« Previous | Main | Next »

DVB-T2-Lite profile tech standard approved: Transmissions are go!

Post categories:

Keren Greene | 13:40 UK time, Friday, 22 July 2011

Andrew Murphy, Justin Mitchell and Martin Thorp (from left to right) with the transmit antenna above them

Editor's note: DVB-T2, the technology standard that enabled High Definition on Freeview now has a new profile which has been provisionally called T2-Lite. This new profile will allow simpler receiver implementations for very low capacity applications such as mobile broadcasting. This new profile can be mixed with conventional T2 signals in a single multiplex, to allow separate optimisation of the individual components.The T2-Lite component could provide audiences with a reliable live 'broadcast' TV or radio experience on their handheld devices. Using this new profile live broadcasts can be delivered to multiple audience members at a fixed cost to the content provider. Justin Mitchell Lead Engineer at BBC R&D explains the technical know-how behind it - KG

On 7 July 2011 we began transmissions of DVB-T2-Lite from the roof of BBC R&D’s South Lab opposite TV Centre in West London. As DVB-T2 is widespread throughout the UK, you might wonder what’s unusual about this? Well, the DVB-T2 specification has just been updated with a new profile which we are evaluating. This marks the start of a thorough technical evaluation by us in BBC R&D and is especially exciting because the specification that defines the format of the signals to be transmitted has only just been approved as standard by the DVB Steering board! Just as with the existing profile - which went on to become 'Freeview HD' in the UK - we are pleased to be there from day one with transmitter (modulator) and receiver (demodulator) designs that we can use to validate the standard and also to licence to manufacturers interested in developing products for mobile applications.

The new T2-Lite profile is designed to make use of the same reliable features we are familiar with from DVB-T2, but by a careful selection of a sub-set of modes, allows for receivers to be implemented using much smaller and more efficient silicon chips. So T2‑Lite will efficiently deliver TV and radio to mobile devices such as phones and tablets (for which power consumption is an important issue) and in-car at the same time as providing services to existing fixed receivers. It's obviously early days but we are interested to find out how this new technology may play a part in delivering BBC content (and maybe one day content from other broadcasters)  to an ever-growing 'mobile audience'.

 

Justin Mitchell with the prototype modulator.

Justin Mitchell with the prototype modulator.

This new profile is defined in version 1.3.1 of the DVB-T2 specification. It was designed so that only minimal changes were needed from an existing DVB-T2 modulator and demodulator to be able to support the new profile, which will encourage its adoption by equipment manufacturers.

In the existing DVB-T2 spec, different services can already been sent with different levels of robustness so that some services might be targeted at roof-top reception (e.g. HD) while others targeted at portable or mobile receivers.  However, in the UK, we have chosen a mode (using 32K carriers) which maximises the data rate for HDTV for stationary receivers.  The close spacing of the carriers in this mode means that the signal cannot be received reliably by mobile receivers.

In the existing DVB-T2 spec, the signal can, if required contain periods of time which can be used to transmit something other than DVB-T2. This feature was included in the spec to future proof it against changes and allows improvements in modulation technology to be incorporated into the system. These periods of time are called future extension frames (FEFs).

However, in the new version of the spec, both the main part and the FEF contain valid DVB-T2 but with different modes (in terms of number of carriers) and varied levels of robustness, allowing both services to mobiles and fixed receivers to be transmitted as part of the same transmission on the same frequency. The system also permits the transmission of a service to mobile devices without a second DVB-T2 service being there.

 

Andrew Murphy, Justin Mitchell and Martin Thorp (from left to right) with the prototype T2-Lite receiver.

Andrew Murphy, Justin Mitchell and Martin Thorp (from left to right) with the prototype T2-Lite receiver.

The new profile allows most of the flexibility of the DVB-T2 spec, but to maximise its effectiveness for mobile and minimise the requirements for the receiver, it has the following differences:

  • It has a maximum bitrate of 4 Mbits/sec
  • Limits the FFT size to exclude 1K and 32K
  • Prohibits the use of rotated constellations in 256-QAM
  • Allows only short FEC frames (Nldpc = 16200)
  • Adds two new even more robust code rates (1/3 and 2/5),
  • Limits the size of the time interleaver memory to approximately half that of standard DVB-T2

And…

  • Reduces the number of permitted mode combinations, prohibits the use of PP8 and provides the capability of scrambling the L1 post preamble signalling bits.

As part of the digital switch-over programme, the UK has already rolled out a nationwide DVB-T2 multiplex which provides several HD terrestrial channels. For this technical trial of T2‑Lite, we’ve combined an HD multiplex intended for reception on fixed receivers with a more robust mobile service which could be television, radio or data or any combination of these. In the UK, we currently use the mode 32K 1/128 256-QAM 2/3 which gives a bitrate of 40.21 Mbit/sec in an 8 MHz multiplex. In our technical trial, we’ve used the same mode for the HD part of the multiplex but have added a Future Extension Frame (FEF) containing the mobile service. The HD part of the multiplex consists of a DVB-T2 frame which is 216.9 ms in duration followed by a FEF of 44.6 ms. This FEF contains the mobile service.

The mobile part of the service is transmitted in a more robust mode with a smaller FFT size. We have chosen 8K 1/32 QPSK ½ with L_DATA = 46. This gives a bitrate of 1.02 Mbit/sec for the mobile service. The HD part of the service is contained within a FEF of the mobile service. This means that the HD service and mobile service are both FEFs of each other.

The screenshot of the oscilloscope shows how the FEFs combine. The yellow signal represents the HD signal and the blue trace represents the mobile signal.

 

An oscilloscope plot showing how the HD (top trace, in yellow) and mobile (bottom, in blue) services slot together; each sits exactly in the gap left in the other service and are combined inside the modulator to create a single signal.

An oscilloscope plot showing how the HD (top trace, in yellow) and mobile (bottom, in blue) services slot together; each sits exactly in the gap left in the other service and are combined inside the modulator to create a single signal.

 

The evaluation is being carried out on UHF channel 53 (730 MHz) and is being carried out under a test and development transmission licence issued by OFCOM. This is entirely separate from the BBC HD service and will have no effect on it.

For those of you interested in the technical details of the SI used in our evaluation, the base stream (at 33.36 Mbit/sec) contains 2 HD services. The network_id of the base stream is 12512 and the T2_system_id is 57568. In the mobile stream (at 1.02 Mbit/sec), the network_id is 12513 and the t2_system_id is 57569.

The modulator uses two Xilinx Virtex 5 FPGAs on Synopsys HAPS-51 cards together with some circuits designed by BBC R&D to provide the transmitted signal at UHF. The demodulator uses three Xilinx FPGAs on Synopsys HAPS-51 cards to receive the new signals. The service to fixed receivers is displayed on an existing Freeview HD television.

My project team (Martin Thorp, Andrew Murphy, Chris Clarke, John Elliott, Tom Ellinor, Nicholas Jayaratnam, Karen Kingston-Lee, and Colin Howes) and I, are thrilled to have been able to begin transmission and reception of T2-Lite the day that the specification was approved by DVB as we were chomping at the bit to get it all going,

We are planning to demonstrate this technology at IBC in Amsterdam in September on the DVB stand (1.D81) so if you are going to be there come and visit us. We will have a live over-the-air demonstration of both the transmitter and receiver and will be more than happy to have a chat about how it’s all been going.

 

Comments

  • Comment number 1.

    If there is going to be a "DVB-T2-Lite" profile, which is a lower quality profile designed for mobile phones etc., couldn't there also be a "DVB-T2-High Quality" profile introduced that we could have 1080p50, 4K, SHV etc.?

  • Comment number 2.

    I have not studied DVB-T2 light but at I cannot see how it is possibe to use it for mobile devices. Mobile phones do use these high frequencies but only work because of there small cells. Phones usually are in contact with several cells at the same time so can choose the best cell. Freeview HD uses a relativly small number of transmitters with highly directional receiving aerials. The idea that this problem can be overcome with extra error correction seems some what optimistic. Most Freeview transmitters are horizontally polarized so not really ideal for car reception.

    Please can you tell me what the spectral efficiency of your trial is in terms of kbps per khz?

    What we don't want is another disaster like DAB with its poor audio quality and error correction.

    It has seemed to me the BBC R&D budget would be better spent on something like DRM for radio services. Most mobile phones have a perfectly good FM tuner in any case.

  • Comment number 3.

    @trevor: Are you out of your MIND? DRM on radio... seriously... WHY?

    Sounds interesting, but wouldn't it be better if creating a new digital mobile TV/radio service to use spare VHF spectrum like Band I, or Band II if/when FM shuts down, rather than taking a significant amount of bitrate that could be much better used for a fifth Freeview HD service? After all, assuming your figures you just posted are representative of what the service would be like, you'd be sacrificing nearly 7Mb/s for the sake of just over 1Mb/s for a mobile service - that's not many video streams, and if it's audio you may as well use DAB.

  • Comment number 4.

    @Muzer0

    I think you may have misunderstood DRM. DRM stands for Digital Radio Mondial which is a radio system which can be used in the LW, MW, SW and FM bands. Infact it can fit into the white space between FM transmitters in Band 2. It is about 2.8 times more spectrally efficient that DAB. Add AAC and you can fit 5 times as many stations in the same space. The BBC has done reseach into this but have been very quite about it.

    FM is not shutting down. The BBC wants to shutdown it's FM national transmitters but as everyone knows it won't be able to it because most people don't want the inferior DAB system.

  • Comment number 5.

    HD1080: You're confusing how the bitrate is provided with how it is used. The DVB-T2 scheme can be used to carry practically anything. It is not required to provide HD pictures. The choice of frame size and rate - and encoding method - is made at the video encoder and can be carried in either DVB-T2 or DVB-T2 'Lite' so long as there is enough capacity from the selected transmission mode. Normally multiple services will be carried on one multiplex (e.g. BBC One HD, BBC HD, ITV1 HD and 4hd, in England).

    The changes for DVB-T2 Lite are to save battery power, by requiring less complex circuits, and reduce the doppler problem - that a receiver moving relative to the transmitter experiences a frequency shift, which changes the relationships of the carriers and therefore causes interference that wouldn't occur if the receiver was stationary. If the receiver is moving quickly enough, it can't decode the signal. For even standard definition TV, you can be going too fast on an Intercity train or even in a car (perhaps not legally!) for it to work.

  • Comment number 6.

    But surely they are using more bandwidth to give a more robust signal for mobile digital TV. Surely stationary TV could use that bandwidth for a better quality picture than currently (including higher definition picture) and wouldn't need to be as robust a signal.

    So surely this DVB-T2 light is wasting bandwidth to give a small low quality mobile picture (using more bandwidth to give it a more robust mobile picture) that could be used for a much higher quality stationary digital TV system that wouldn't need as robust a signal.

  • Comment number 7.

    HD1080: This is not a change to the Freeview HD signals transmitted from those transmitters which have switched over, and the early HD services at Crystal Palace, Emley Moor, Pontop Pike and Lichfield. This is a short-range low-power test from one site. The public Freeview HD multiplex is running at the highest capacity possible, within the rules of DVB-T2, for equivalent coverage to the other two Public Service multiplexes (for sites that have switched over). This is just enough, at the moment, for four HD TV services.

    The BBC were required to make three slots available, for the regional ITV company, Channel 4 or S4C (in Wales), and Channel 5. At the time Freeview HD launched, Channel 5 turned down the offer, allowing the BBC to reclaim the capacity and launch BBC One HD. It's hoped that improved compression will allow a fifth slot to be created by the end of next year, which again will be offered to Channel 5 first.

    More capacity could only be achieved by using less error protection, which would require power increases to keep the same coverage - which would need to be co-ordinated between different regions in the UK and with our neighbours, primarily Ireland, the Netherlands, Belgium and France. Beyond a certain point, adding a new multiplex would be easier, though again any new frequency allocations would have to be co-ordinated across the country and across Europe, and many frequencies now used for TV are being sold off for mobile phone services.

    This trial is to see how *technically* feasible it is to slice up the transmission into high and low-capacity parts. Freeview already carries BBC National Radio stations which take up about 5% of the BBC A multiplex (for regions that have switched over). However, the receiver has to have fully robust stationary reception in order to play back the radio portion. The time-slicing used in this trial would deliver three-quarters of the HD TV capacity *and* allow the radio stations to be received by portables, from the same frequency allocation. The capacity delivered in the mobile part is too low for TV.

    The background is that the radio spectrum regulator, Ofcom, keeps talking about releasing more and more spectrum for mobile phones and wireless broadband, even where that isn't a sensible use for that frequency range (87.5 - 108 MHz will deliver no useful broadband service), and the broadcasters may find themselves having to cram things in where they can, rather than having ranges dedicated to radio and TV.

  • Comment number 8.

    why the data rate is limited to 4 Mbit/s in T2 lite?

    you have defined the followed configuration " 8K 1/32 QPSK ½ with L_DATA = 46". and This gives a bitrate of 1.02 Mbit/sec for the mobile service. my question is how did you calculate this data rate ?

  • Comment number 9.

    I think it's great that you are experimenting with DVB-T2-Lite.
    However I think it is a shame that nobody seem to be considering using it in Band III. There seems to be a number of unused DAB channels, and as far as I understand DVB-T2 has a mode with a bandwidth suitable to be used in these channels. So I assume that T2-Lite could also be used in Band III channels in this way.

  • Comment number 10.

    HD1080: DVB-T2 lite is a much more efficient use of available bandwidth than DAB.

  • Comment number 11.

    @ RichardEvans67

    There are no unused blocks in VHF Band III. Most of the blocks in sub-bands I and II are used by PMR & PMSE providers and these users are protected until 2015. In their latest review document (22 June) Ofcom identified Block 5A as being available for local DAB in some areas. If any others become available these will also be used for local DAB to minimise frequency re-use and reduce co-block interference between multiplexes.

    Block 11A is spare as Channel 4 DAB handed the licence back without launching the service, but Ofcom still hope to use it for a national DAB service.

  • Comment number 12.

    To answer the question in comment 8 asked by “DVB-T2 Man”

    First the data rate is limited to 4 Mbit/s in T2-Lite because Section I.10 of the DVB-T2 specification says “Additionally, for the TS case, the maximum input TS bit rate shall be 4 Mbit/s.” The reason this limitation is imposed is to allow for simplifications in the FEC decoder implementation.

    To calculate the data rate of the T2-Lite service, you need to calculate how many information bits are transmitted and divide that by the amount of time it takes to transmit them.

    The information bits are broken up into FEC blocks and, in this mode, there are 38 FEC blocks in an interleaving frame.

    The mode we are using is QPSK ½ which according to table I.1 means that Kbch = 7032. Now each FEC block has an 80 bit header, so the number of information bits in a FEC block is 7032-80 = 6952 bits. So the total number of information bits in 38 FEC blocks is 6952 * 38 = 264176 bits. However type B in-band signalling is mandatory in T2-Lite, so the number of bits in an interleaving frame is reduced by 102 which gives 264176 – 102 = 264074 bits.

    The next question is how long does it take to transmit those information bits.

    If we work in units of T (7/64 us), then the duration of the T2-Lite frame is ((46+2)*8448)+2048 = 407552. There are 46 data symbols and 2 P2 symbols (in 8K). The FFT size in 8K is 8192 and the guard interval is 1/32 which means that the number of samples of T in a symbol is (33/32)*8192 = 8448. Finally the P1 symbol is 2048 samples of T in duration.

    The length of the FEF (which is the T2 base) is 33024*60+2048 = 1983488.
    So the total duration is 407552 + 1983488 = 2391040 T = 261.52 ms.

    Finally, because we are operating in high efficiency mode, the data rate is increased by (188/187) because of the removal of the transport stream sync bytes.

    So the data rate = (188/187) * 264074 / 0.26152 = 1.015166 Mbit/sec.

  • Comment number 13.

    Thank you Justin for the clear explanations about data rate calculation.

    @ RichardEvans67 and Brian Gregory :

    According to DVB.org, Open Channel company in Denmark plans to start DVB-T2 Lite tests in 1,7 MHz bandwidth channels.

    See http://www.dvb.org/news_events/news/t2-lite-radio/index.xml

    Bearer performances for DVB-T2 Lite outperform existing DAB systems (still based on old Eureka 147 developments). It could be an efficient way for getting more capacity or a better quality of coverage, specially in countries where nothing has been still deployed for digital radio.

    Comments from Justin (and others) are welcome !

  • Comment number 14.

    Question to Andrew: Hi.
    1) does Lite version has common PLP and Data PLP?
    2) MUX of TSPS and TSPSC: does it require to alter PMT table to include common PLP TS PID also part of services that are connected to data PLP of that particaulr channel?
    3) Does L1 signal still required for Lite version. SInce most of the parameters are fixed then why we need L1 signal
    thanks a lot in advance
    [Personal details removed by Moderator]

  • Comment number 15.

    on CommonPLP:
    1) Is it correct in saying "Common PLP" will carry only EIT and SDT TS packets?
    In case yes, suppose one component is common to all channels and also have same time synch. In this case, ( as a rule 1), we can seprate this component and put it common PLP. If we do this then, is it require toalter PMT. or is it true that data PLP PMT will have information on Common PLP service as well ( this might be not true in general)... thus question is ...is it require to alter or revise PMT and PAT, we we MUX TSPS and TSPSC?

  • Comment number 16.

    Q on CSP implementation of RX
    Hi , in a function "t2_rx_dvbt2blrotcondmap" , after de-rotation there is need to put back imag part of array . Fro example following code
    % undo imag shift
    u2 = reshape(imag(DataRx.data), CELLS_PER_FEC_BLOCK, []);
    u2 = u2([2:end 1], :);
    u2 = reshape(u2, 1, []);
    appears be doing exactly what is required at receiver end. But
    VV007-16KFFT_TP08a_CSP.txt appears to not matching. Is there any issues in test vecot at TP08a.txt. But above code appears to be clean and good.. any advice on the above.
    thanks in advance /jk

  • Comment number 17.

    I have executed transmitter. I am not getting VV007-16KFFT_TP016_CSP.txt file which is not mentioned in DVB-T2-ReferenceStreamsDocumentation and its not showing any error too. May I know what to do.

    Thanks and Regards,
    Anbarasu.P.

  • Comment number 18.

    I got error in the files
    VV007-16KFFT_TP025_CSP.txt and VV007-16KFFT_TP032_CSP.txt
    when i compared with the Reference streams which are given in ftp://ftp.kw.bbc.co.uk/t2refs/.

    The mistake i found in the power not in values.
    eg: 7.00002e00
    7.00002e000
    Kindly help me to clear it.

    Thanks and Regards,
    Anbarasu.P.

 

More from this blog...

BBC iD

Sign in

BBC navigation

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.