of this month.
DVB-T2 is a great improvement on the first generation of DTT standards now in use around the world. With DVB-T2’s proven increase in transmitted data capacity in an existing channel and its ability to individually optimise performance for 3DTV, HDTV, SDTV and TV to mobiles in the same multiplex, it must be the first choice for any country still deciding on a DTT standard.
It is also likely that DVB-T2 multiplexes will be used side by side with DVB-T multiplexes in many mature digital countries, as they are in the United Kingdom, and will gradually replace DVB-T. Fortunately DVB-T2 uses the same infrastructure as DVB-T and brings with it at least a 50 percent and in some cases a 67 percent improvement in payload capability as well as improved robustness. This increase in payload is now vital if terrestrial delivery services are to compete successfully with the 3DTV and HDTV services delivered by cable and satellite operators. Set-top boxes are already below the £85 (€100) in the U.K. market (July 2010) and will drop much more quickly as the digital switchover continues.