BBC submission to Ofcom on the choice of the UK ...

BBC submission to Ofcom

on the choice of the UK transmission mode

for high-power Multiplex B using DVB-T2 modulation

R edac ted vers ion

September 2009 (following draft of 28 August 2009)

BBC SUBMISSION TO OFCOM ON THE UK DVB-T2 TRANSMISSION MODE FOR MULTIPLEX B


Contents

1. EXECUTIVE SUMMARY .................................................................... 1 1.1 Aim of this BBC submission to Ofcom ............................................................... 1 1.2 Headlines from the submission .......................................................................... 1 1.3 Conclusion ......................................................................................................... 2

2. CONTEXT .......................................................................................... 3 2.1 Introduction ........................................................................................................ 3 2.2 Benefits to citizen-consumers of DVB-T2 compared with DVB-T ...................... 4

3. LABORATORY AND FIELD TESTS ................................................... 5 3.1 Laboratory tests ................................................................................................. 5 3.2 Field tests .......................................................................................................... 6 3.3 Summary of DVB-T2 vs. DVB-T performance ................................................... 7

4. COVERAGE OF DVB-T AND DVB-T2 ............................................... 9 4.1 Context for an analysis of coverage .................................................................. 9 4.2 Coverage scenarios for DVB-T2 ..................................................................... 10 4.3 Impact of the long delays echo performance on coverage in an SFN ............ 13 4.4 The BBC’s recommended approach to implementing the UK transmission mode ...................................................................................................................... 13 4.5 Why an alternative approach – moving from a lower to a higher capacity mode once launched – is not feasible ............................................................................. 14 4.6 Communication and mitigation ........................................................................ 14

5. TECHNICAL SUMMARY ACROSS PRIORITY MODES .................. 16 6. HD FORMAT TRENDS ..................................................................... 18

6.1 Trend towards larger screen sizes ................................................................... 18 6.2 The need for 1920x1080 resolution as screen sizes continue to increase ...... 19

7. BIT RATE ALLOCATION .................................................................. 21 7.1 Bit rate management across a multiplex .......................................................... 21 7.2 Accommodating HD services in available capacity .......................................... 22

8. SUMMARY OF BBC SUBMISSION TO OFCOM ON THE UK DVB-T2 TRANSMISSION MODE ...................................................................... 23 9. CONCLUSION ................................................................................ 27 CONFIDENTIAL APPENDIX: KEY FINDINGS FROM DVB-T2 TRIAL . 28 A1. RECEIVER TESTS ........................................................................ 28 A1.1 Laboratory tests ........................................................................... 28 A1.2 Field tests..................................................................................... 28



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1. EXECUTIVE SUMMARY

1.1 Aim of this BBC submission to Ofcom The BBC is working with Ofcom and other industry partners to roll out the Freeview HD platform from December 2009, using the new DVB-T2 modulation standard along with MPEG-4 video encoding. The regulator and the BBC set up a trial to test and validate the various parameters associated with the DVB-T2 standard and specification. Parameters have been tested using prototype receivers, and the results are informing the major technical decisions that will support the platform’s operations. The current priority is to decide on the UK DVB-T2 transmission mode – ensuring that UK household coverage meets the current planning targets while delivering the highest possible on-screen quality to viewers of the largest and most accessible UK digital television platform. This submission: • summarises the laboratory and field test results for seven transmission modes

selected by Ofcom explaining why three modes (Modes 2, 4 and 7) were prioritised for detailed analysis and assessment;

• reviews the implications for UK coverage against the Joint Planning Project (JPP) targets;

• sets out the case for high-resolution video; • emphasises the importance of effective bit rate allocation to support the

development of the Freeview HD portfolio, so that at least five PSB HD services can be accommodated; and

• recommends to Ofcom a transmission mode, based on the evidence and analysis presented in this submission.

[ ]

1.2 Headlines from the submission Laboratory tests demonstrated that DVB-T2 met or exceeded the majority of fundamental test targets. In particular, T2 receivers significantly exceeded Digital Television Group (DTG) performance targets in terms of impulsive noise. Field tests using a prototype DVB-T2 receiver demonstrated that, in terms of effective sensitivity, all Ofcom modes except one performed in a similar way to a typical, fifth-generation DVB-T set-top box. Coverage predictions indicate that all prioritised modes perform in excess of the JPP planning parameters and will safely achieve 98.5% coverage. Marginal coverage


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variances are offset by the significant increase in resilience to impulsive noise which T2 delivers compared with DVB-T. The technical evidence, plus the coverage analysis, shows that – even at an early development stage – DVB-T2 meets JPP targets. For enhanced DVB-T coverage, DVB-T2 delivers almost identical coverage to DVB-T (within 0.19% to 0.33%). However, DVB-T2 offers the significant advantage of much more resilience than DVB-T to common DTT on-screen interference effects. Other picture quality factors have also been considered, as it is not only preferable but also important for the platform to launch with the highest possible picture resolution while having the capacity to add services as the platform develops. This approach benefits citizen-consumers as well as justifying broadcasters’ ongoing investment in HD, production and transmission technologies. Ofcom’s Mode 7 appears to offer the maximum flexibility to deliver high quality, on-screen content to 98.5% of UK households. While recognising that any lower-capacity mode might offer coverage advantages, the BBC strongly advises Ofcom that it would not be feasible to start at a more cautious initial transmission mode and then increase to a higher capacity mode after a review. The significant negative coverage impact would disappoint consumers and undermine Freeview HD’s development. The BBC proposes a three-stage approach to implementing the UK’s transmission mode. This approach would enable Mode 7 to be implemented from the platform’s launch, followed by reviews at agreed intervals: after six to eight months, after eighteen months and at the end of switchover to monitor any risks of a coverage shortfall. This would provide the opportunity both to check this higher capacity mode in operation in real world conditions (rather than under development conditions) and, if necessary, subsequently move to a lower capacity mode.

1.3 Conclusion The BBC recommends that Ofcom confirms Mode 7 (40.2 Mbit/s) as the UK transmission mode on the high-power Multiplex B using DVB-T2 modulation. This mode not only meets Ofcom’s coverage planning commitment but it also ensures that – from launch in December 2009 – the Freeview HD platform is as attractive as possible in comparison with other platforms offering HD content. This mode offers the greatest degree of future-proofing, ensuring that Freeview HD becomes increasingly competitive during its three year roll-out, as extra services can be added at a consistent resolution, reflecting broadcast developments and screen-size trends. It makes sense for the platform to reflect HD’s high-end appeal from its launch. Broadcasters and industry partners have invested significantly in Freeview HD content and equipment, and consumers are already positive about the exciting possibility of free-to-air access to HD content via the UK’s largest digital television platform.


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2. CONTEXT

2.1 Introduction HD is rapidly becoming the new production and broadcast standard, the benefits of which should be shared with as many viewers as possible. The move to accommodate DVB-T2 on an upgraded multiplex will provide a simple, free-to-air option for accessing high quality PSB HD content. Since February 2009, Ofcom and the BBC have been co-ordinating a trial, working with a group of manufacturers to test various parameters associated with the DVB-T2 standard and specification. The current priority is to decide on the UK DVB-T2 transmission mode. Ofcom’s commitment is to deliver maximum spectrum efficiency balanced with acceptable coverage and picture quality. As the multiplex operator and the leading PSB content generator, the BBC aims to secure the mode which will deliver maximum viewer value and help to secure the DTT platform's future. Ofcom’s and the BBC’s aims are complementary. Out of the proposed tests (Draft Trial Plan - Test schedule) certain tests have been prioritised within the T2 Programme’s timescale1, the results of which inform this summary paper. Figure 1 below sets out the seven Ofcom modes which were tested (ranging in capacity from 32.4 Mbit/s to 44.8 Mbit/s).

Figure 1: Ofcom Modes

Ofcom Mode 1 2 3 4 5 6 7 DVB-T2 Validation and Verification Group Mode

VV0014 VV0012 VV00013 VV0001 VV0015 VV0016 VV0003

Data rate (Mbit/s) 32.4 34.6 36.1 36.1 39.8 44.8 40.2

Number of Carriers 8K 8K 8K 32K 8K 32K 32K

Modulation 64QAM 64QAM 64QAM 256QAM 256QAM 256QAM 256QAM Inner coding 3/4 4/5 5/6 3/5 3/5 3/4 2/3 Guard interval 1/32 1/32 1/32 1/128 1/32 1/128 1/128 While we do not propose applying a weighting factor in favour of DVB-T2 (known throughout this document as 'T2'), it is worthwhile to highlight that the reference receivers used for the tests were fifth-generation DVB-T technology. There is bound to be a performance differential between current T2 receivers (which are pre-first generation prototypes) and later generations of T2 receivers.

1 The T2 Programme is the partnership co-ordinated by the BBC and supported by Ofcom, with the aim of launching Freeview HD from December 2009.


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2.2 Benefits to citizen-consumers of DVB-T2 compared with DVB-T In terms of value to citizen-consumers, T2 enables the delivery of, among other things: • HD services in DTT capacity – with Freeview being the largest and most

accessible platform, even before DSO; • improved picture and sound quality; • a minimum 30% capacity increase over DVB-T, with the same coverage planning

parameters; • access to HD services on DTT, using the existing transmission infrastructure and

receiving aerials; • a mechanism for service-specific robustness; • time-interleaving, which provides a significant improvement in impulsive

interference performance; and • a mechanism to reduce the peak-to-average power ratio.


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3. LABORATORY AND FIELD TESTS

3.1 Laboratory tests In line with the DVB-T2 trial objectives, a series of detailed ‘bench’ tests were carried out in the laboratory. The performance targets for 3 DVB-T modes are shown in the chart below (taken from table 10.2 of Section 10.3 of the DTG D-Book v6.0).

Figure 2: D-Book v6.0 performance targets [ ]

For DTG modes 1 and 2, these requirements are the same as the previous version of the D-Book. These requirements have not been updated as they will not be used after switchover. For modulation Mode option 3, new requirements have been introduced, whose values are being used as a reference for T2 prototype receiver results. The main tests were grouped as follows: • basic RF tests: C/N with Additive White Gaussian Noise (AWGN) and sensitivity; • the family of echo or multipath measurements; and • impulsive noise measurements. Basic RF tests – headline results In terms of RF sensitivity, all but one of the Ofcom modes met the target figures. Multipath tests – headline results Another set of measurements tested the performance of T2 receivers when echoes were present. With the exception of one of the Ofcom modes, receivers met the target DTG performance figures with Doppler tests as well as with single echoes when the relative path was within the guard interval. As had been predicted from previous theoretical analysis, tests demonstrated that T2 receivers were unable to support echoes of 200µs and above for the modes tested. However, this does not represent a problem, as such delays would not occur in a multi-frequency network (MFN) and are unlikely to occur in single frequency networks (SFNs) as implemented in the UK. Impulsive Noise – headline results Tests have demonstrated that DVB-T2 is significantly more resilient than DVB-T in terms of impulsive interference. [ ]


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In summary, the T2 laboratory tests have shown that the new T2 standard meets or exceeds the majority of fundamental test targets. In particular, T2 receivers significantly exceed DTG performance targets in terms of impulsive noise.

3.2 Field tests 25 locations with different characteristics were selected for the field tests. Figure 3, below, shows a coverage plot of the Crystal Palace DVB-T2 test transmitter and the location of the test sites.

Figure 3: Coverage plot for the Crystal Palace trial, showing all test locations

Locations were chosen to give a range of impairments in terms of signal level, multipath and Doppler. All 25 locations were assessed to inform the overall conclusions of the field tests [ ] . Four particular locations were chosen to complete a detailed analysis in this document. Two of those locations presented significantly challenging reception conditions. Factors influencing difficult reception included: location affected by reflections from buildings; location obscured by tall buildings; presence of long-delay


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echoes; high levels of co-channel PAL analogue interference and adjacent channel interference. The other two additional locations presented optimal reception conditions: one of them had a clear line-of-sight to Crystal Palace and presented a very clean channel with high received signal levels. The other good location was a typical, densely populated suburban location with a mixture of low-level and mid-delay reflections. The measurements carried out in those locations showed the effective sensitivity of the T2 prototype receiver for quasi-error-free operation against the performance of a typical DVB-T set-top box and a latest generation DVB-T test sample. The current sensitivity values used by the JPP for its spectrum planning and coverage reviews were also used to compare DVB-T2 and DVB-T receivers’ performance. It made sense to identify and prioritise particular modes which would deliver value from ongoing, detailed comparison and analysis at specific locations: • Mode 7 was prioritised, as it offered a bit rate which would comfortably

accommodate five services. • Mode 6 was not prioritised, as the high capacity could have too significant an

impact on coverage. • Mode 5 was not prioritised, as its bit rate was too close to the prioritised Mode 7. • Mode 4 was prioritised, as its bit rate was sufficiently different from Mode 7 to

provide a clear comparison – while also offering the potential of accommodating at least four HD services (on current bit rate projections).

• Mode 3 was not prioritised, as its bit rate is identical to Mode 4, and the 32K number of carriers was preferable to 8K.

• Mode 2 was prioritised, because under this mode a fifth generation DVB-T receiver’s performance is almost identical to the theoretical best DVB-T2 receiver.

• Mode 1 was not prioritised, as the bit rate appeared too low to accommodate even four HD services.

[ ]

3.3 Summary of DVB-T2 vs. DVB-T performance The effective sensitivity of the T2 prototype receiver2

2 The first draft of this BBC submission reflected confidential results from one prototype receiver. During the completion of this document, additional data were supplied from another receiver, demonstrating very similar results and reinforcing the original submission.

averaged over the 25 locations was measured in order to assess the performance of the T2 prototype receiver for quasi error-free operation. The T2 receiver performance was compared against three different implementations of a DVB-T receiver: a modern iDTV, a typical set-top box (STB) and a latest generation test sample. For further comparison with DVB-T, the following measurements were used as references: the Gaussian sensitivity performance requirement in the D-Book; the sensitivity used for UK spectrum planning and coverage reviews – which determines the post-DSO UK population coverage of 98.5%; and a ‘theoretical limit’ derived from receiver characteristics that represent the best achievable implementation margin with optimal noise figure and theoretical protection requirement.


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Points to note in the field test results are that: • The latest generation DVB-T receiver (Rx_TS) performed extremely well – the

average being very close to the theoretical limit (in fact, the best measurement is actually at the theoretical limit).

• D-Book requirements were exceeded by the majority of the Ofcom modes in all 25 locations.

• All Ofcom modes performed significantly better than the JPP parameters. • The majority of the Ofcom modes performed within 1.5dB of a typical DVB-T set-

top box. The Appendix shows a detailed summary of all modes compared to different reference targets. Figure 6 in Section 5 shows a comparison of C/N for DVB-T and DVB-T2 at Modes 2, 4 and 7.


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4. COVERAGE OF DVB-T AND DVB-T2

4.1 Context for an analysis of coverage For all of the DVB-T and DVB-T2 tuners/demodulators assessed in the laboratory tests and field trials, the measured results have been extremely close to the theoretical best. This indicates extremely good performance from both the RF tuner and the demodulator. While such low implementation margins are what we should expect for the more mature technology of DVB-T, the similarly excellent performance of early DVB-T2 equipment suggests that many of the lessons learnt from DVB-T have been equally appropriate for DVB-T2. Nevertheless, Figure 6 in Section 5 shows that, although DVB-T has probably now achieved its ultimate performance, there is still some potential for modest improvement in DVB-T2. In terms of the coverage of both DVB-T and DVB-T2, the back-stop limit to performance is determined by the theoretical carrier-to-interference for each variant of each of the standards (as this determines both the minimum signal level requirement and the protection ratio for each mode). This value is shown in the DTG D-Book as the simulation of a receiver with perfect channel estimation. Provisional values for T2 from D-Book 6.1 are shown in Figure 4 below. Also shown are the differences relative to DVB-T. Figure 4: C/I and the difference relative to DVB-T 64 QAM 2/3

[ ] As measurements show close alignment with the C/I values shown, the difference in UK population coverage between DTT transmissions can be relatively easily calculated using the UK Planning Model with the parameters in Figure 4. However, as all values – including, of course, those for DVB-T – are so much better than those currently used by the JPP, doing so would create an anomaly in that the coverage predicted for both DVB-T and DVB-T2 would be significantly better than the 98.5% to which we currently work in the UK. However, for DVB-T we do not believe that we are being overly pessimistic in the planning parameters. Reports of the quality of coverage throughout seven years of DVB-T roll-out lead us to believe that, for good quality domestic aerial installations, the planning parameters are more or less correct – but that, for poor quality domestic aerial installations, the planning parameters may actually be rather optimistic. The former ITV Digital and the Confederation of Aerial Installers (CAI) produced a set of installation guidelines requesting a further 10 dB of signal margin as necessary to overcome domestic impulsive interference. Similar values for an additional margin were also obtained from an extensive set of measurements completed by BBC Research & Development and documented in the


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BBC Technical Note R&D 1940 (Laboratory-based assessment of impulsive interference effects to Digital Terrestrial Television). Two noteworthy conclusions from this work are that:

• No significant increase above the Gaussian limit was needed for a well engineered installation with a double-screened cable and a balun antenna but with no outlet plate (a downlead connected directly to the receiver).

• An identical installation, but using a poorer quality single-screened cable, required an increase of around 12 dB above the Gaussian limit.

Although improved domestic installations and later implementations of DVB-T demodulators have reduced this margin, impulsive interference continues to be a limit to DVB-T’s performance in many circumstances. So what is a realistic comparison of DVB-T and DVB-T2 coverages? Disregarding acquired wisdom on DVB-T planning parameters seems not to be sensible – but, on the other hand, measurements indicate much better performance. However, if this difference is due to impulsive interference, it is encouraging for the likely domestic performance of DVB-T2 – as laboratory and some field results show almost complete immunity to impulsive interference. Initially it seems prudent to discuss both coverage issues independently, based on one of the following options: • an assessment of DVB-T2 coverage against the existing JPP and D-Book

planning parameters; and • a consideration of the implications if newly available dual-standard DVB-T/DVB-

T2 domestic receivers realise the recently measured high performance for both DVB-T and DVB-T2. This can be assessed under two conditions: o where such high performance receivers are used outside the JPP predicted

UK coverage areas; and o where they are used inside predicted coverage areas but with sub-standard

receiving installations (a review of current Ofcom research from Whitehaven suggests that, after DSO, between 10% and 15% of homes there experience intermittent DTT reception).

Figure 6 in Section 5 shows the derivation of minimum signal levels using JPP planning parameters for DVB-T and JPP parameters along with D-Book v6.1 values for DVB-T2. It should be noted that the planning parameters in Figure 6 for DVB-T are those currently used by the JPP in the evaluation of UK coverage; for DVB-T2 they are derived from JPP figures adjusted by implementation margins shown in D-Book 6.1. The final sensitivity values so obtained, for both DVB-T and DVB-T2, are different to those in the D-Book which is intended as a target for manufacturers rather than for planning purposes.

4.2 Coverage scenarios for DVB-T2 a) DVB-T2 coverage against the existing JPP and D-Book parameters


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For Digital Switchover in the UK, the spectrum planners were required to plan digital television so that the core coverage of the public service multiplexes equalled or exceeded the existing analogue television coverage. Coverage of the analogue television network was computed as one of the work packages in the Government’s Digital Action Plan, and the result was the now well-known figure of 98.5%. Despite many versions of the UK DSO Plan, we still consider the figure of 98.5% to be the necessary minimum coverage. From Figure 6 in Section 5 it can be seen that Modes 2, 4 and 7 realise performances in excess of the JPP planning parameters and that, according to established JPP methods, they will safely achieve 98.5% coverage. b) Coverages if domestic receivers realise the recently measured high performance for both DVB-T and DVB-T2 Laboratory and field trials show that current implementations of DVB-T and DVB-T2 receivers may perform significantly better than has been assumed by the JPP. The consequences of improved performance would be seen in two coverage categories.

Figure 5: UK population coverage – DVB-T vs DVB-T2

(i) Outside JPP predicted coverage This occurs in locations which are outside the JPP predicted coverage and where a consumer – either without reference to, or disregarding the guidance of, the coverage post-code database installs dual-standard DVB-T/T2 equipment. As the performance for DVB-T is slightly better than for DVB-T2, there is the possibility that DVB-T will work while T2 will not. The following table (Figure 5) shows the UK population coverage assuming enhanced performance for DVB-T, derived from the average of the best DVB-T implementation over the 25 locations; and it also shows the same for the slightly lower performance of DVB-T2 again averaged over the 25 locations. For further comparison a prediction was made for DVB-T2 with a further 1 dB degraded performance. The differences between these figures indicate the number of households in the UK – outside the quoted coverage areas – which may in theory experience difficulties with DVB-T2 reception.

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PSB 3 Coverage

households

PSB 3 Coverage % UK population

DVB-T (using sensitivity[ ]) 27,465,648 99.59 DVB-T2 (using sensitivity[ ]) 27,412,277 99.40 DVB-T2 (using sensitivity[ ]) 27,373,745 99.26 Difference T2 [ ] 53,371 0.19 Difference T2 [ ] 91,903 0.33

3 These are computer modelling projections, not in-home test results


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As expected, the UK coverage has risen dramatically from 98.7% (computed from JPP planning parameters) to 99.59%. The difference in coverage at this high end of the range is only of the order of 53,000 households – a noticeable but not major coverage impact. For a further 1 dB degradation in DVB-T2 performance, the difference in coverage increases to 0.33% (around 90,000 households).

• low wanted signal level caused by low aerial gain and lossy (waterlogged) downlead;

(ii) Inside JPP predicted coverage We have always known that the performance of domestic receiving installations is inversely related to the signal level in the location: i.e. in high signal areas, domestic receiving installations operate below the planning standard – either because they were poorly specified in the first place, or they were allowed to degrade with time. In low signal level areas, installations are generally of high quality – sometimes even better than the planning standard. Either way, the final result seems to be that the signal fed into the television is just enough for adequate reception. Consumer research into the quality of DTT reception in the Whitehaven area (following the Copeland DSO Pilot) has identified significant numbers of householders – 10% to 15% within predicted coverage – who report that they have experienced poor picture or sound quality. Ofcom has expressed concerns that the numbers identified in Whitehaven who receive marginal DVB-T may be replicated UK-wide with DVB-T2 – on the basis that, as measurements have shown that DVB-T2 requires slightly more signal than DVB-T, it may not be decodable for large numbers of the population. To address these concerns it is worth gaining some insight into the reasons for the large number of viewers reporting sub-standard reception. As the viewers are in predicted coverage areas, there is no doubt that the underlying cause is domestic aerial installations performing below the planning standard. One reason for these high values of 10% to 15% may be the nature of DTT viewing in households that were included in the sample from which the figures were derived. We have learnt that, in households using satellite for their main television, the results for DTT from the second television were used. These results, then, may have been for a portable set-top antenna rather than for a fixed antenna. It is often true to say that an aerial upgrade can solve the problem. However, viewers’ perceptions of their new HD service will be unfavourable if it performs worse than their existing DTT – so, even given that the aerial installation may be poor, we need to look further into the possible mechanisms for interference. In the majority of cases these can be categorised as one of, or a combination of, the following:

• high co-channel interference caused by tropospheric propagation from other broadcast stations, made worse by poor antenna directivity; and

• increased susceptibility to impulsive interference from switching electrical equipment (i.e. thermostats and motors) – often caused by poor down-leads.

In general, a wanted signal level remains at a relatively constant level, so this in itself is not a cause of intermittent reception. A more likely scenario is that a low wanted signal level reduces the margin against co-channel or impulsive interference. The


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characteristics of these two mechanisms for interference are different: co-channel interference is experienced over extended periods, normally occurring in anti-cyclonic conditions during the summer months; whereas impulsive interference is in very short bursts, often attributable to a particular electrical event such as a thermostat switching or a motor starting. Consequently, it is normally quite easy to discriminate between them. Unfortunately, in this respect, the reports from the Copeland research are not easy to interpret, and they do not assist in identifying which type of interference predominates. However, there are two pieces of information relating to Whitehaven which do help: • At the start of the Copeland pilot, there were very few significant co-channel

interferers – the main stations being Cairn Hill, Cambret Hill, Llanddona and Black Hill. Over the Whitehaven coverage area, the tropospheric field strengths (1% of the time) from these stations are not particularly damaging, with excess signal margins still of the order of 20 dB.

• Copeland DSO took place during October to November 2007. Tropospheric propagation is unlikely to occur during these months.

So the evidence seems to point to impulsive interference from domestic electrical equipment as being the most likely cause of the majority of the reported intermittent picture quality, especially true if some results for set-top antennas were included. This being the case, the deployment of DVB-T/DVB-T2 equipment in such areas will – far from causing disillusion with the introduction of DVB-T2 – generate positive comparisons with DVB-T, as T2 offers almost total immunity to impulsive interference.

4.3 Impact of the long delays echo performance on coverage in an SFN The UK Planning Model was used to identify the impact of the long delay echo performance of T2 on coverage in the UK network containing SFNs. Results show that the coverage of the network would be reduced by only 0.04% and this impact is negligible.

4.4 The BBC’s recommended approach to implementing the UK transmission mode The BBC recommends a three-stage approach to implementing the UK’s transmission mode. This approach would enable a potentially higher capacity mode (7) to be implemented from the platform’s launch, followed by three mode reviews at various stages in operation – providing the opportunity to move to a lower capacity mode instead, if that were seen as the most suitable decision based on feedback from the public. (It is worth noting that this is only viable if the launch mode is high enough the capacity for subsequent reduction after the platform is established.) The proposed three-stage review process is as follows: Stage 1: 6-8 months after launch


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After the June 2010 World Cup it is forecast (through joint Freeview and T2 Programme predictions) that there will be a significant number of T2 receivers (STBs, digital television recorders/DTRs and iDTVs) in the market. It would therefore be appropriate to do a initial assessment of potential reception problems4

• in-home measurements at different locations;

identified in that period. The proposed review mechanisms include:

• providing reception advice • feedback from CAI members and the public from broadcasters help lines and

websites; • a review of a sample of the installed receiver base; and • a review of the outcomes of the mitigation tactics (see Section 4.6 below) – both

in good reception areas and in marginal areas. Stage 2: 18 months after launch At this stage, Ofcom could carry out a formal review based on: • a BBC submission to Ofcom, including information from the sources listed under

Stage 1 above, plus manufacturer and retailer feedback; • surveys by BBC Distribution at different transmitter locations; and • Arqiva’s technical reports, noting transmission and reception-related issues in the

T2 infrastructure. Stage 3: when DSO is complete (at the end of 2012) Once switchover is complete, it would be appropriate to complete a full mode implementation review.

4.5 Why an alternative approach – moving from a lower to a higher capacity mode once launched – is not feasible The BBC strongly advises Ofcom that it would not be feasible to start at a more cautious initial transmission mode with the intention of increasing to a higher mode after a review. This is because the potential coverage impact for existing receivers would result in consumer confusion and dissatisfaction – as people just outside the coverage areas would lose HD services that they were previously able to receive – and they might consider moving to another platform in order to access HD services (which would involve a financial penalty, such as the cost of a new receiver, dish and/or cable installation and, possibly, a subscription). Even for newer receivers, the improvement in performance against launch T2 receivers would be insufficient to offset the loss in coverage caused a mode change from 36.1 to 40.2 Mbit/s.

4.6 Communication and mitigation As noted in Section 4.2, there is the potential for a small coverage difference between DVB-T and DVB-T2, which could cause some consumer confusion. We

4 It is worth noting that many reception problems will be linked to the five early sites as a result of reduced power operation until DSO is completed for these transmitters.


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need to be clear about how coverage issues will be communicated to viewers and which mitigation techniques are available to them if their reception is not satisfactory. The coverage predictions of both PSB 1 (DVB-T) and PSB 3 (DVB-T2) at a viewer’s house can be determined from postcode checkers on the Freeview, Digital UK and BBC websites. These will be updated to show Freeview HD coverage based on existing JPP planning parameters well before launch. If, following a positive coverage check (i.e. their location is in coverage), a viewer experiences reception problems, the websites will suggest that they consult a qualified aerial installer to check their receiving installation. This is the same message that has been given to date for DVB-T reception problems and, therefore, will be equally appropriate for DVB-T2. Should a viewer choose to install equipment despite a negative coverage check, there is a possibility that they could receive both DVB-T and DVB-T2 satisfactorily (especially if the enhanced performances of DVB-T and DVB-T2 are realised). Unfortunately, there is also the possibility that this could lead to the situation where differential performance causes reception difficulties with DVB-T2. Communication in this situation is rather more complicated as normal advice would be not to attempt installations out of predicted coverage. Nevertheless, mitigation – such as using superior performance domestic receiving aerials, possibly employing masthead pre-amplifiers – may be useful. We suggest that, if differential coverage becomes an issue, the more likely scenario will be that some problems occur with DVB-T due to impulsive interference, while DVB-T2 remains solid. Advice in this case would firstly be to identify and fix any faulty electrical equipment; and, secondly, to improve the receiving installation (aerial and download). Viewers’ can access advice via: • the DUK and BBC helplines, including the BBC’s Reception Advice service; • CAI members; • local retailers, especially smaller independent retailers, who can often provide

recommendations on equipment installation and upgrade; and • consumer websites. Once there is a significant, installed receiver base, further in-home trials can be conducted to generate more refined information on potential problems (see Section 4.4 above). This information can be set alongside the feedback to the BBC and DUK helplines in order to target further investigation and any appropriate mitigation.


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5. TECHNICAL SUMMARY ACROSS PRIORITY MODES Figure 6 below shows the derivation of minimum signal levels using JPP planning parameters for DVB-T and JPP parameters along with D-Book v6.1 for DVB-T2. [ ] It should be noted that the planning parameters in Figure 6 for DVB-T are those currently used by the JPP in the evaluation of UK coverage; for DVB-T2 they are derived from JPP figures adjusted by implementation margins shown in D-Book 6.1. The final sensitivity values so obtained, for both DVB-T and DVB-T2, are different to those in the D-Book which is intended as a target for manufacturers rather than for planning purposes. Figures in red are a suggested theoretical best for implementation margins and a receiver noise figure of 4 dB. ‘Possible improvement’ shows the difference between theoretical best minimum signal level and the lowest minimum signal level measured during field measurements. JPP Document 1 – outlining the planning parameters to be used in the UKPM for DVB-T – contains the following explanatory text: “The quoted system C/N ratios are taken from Chester 97 and are applicable for fixed reception, assuming a Ricean channel. The quoted receiver implementation margin is extrapolated from the ETSI DVB-T specification and other sources, ITU-R Recommendation BT.1368-313 and European Broadcasting Union Broadcast Planning Note (BPN) 047. Measurements made using available set-top boxes and professional decoders show that the minimum failure C/N in real conditions is somewhat higher than would be expected by these references, hence the addition of an allowance for ‘real’ conditions”. Chester System figures are ‘theoretical’ for QEF i.e. post Viterbi BER of 2 x 10-4 .


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Figure 6: Comparison of C/N for DVB-T and DVB-T2 [ ]


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6. HD FORMAT TRENDS

6.1 Trend towards larger screen sizes The second factor influencing mode selection (beyond laboratory and field tests of coverage/reception) is the need to accommodate consumers’ increasing propensity to purchase larger television displays. With flat-panel displays (plasma or LCD) now common, there is a growing appetite for larger screens. According to Ofcom’s latest Communications Market Report (published in August 2009), almost one-third of all TV sets sold in 2004 had a screen size of less than 15”. However, this had fallen to 7% in the year to April 2008. Figure 7below illustrates the steady upwards trend to April 2008 in the proportion of mid-size TV displays sold.

Figure 7: TV sales trend based on 26” screens as reference (GfK data)

Latest figures (July 2009), as represented in Figure 8 below, show that screens of 30” and above currently comprise almost 50% of TV sales compared with around 30% in early 2005. It should be noted that very large screens (e.g. 50” and above) were very scarce and prohibitively priced in 2005 – and, therefore, there was limited opportunity to purchase them. They are now more frequent purchases, due to availability and the preference in some homes to set up home cinema arrangements to benefit from HD/Blu-ray content.


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Figure 8: Percentage of TV set sales of 30” and above (GfK data, July 2009)

0

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30-32" 33-39" 40-42" >42"

6.2 The need for 1920x1080 resolution as screen sizes continue to increase The chosen transmission mode should support a picture resolution that maximises the delivery of HD benefits on viewers’ increasingly large and high-quality displays. Figure 9 below illustrates the incentive to move from 1440x1080 towards picture resolution 1920x1080, as screen sizes continue to increase above 50”.

Figure 9: Horizontal sampling of TV standard required for different screen sizes, viewed at 2.7m5

5 These data were based on viewers with ‘average’ eyesight sitting an average 2.7m from the screen. Some viewers may be closer, and some viewers may have better than average vision.


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Broadcasters are also investing in their production facilities, replacing older 1440x1080 resolution cameras and tape-decks with more recent 1920x1080-capable units. Many satellite broadcasters have already switched from 1440x1080 to meet the consumer demand for ‘full HD’6

6 Ofcom and the BBC have regarded 1440x1080 resolution as an option. However, since the original proposals for clearing Multiplex B, other platforms have started to move to higher resolutions. In addition, the original HD on DTT trial and subsequent research has shown consumer demand for at least five PSB HD services. For both these reasons, plus the desire to make the multiplex economics effective for PSBs in terms of the carriage of HD, convince the BBC that Mode 7 (40.2 Mbit/s) is in the best interest of the platform. The BBC also firmly believes that targeted coverage will be achieved at this transmission mode.

. Additionally, the BBC noted Ofcom’s enthusiasm for 1080p50 input/output during its consultation in January 2009. On that basis, 1080p50 output was included in the T2 receiver technical specification. Therefore, by providing the highest picture resolution with the encoders, we have ensured a consistent HD experience across devices. Concurrently, the trend on the satellite platform is to move towards 1920x1080 – so it is therefore a priority to ensure there is comparable picture quality for DTT viewers. According to TV sales data from GfK, in January 2005 around 40% of all TV sets sold had screens of less than 20”. Three years later, in January 2008, one-third of all TV sets sold had screens of 35” and above. Based on the same trend, it is reasonable to predict that, by January 2011, a similar proportion (i.e. 30%) of TV sales will be 50” and above – assuming a 15” increase over the next three years (similar to January 2005 to January 2008). This is without even factoring in the ongoing increase in big screen availability. Screens of 50” and above (particularly between 53” and 59”) ideally require the 1920x1080 format for maximum on-screen benefit. By the end of switchover (two years later, by January 2013), it is reasonable to assume that a significant proportion of sets sold will be around 55” and above, requiring the 1920x1080 format. However, the higher format requires a higher bit rate; 1920x1080 video takes, on average, 20% more bit rate than 1440x1080. In order to deliver the additional bit rate required on the largest screens, it is reasonable to look first for coding technology improvements. However, Section 7 below illustrates that additional improvements from coding technologies will be small as MPEG-4 is becoming mature, so we also need to look elsewhere.


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7. BIT RATE ALLOCATION

7.1 Bit rate management across a multiplex Figure 10 below shows an example of the bit rate allocation for all the components of the HD services in a four service pool. On average, the bit rate associated with the video is around 9 Mbit/s per service; and each AAC audio stream requires 365 Kbit/s (net). It is worth noting that, for the third service, an additional AAC joint stereo audio stream has been taken into account (for S4C). Each subtitle stream requires 200 kbit/s; and the third service has two streams to support two different languages. There is one HE-AAC mono audio description stream per service, requiring 72 kbit/s (net). For MHEG-related applications, around 100 kbit/s are allocated to each service. There is one PMT (Programme Map Table) for each service present in the multiplex, which takes 18 kbit/s. The total of all these components is 9,728 Mbit/s for services with one audio and subtitle stream; and 10,064 Mbit/s for services with two audio and subtitle streams. In addition to the components mentioned above, further capacity is required for SI data and multiplex management information. Therefore, the sum of the bitrates of all components in a four service multiplex is 40,158 Mbit/s, in which the video component takes around 36 Mbit/s and the rest of the components a total of 4 Mbit/s.

Figure 10: Bit rate allocation in a multiplex 7

Video

Main AudioAD

SubtitlesMHEG service specific 1 & 2

MHEG BootPMT

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ice 1

8970 (VBR)

365722001003

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ServiceTotal:9728

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ice 2

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Service Total:10064

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ice 2

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Service Total:9728

PAT + Cross carried SI 508Other PSI/SI

Mux Headroom 402.15 1% of final mux rate

7 This diagram has been prepared for scenario planning and illustrative purposes only. It does not represent a plan for bit rate allocation or bit rate management.


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7.2 Accommodating HD services in available capacity [ ]

In conclusion: • A transmission mode of 36.1 Mbit/s (Mode 4) would require [ ] predicted

coding improvements in order comfortably to accommodate four HD services at 1920x1080 resolution.

• A transmission mode of 40.2 Mbit/s (Mode 7) from launch in December 2009 allows Freeview HD on multiplex B to deliver three and, subsequently, four HD services by the end of 2010 – at a 1920x1080 resolution that reflects content origination, broadcasters’ encoding and consumers’ screen-size choices.

• Only a 40.2 Mbit/s multiplex would deliver five PSB Freeview HD services – unless coding efficiencies go well beyond anything that manufacturers currently predict and technical trends indicate as possible.

Figure 15 below summarises the advantages and disadvantages – in terms of capacity at different picture resolutions – of 36.1 Mbit/s (Mode 4) and 40.2 Mbit/s (Mode 7).

Figure 15: Advantages and disadvantages of 36.1 Mbit/s and 40.2 Mbit/s

Mode Advantages Disadvantages

36.1 Mbit/s

• No risk of reception differences between the best DVB-T and DVB-T2 receivers

• Not able comfortably to accommodate four HD services until [ ] predicted coding improvements take place

• Impossible to accommodate five HD services unless coding efficiencies go well beyond anything that manufacturers currently predict

• Resolution would need to be reduced, despite trends in content encoding towards 1920x1080

40.2 Mbit/s

• Can accommodate four HD services by December 2010

• Could accommodate five HD services [ ]

• Resolution can be maintained at 1920x1080 throughout Freeview HD’s development

• Reflects the commitment to grow the platform by the London Olympics and beyond

• The launch mode could be reviewed once there are enough receivers in operation across various regions


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8. SUMMARY OF BBC SUBMISSION TO OFCOM ON THE UK DVB-T2 TRANSMISSION MODE This final section summarises the evidence presented in previous sections and highlights the BBC’s recommendation to Ofcom on the UK DVB-T2 transmission mode. Laboratory tests demonstrated that DVB-T2 meets or exceeds the majority of performance targets. In particular, T2 receivers significantly exceeded DTG performance targets in terms of impulsive noise. In the field tests, the latest generation DVB-T receiver performed extremely well, comparable to the theoretical limit. In terms of the DVB-T2 prototype receiver, all Ofcom modes except one performed better than the reference D-Book target. Additionally, all Ofcom modes performed significantly better than the JPP parameters. The majority of Ofcom modes also performed very close to a typical DVB-T set-top box. Coverage predictions indicate that all modes realise performances in excess of the JPP planning parameters and that, according to established JPP methods, they will safely achieve 98.5% coverage. However, it is worth noting that current implementations of DVB-T and DVB-T2 receivers may perform significantly better than what has been assumed by the JPP. Considering an enhanced performance of DVB-T and applying a slight degradation to DVB-T2, the difference in coverage predictions is only of the order of 50,000 households. For an additional 1 dB performance degradation for DVB-T2, the coverage difference between enhanced DVB-T – representing 99.59% UK population coverage – and DVB-T2 is 0.33% (around 90,00 households). However, this marginal coverage difference is offset by the significant increase in resilience to impulsive noise which DVB-T2 delivers over DVB-T. (Details of coverage differences are set out in Section 4.2.) So the technical evidence, plus the coverage analysis, shows that – even at an early development stage – T2 delivers almost the same coverage as DVB-T, but with significantly increased in-home resilience to common DTT on-screen interference effects. On this basis – i.e. of T2’s comparability with (and, even, advantages over) DVB-T, another significant factor which enhances HD’s value to citizen-consumers can then be considered: picture quality. Picture quality is determined by the following factors: • the available bit rate (which is directly related to the transmission mode and

therefore the number of services achievable); • the number of services required to meet viewers’ demand and ensure that

Freeview HD is competitive alongside other HD platforms;


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• the increasing move to high-quality HD production across all genres8

• broadcasters’, production and distribution equipment choices; and ;

• trends in viewers’ screen size preferences and purchase decisions.

In view of the factors above, it is not only preferable but also important for the platform to start at the highest possible picture quality within the maximum possible bit rate. Mode 7 (40.2 Mbit/s) enables the highest resolution from launch, ensuring consistency as the platform develops and adds extra services. This is of significant benefit to citizen-consumers, as well as justifying the broadcasters’ ongoing investment in, production and distribution equipment technologies. While Mode 7 (40.2 Mbit/s) is a strong candidate to deliver the best balance of coverage and on-screen quality, it is appropriate to assess the risks of both the priority modes. Mode 4 (36.1 Mbit/s) While this mode offers comparable coverage to the best available DVB-T receivers, the risk to the regulator is that it would not be possible to increase to a higher capacity mode at a future date (even if it looked as though that higher bit rate could deliver beneficial service and/or picture quality improvements). This is primarily because the potential coverage impact for launch receivers would be so significant that it would result in consumer confusion and dissatisfaction – as people just outside the JPP coverage areas would lose HD services that they had previously been able to receive. Indeed, they might consider moving to another platform in order to access HD services again (which would involve a financial penalty to them, such as the cost of a new receiver and dish and/or cable installation plus, possibly, a subscription). The projected improvements in performance of DVB-T2 receivers in later generations would never be sufficient to reclaim the lost coverage for a mode change. Any such process would involve a huge potential reputational risk to the Freeview platform and brand, to the BBC as multiplex operator, to the industry (whose receivers could be blamed for reception changes) and to the regulator. In addition, this mode would definitely require the predicted coding improvements in order comfortably to launch four PSB HD services at the preferred resolution. The risk is that the platform relies on the encoder manufacturers to deliver [ ] improvements [ ], with a consequent risk to picture quality. [ ] Mode 7 (40.2 Mbit/s) This mode primarily involves the risk of a marginal coverage shortfall at launch followed by a requirement for a potential move from 40.2 Mbit/s to a lower capacity mode (e.g. 36.1 Mbit/s) after a review period – but any such risk would be significantly mitigated by the fact that coverage could only increase under such a scenario. 8 1440x1080 resolution has been regarded as an option – but consumer demand for high-resolution HD services on large screens, plus the need to ensure effective PSB HD carriage finances, means that 1920x1080 is preferable. Mode 7 accommodates 1920x1080 as well as delivering targeted coverage.


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From launch in December 2009, Mode 7 would allow Freeview HD on multiplex B to deliver three and, subsequently, four HD services by the end of 2010 – at a 1920x1080 resolution that reflects content origination, broadcasters’ encoding and consumers’ screen-size choices. Only this mode would deliver five PSB Freeview HD services – unless coding efficiencies go well beyond anything that manufacturers currently predict and technical trends indicate as possible. For ease of comparison, Figure 16 below summarises the comparative advantages of the three priority modes. On balance, Mode 7 (40.2 Mbit/s) offers a strong mix of advantages – although Mode 4 (36.1 Mbit/s) could be considered a sensible fall-back after a review period, once there are enough T2 receivers in operation to assess all factors. Mode 2 (34.6 Mbit/s) does not offer advantages in terms of service development or picture quality, although it does deliver targeted coverage.


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Figure 16: Comparison of key measures in assessing the three priority modes

Lab Test: C/N with AWGN

(Figure 6, Section 5)

Field test: Best

decoded receiver

input (Figure 6)

Coverage (Section 4)

Picture Quality at capacity for four HD

services (Section 7)

Picture Quality at capacity for five HD

services (i.e. to grow platform)

(Section 7)

Risks

Mode 2 (34.6

Mbit/s) [ ] [ ]

• Safely achieves 98.5% in line

with JPP parameters

Very difficult to accommodate four HD services at 1920x1080, even when [ ] predicted encoding improvements happen

Not enough capacity to accommodate five HD services at 1440x1080, let alone at 1920x1080

Five HD services would never be achieved at high resolution

Mode 4 (36.1

Mbit/s) [ ] [ ]

• Safely achieves 98.5% in line with JPP parameters

Unable comfortably to accommodate four HD services at 1920x1080 resolution until [ ] predicted encoding improvements happen

Only just enough capacity to accommodate five HD services at 1440x1080, but not enough for 1920x1080

The marginal extra coverage advantage would be more than offset by picture quality and format reductions – which would undermine Freeview HD’s appeal. Five HD services would never be achieved at high resolution

Mode 7 (40.2

Mbit/s) [ ] [ ]

• Safely achieves 98.5% in line with JPP parameters

• Considering an enhanced DVB-T coverage of 99.59% and applying a 3 dB degradation in coverage to DVB-T2, the maximum coverage difference would be 0.33% (around 90,000 households). See Section 4.2

Four HD services accommodated comfortably at any point during 2010 at 1920x1080 resolution (without the need for additional coding improvements)

Five HD services accommodated comfortably [ ] at 1920x1080 resolution – supporting broadcasters’ encoding and consumers’ screen size preferences

The marginal reduction in theoretical coverage (on worse case calculations) is unlikely to be seen as significant in the context of high quality pictures for 99.4% of UK households


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9. CONCLUSION In summary, Mode 7 (40.2 Mbit/s) offers the most flexibility to deliver the highest number of services at the best quality to the targeted coverage percentage – that is, 98.5% of UK households. This not only meets Ofcom’s planning commitment but it also ensures that – from launch in December 2009 – the Freeview HD platform is as attractive as possible in comparison with other platforms offering HD content. This mode will also offer future-proofing, ensuring platform competitiveness during its three year roll-out through the addition of extra services at a consistent resolution. Additionally, as multiplex operator, the BBC has planned its operations on the basis of five services launched by the end of switchover. Fewer services in restricted capacity would cause financial challenges which could damage the platform’s development. It makes sense for the platform to reflect HD’s high-end appeal from its launch. Broadcasters and industry partners have invested significantly in Freeview HD content and equipment, and consumers are already excited by the possibility of a cost-effective means of accessing the increasing volume of HD content. This BBC submission therefore recommends to Ofcom a transmission mode that will deliver the highest quality on-screen experience to the UK households within the targeted coverage parameters. The BBC requests that Ofcom confirms Mode 7 (40.2 Mbit/s) as the UK DVB-T2 transmission mode on the high-power Multiplex B.


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CONFIDENTIAL APPENDIX: KEY FINDINGS FROM DVB-T2 TRIAL [ ]

A1. RECEIVER TESTS

A1.1 Laboratory tests [ ]

A1.2 Field tests [ ]

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