DVB-T2 to enable HDTV post-switchover

DVB-T2 is the name for a new digital terrestrial TV standard which is an upgrade of the current DVB-T system that is used for Freeview, and quoting from an article on the DigiTag (Digital TV Action Group) website about the new DVB-T2 system:

Judging by the above quote, they are focusing on the fact that DVB-T2 is backwardly compatible with existing aerials but that it will still provide a 30% increase in capacity -- if DVB-T2 adopts MIMO as an option, then it should be able to provide a 100% increase in capacity compared to DVB-T (see below for a brief discussion about MIMO), but that would require everybody to install new aerials. So, I'll concentrate on how DVB-T2 would affect the amount of capacity available on Freeview to see how much additional capacity should become available in order to allow HDTV channels to launch.

Freeview capacity pre/post switchover

The Freeview system currently consists of the following multiplexes:

Once analogue TV has been switched off, the transmission powers of the multiplexes will be able to increase (they're currently running at lower power to avoid interfering with the analogue TV signal), and the multiplexes that are using 16-QAM will change to using 64-QAM, which will increase their multiplex capacity by 6 Mbps. Therefore, taking into account the increase in capacity from multiplexes changing from 16-QAM to 64-QAM and the 30% increase that should result from the use of DVB-T2, the post-switchover capacity of Freeview should be:

Post-switchover Freeview capacity  =  6 multiplexes  x 24 Mbps  x  1.3  =  187 Mbps

Therefore, there should be 67 Mbps of additional capacity available on Freeview once analogue TV has been switched off, and assuming that an HDTV channel will use a bit rate of 8 - 10 Mbps, this would allow between 6 to 8 HDTV channels to be broadcast on Freeview. However, only those people who currently can receive Freeview would be able to receive all of the HDTV channels that will launch, because the commercial multiplex operators have chosen not to increase the coverage of their multiplexes from the current 73% population coverage. The multiplexes are also changing post-switchover to PSB (public service broadcasting) multiplexes and non-PSB multiplexes rather than the current BBC or commercial set-up. For more details about this see the Digital UK website.

The fact that HDTV will be able to be carried post-switchover is obviously not very good news for the HDforAll campaign that has been set up primarily by the BBC, ITV and Channel 4 to lobby Ofcom and the government for the broadcasters to be given additional spectrum that will be freed up once analogue TV has been switched off so that they can broadcast their channels in high-definition -- Ofcom said that they would be able to broadcast their channels in hi-def, but the broadcasters disagreed.

So, clearly it will be possible to broadcast HDTV post-switchover, and if the broadcasters want extra capacity for Freeview over and above the 187 Mbps capacity calculated above, they could increase the capacity of DVB-T2 further to 288 Mbps, although this would require that everybody would have to buy a new aerial, and I dare say that many would just migrate to satellite, cable or using the Internet instead, so I doubt the BBC et al would risk that, because Freeview is seen by the BBC as a way of hanging onto the licence fee, because Freeview receivers don't have card slots on them, which reduces the risk of the BBC changing to being subscription-based -- this was the reason why the BBC launched Freeview in the first place, because as Greg Dyke said in his book, it was a way to "flood the UK market with dumb set-top boxes" that don't allow subscription payments, and therefore it gave the BBC "another decade of secured funding".

To be honest, a decade from now we'll have lots of HDTV channels available via the Internet being delivered via our super-fast 50 - 100 Mbps broadband connections, and Freeview will by then look an extremely antiquated system. So for once I actually agree with Ofcom that it's better not to gift this spectrum to the broadcasters -- and for the record, I watch TV via Freeview and I'm very much in favour of HDTV being provided via Freeview. But I just think it's a bad long-term policy to give this spectrum to the broadcasters, because terrestrial TV eats up about 400 MHz of prime spectrum that's well-suited to mobile systems, whereas with TV only being received on stationary receivers in homes it could be broadcast at any frequency -- e.g. satellite transmissions use frequencies of about 12 GHz -- so it's better to transmit TV at these higher frequencies where there isn't much demand for spectrum.

DVB-T2 - technologies

Details about which new technologies DVB-T2 will use have been very thin on the ground so far, not least because the DVB Project only issues a Call for Technologies in April this year, but the following technologies have apparently been discussed in the DVB-TM-T2 group that's developing DVB-T2:

• 16K (~16,000 subcarriers) and 32K subcarriers in addition to the existing 2K and 8K subcarriers modes -- the higher the number of subcarriers the longer the OFDM symbols will be, and this will improve the spectral efficiency (i.e. a higher bit rate can be transmitted) because the guard interval in between the useful OFDM symbols will consume a smaller percentage of the overall symbol duration (the overall symbol duration is the guard interval duration + OFDM symbol duration), and no data is transmitted during the guard interval. A higher the number of subcarriers also improves performance when there is impulsive interference.
• 256-QAM in addition to the existing QPSK (same as 4-QAM), 16-QAM and 64-QAM signal constellations -- the higher the number of constellation points the higher the bit rate can be (the number of bits carrier per subcarrier per OFDM symbol is log₂(number of signal points).
• LDPC (low density parity check) error correction coding -- LDPC is one of the new types of error correction coding that employ the 'turbo decoding principle' and which are almost optimal in that they perform within a fraction of a decibel of the Shannon Capacity limit (over the additive white Gaussian noise channel), which is theoretical limit beyond which error-free communication systems is impossible. The other main type of near-optimal error correction coding is 'turbo coding', but turbo coding is under patent, whereas LDPC is not, which is probably why LDPC is being preferred. LDPC coding is also used on the DVB-S2 satellite system.
• MIMO (multiple-input multiple-output), which refers to the use of more than one antenna at both the transmitting and receiving ends, and if you've noticed that Pre-N Wi-Fi kit has two antennas instead of one, this is because the 802.11n Wi-Fi standard is using MIMO with two antennas at each end. The advantage of using MIMO is that the capacity increases according to the minimum of the number of antennas at either end of the link. For example, if there are at least two antennas at both the transmitting and the receiving ends then the capacity doubles relative to the theoretical channel capacity when there's only one transmitting and/or receiving antenna. The BBC R&D department performed some tests using MIMO last year where they transmitted a normal DVB-T signal on one polarisation (i.e. either horizontal or vertical) and a signal that had been pre-processed for MIMO transmission on the other polarisation, and they found, as theory predicts, that the capacity doubled compared to transmitting a single DVB-T signal. As well as the 802.11n Wi-Fi standard using MIMO, NTT DoCoMo has developed a 4G mobile phone prototype system that uses MIMO to increase the capacity of the systm, and they've managed to transmit at a staggering 5 Gbps in a 100 MHz channel with the receiver travelling at 10 km/h using 12 antennas at both the transmitter and the receiver. This equates to a spectral efficiency of 50 bits/s/Hz -- to compare, DAB only has a spectral efficiency of 0.66 bits/s/Hz!

New MPEG-4 H.264 video codec

The new MPEG-4 H.264 video codec will also be used alongside the DVB-T2 transmission standard:

• MPEG-4 H.264/AVC video codec -- this is the new video codec that's being used for HDTV channels on satellite, and this codec is approximately twice as efficient as the MPEG-2 video codec that's currently used for standard-definition TV channels on Freeview, satellite and cable.