This is a short introduction to the DVB-T2 standard, highlighting differences from the original DVB-T system as used by N.Z Freeview.
This is for those interested in the RF technology of this flavour of digital television and particularly, how it differs from the first generation DVB-T system. My earlier article on how the DVB-T system works; comparing it to analogue, is available here
DVB-T2 was announced by the DVB consortium in late 2008. It is an extension of DVB-T in that it is an OFDM (Orthogonal frequency division multiplexed) system using QAM on a large number of carriers. There are a wide number of possible variations of almost every parameter so broadcasters can fine-tune to get the highest data capacity possible while retaining an appropriate level of robustness for their application. The T2 standard offers an increase of data capacity compared to plain DVB-T, which may be as high as 67% but in practice is somewhat less than that. Following is a table showing the essential differences between DVB-T2 and DVB-T.
The increase of data capacity is achieved by incremental improvements although rather a lot of the increase is due to the more efficient error-correction algorithms. The original convolutional coding plus Reed-Solomon schemes have been completely replaced by the new LDPC (Low density parity check) plus BCH coding (Bose-Chaudhuri-Hocquenghem). The use of these methods is said to be 30% more efficient at correcting data errors in QAM systems. Other changes include:
The best payload capacity in DVB-T of 31.66Mb/s has now been increased to 50.34Mb/s, an increase of 59%. Broadcasting in N.Z on the UHF band uses 8MHz channels although other countries use 7MHz and even 6MHz.
One last trick up the sleeve of DVB-T is PLP (physical layer pipes). A broadcaster could assign individual programme streams to several 'pipes' which each have their own set of parameters. So, particular programmes could, for example, be sent with more error correction making them more robust that other programmes.
Parameters to provide roughly equivalent transmission robustness to Freeview are as follows:
Actual number of carriers for 32k (ext) mode is 27841. These fit into a 7.77MHz total channel bandwidth, so carrier spacing is 279Hz. These parameters result in a total useful payload capacity of 38.916043Mb/s. Compare that payload capacity with Freeview (using plain DVB-T) which is only 26.35Mb/s.
Below is a 256QAM constellation diagram showing the rotation method in DVB-T2
Count the 256 positions if you are keen. Each constellation position carries 8 binary bits.
SKY make their Igloo service available via their Pace branded set top boxes which have been around since mid-2012. These connect to your UHF antenna and to your TV set via an HDMI cable preferably. An internet connection is required for the on-demand service, which is streamed rather than being over the air.
Any comments or questions please use the form on the CONTACT page.
Axino-tech Consulting & Services , December 2011.