report on the second phase of digital audio broadcasting

Report onThe Second Phase of

Digital Audio Broadcasting Technical Trial

Introduction

Having studied the results of the first phase of Digital AudioBroadcasting (DAB) technical trial, the DAB Technical Sub-Committeeconsidered that a second phase of DAB trial should be carried out togather more technical information to address the gaps identified in thefirst phase report. With the approval of the Steering Committee, thesecond phase of trial was conducted in the period from mid-April to earlyMay 1999.

2. The scope of the second phase trial covers the following areas:(a) to gather more information about mobile reception;(b) to find out the protection levels required for mobile

reception;(c) to explore the feasibility of transmitting text and graphics;

and(d) to compare indoor receptions of simulcasted DAB signals

against FM signals.

3. This paper reports the arrangement of tests and the findings ofthe Technical Sub-Committee from the second phase of the trial.

Equipment Set-up and Test Methods

Test parameters4. Technical parameters such as sound quality, bit-error-rate andfield strength under different error protection levels, mobile reception andindoor reception environments were evaluated. The configuration of thetwo-transmitter single frequency network remained the same as that usedin the first phase. All the tests were conducted at full functioning of thetrial network during which the two transmitters were on-air at the sametime.

Programme channel arrangement5. To facilitate the comparison of DAB sound quality against FMsound quality, the programme content of the DAB multiplex had beenrearranged to include live FM broadcast as shown in the following table:

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ProgrammeChannel

Content Bit-rate1

(kbit/s)Protection

level2

1 (ref.) CD music 192 32 CD music 192 23 CD music 192 44 Live FM 192 35 Live FM 192 36 Live FM 192 3

Field measuring equipment6. The field measuring system used in mobile reception test hadbeen upgraded from Rohde & Schwarz TS9951 previously used in thefirst phase to a newer model TS6200. The configuration of TS6200 issimilar to its predecessor with the exception that one of its elements, thePhilips DAB452 test receiver, has been replaced by the newer modelDAB752. In the first phase, we had adopted the technical parameter"error-flag-rate (EFR)" in the range 1 to 6 as the indicator of the receptionperformance. However, the new model has dropped this six-gradepresentation of EFR and replaced by its actual measured values. Forconvenience, another parameter, the "error-flag-count (EFC)", wasadopted this time as the receiving performance indicator since it isdirectly proportional to EFR. Before the commencement of the secondphase trial, we had conducted stationary reception and mobile receptionexperiments to determine the threshold EFC value for differentiatingbetween satisfactory reception and on-set of noise. The results revealedthat EFC values exceeding 2800, 3500 and 1600 respectively for channels1, 2 and 3 would indicate a range of receiving audio from short radiodropouts to complete muting.

Text and graphics transmission test set-up7. Each audio programme may contain optional ProgrammeAssociated Data (PAD) which are used to convey information insynchronisation with the sound programme. At the Mt. Gough site,PAD were supplied to the DAB source encoder at which the data wereencoded with the audio bit-streams of programme channels 1, 2 and 3.As recommended by the contractor, PAD information were sent at themaximum speed of 16 kbit/s which supported data rates of 1 kbit/s fortext and 15 kbit/s for graphics. A few pieces of textual information andstill pictures were transmitted repeatedly in an endless loop.

1 Bit-rate should not be lower than 160 kbit/s for good quality music.2 Level 3 is generally good enough for normal reception.

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8. Due to the limitation of the field measuring system which doesnot provide indicating parameter about the bit-error-rate on the PADchannel, an alternative assessment method was used. An audio and dataversion of DAB car receiver, the Hannover DAB106D with a separate 4-inch colour LCD monitor, was employed for the test. During fieldmeasurement, text or graphics material was individually selected andvisually assessed along the test routes. Any breaks or interruptions inthe refreshing of screen pages were recorded manually on road maps.

Indoor reception test set-up9. The same car radio, Oritron DAB, with antenna arrangementprevious used for outdoor/indoor reception tests was again used here forindoor reception test. Depending on the size of the buildings selectedfor the test, sampling points were taken as many as possible on floors oflow, medium and high levels. For comparison, the FM broadcast servedby either the Mt. Gough or the Beacon Hill site for that particular areawas also subjectively assessed by listening to portable FM receivers.The reception quality of simulcast DAB programme channels 4, 5, and 6were compared with their corresponding live FM programmes. Thesound quality of DAB were graded A, B and C respectively forconditions of perfect reception, intermittent noise and mute whereas thesound quality of FM were graded Good, Fair and Noisy. In addition, thefield strengths of DAB and FM signals were measured by a Rohde &Schwarz ESVB field strength meter and an Anritsu ML524B measuringreceiver for reference.

Results

10. The following tests were conducted:(a) Field measurement;(b) Text and graphics transmission test;(c) Protection level test for mobile reception; and(d) Indoor reception comparison.

Field measurement result11. Test routes for the field measurement were selected in 10densely populated areas with high density of buildings and narrow roadssuch as Sai Ying Poon, Mongkok and Sham Shui Po. They wereselected in order to broaden the variety of test routes since the previousroutes in the first phase were mainly wide and open roads across districts.The results on programme channel 1 are shown in Annex 1(a) to (c).

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12. The result indicates that majority of the reception along theroutes were satisfactory with a few blind spots in North Point near QuarryBay. It was due to obstruction of signal from Mt. Gough by the hillyterrain of Tai Hang and Braemar Hill. These areas could not besufficiently supplemented by the 37º off-set Southern beam from BeaconHill.

13. A table showing the statistics of the routes travelled is shown inAnnex 1(d). The total length of test routes in 10 areas is about 94.6 kmand among this, about 94 km was well severed. In other words, thepercentages are more than 99%.

Text and graphics transmission test result14. Text and graphics transmission test was conducted concurrentlywith the field measurement. Interruptions to the refreshing of either textor graphics pages happened coincidentally with impairments to thereceived audio. The performance was similar to that of the fieldmeasurement. It was noticed that there was slight delay in theinterruption of page refreshing observed by the viewers. It is because ofthe characteristic of the receiver which keeps on displaying the last pageat time of failure in reception of the updated information. It can bededuced that the percentage of reception had also reached 99% of thetotal travelled distance.

Protection level test result15. Four test routes were selected at marginal service areas which lieat the edge of the trial network coverage in Kwai Chung, Ma On Shan,Quarry Bay and Repulse Bay in order to find out the effect of differentprotection levels to the received quality under mobile receptionenvironment. Each of the route was repeatedly travelled three timesrespectively for reception of programme channels 1, 2 and 3. Theresults are presented on a table in Annex 2. The percentages of grade Areception quality were calculated for comparison. It agrees withtheoretically predicted performance as programme channel 2, which wasencoded with the highest protection level, had achieved the highestpercentage. It was then followed by channel 1 and 3 which achievedsubsequent lower scores.

Indoor reception comparison result16. Due to the strong dependence of the building penetration losson different types of buildings, the indoor reception test in the secondphase was more comprehensive that covered large variety of buildings

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and areas. Within the expected coverage of the trial network, a total of25 locations including 16 public housing estates, Government officebuildings, private organisation office buildings and shopping malls wereselected for the test. The result was summarised on a table in Annex3(a).

17. The average percentages of reception at the 430 sampling pointsin 25 locations by FM receiver and DAB receiver are 90% and 72%respectively. The indoor DAB reception was in general lower thanconventional FM broadcast. This is mainly due to the difference ofradiated powers. The radiated power of DAB is 607W while the FMradiated power is 3000W. In addition, FM receivers can still work atlow field strength (as low as 40 dBµV/m) at the expense of lower signal-to-noise rate at the audio.

18. Based on the performance of the car mounted receiver used inthe test, which has a sensitivity of –85 dBm, the field strengths collectedhave further confirmed that the minimum required field strength forindoor reception is about 51 dBµV/m. Of course, a better or lowerminimum required field strength may be achieved with other receivers ofbetter sensitivity. Statistical figure derived from the frequencyhistogram of indoor field strengths in Annex 3(b) shows that thepercentage of field strength equal to or greater than 51 dBµV/m is71.63% which is very close to the average percentages of reception.

Conclusion

Outdoor coverage19. Over 99% of the total test routes got a satisfactory reception.Since the test routes selected in this phase were conducted amongrelatively narrower roads in areas with high density of buildings, theoverall outdoor coverage by the two-transmitter trial network,theoretically, should approaching 100%. Outdoor reception isconfirmed to be satisfactory and it demonstrates a good combat ofmultipath effect.

Capability of text and graphics transmission feature20. The capability of transmitting text and graphics materials by thePAD feature is affirmative. Interruptions to the refreshing of the testmaterials happened coincident with the locations of audio impairment.Its performance is similar to the reception of sound but without theannoying intermittence or audio muting known by the viewers. Since

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the last received material is stored and kept on displaying withoutcorruption, the viewers may not even know the reception of material hasbeen interrupted. The refresh rate (number of pages per unit time)depends on the complexity of the materials delivered. Experienceshowed that the refreshing of a page of text and a page of graphics wouldtake about one second for the former and over 15 seconds for the latter.Due to the low refresh rate, the PAD feature will only be limited fordelivery of non time sensitive information, such as station identity,enhanced radio commercials, CD covers or song titles etc.

Protection level required21. Result from the protection level test in mobile environmentagrees with theoretically predicted performance. Its performance issimilar to the performance found in the stationary reception outdoors inthe first phase. Protection level 3 is considered to be generally goodenough for normal operations. Indoor coverage and possible solutions22. Taking conventional FM broadcast as a reference, the successfulreception rate of DAB at indoors is somewhat lower than FM in the ratioof 0.8 to 1. In other words, the trial network has achieved an overall of80% of the penetration of those of the current FM broadcast service atindoors. This deficiency would mainly affect the 20% of portablereceivers at indoors but not the types of HI-FI tuner and PC add-onreceivers since they would be assisted by In-Building Coaxial CableDistribution Systems (IBCCDS) or self-installed external antennas. Itshould be noted that the indoor performance would be significantlydifferent from the result if the radiated power at Mt. Gough had beenboosted to the same level as the current FM effective radiated power(ERP) of 3000W. This would be a 7dB increase in power relative toexisting ERP of 607W. Since the DAB coverage is overlapped bysignals of two transmitters, the field strengths would not be increased bythe same amount. But for a conservative estimate of field strengthincrease by 4 to 5 dB, the percentage of indoor reception would betheoretically improved to a certain extent relative to the penetration ofFM service, in accordance with the statistics shown in Annex 3(b). Infact, in future planning of an operative DAB network, the radiated powerof every transmission site should be commensurate with the requiredcoverage of the transmitter, the larger the coverage the higher the power.

23. By the time when portable receivers become popular, remedymeasures would be required to address the shortcoming of indoorpenetration. Three possible solutions are conceivable for improving the

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situation, the installation of external antennas by the audience, themodification of IBCCDS to include distribution of DAB signals and theinstallation of gap-fillers by the broadcasters.

24. The first solution requires the audience themselves to installsimple external antennas (whips of about 5 cm) at balconies or windows.This applies to flat units without the installation of IBCCDS carryingDAB down-converted signals.

25. The second solution would require the installation of L-band toVHF band frequency down-converters and VHF channel amplifiersbefore the cable distribution network of IBCCDS. The amplified VHFsignals are then distributed to individual flat unit via their existing cablenetworks. The VHF frequencies selected for carrying DAB ensemblesshould be compatible with prevalent DAB receivers. The equipmentcosts involved would be borne by the audience as this is similar to currentpractice of distributing FM broadcast signals via the IBCCDS. Atypical 8MHz television channel bandwidth would be able toaccommodate up to five DAB carriers or 30 near CD-quality stereophonicprogrammes for future growth and expansion of DAB services.

26. The third solution would require the broadcasters to add low-power gap filling signal repeaters down to building roof-top level insignal underserved areas. However, the equipment and recurrent costsinvolved would be tremendous and only be justified in light of hugedemand by the public in highly populated residential areas at the timewhen portable DAB receivers are popular.

27. It is anticipated that, by implementing a combination of thesuggested solutions, the indoor penetration deficiency should betechnically solved.

Technical Sub-CommitteeSteering Committee for DAB TrialJuly 1999

[C:\My_doc\DAB\Phase2\Report_Ph2 14.7.99]

Annex 1(a)

Field Measurem

ent Result in H

ong Kong Island

Green : Good reception

Red : Audible impairment

Annex 1(b)

Field Measurem

ent Result in K

owloon



Annex 1(c)

Field Measurem

ent Result in Shatin



Annex 1(d)

Statistics of Field Measurement Result

Test Route Grade A Grade B or C Sub-totalCheung Sha Wan / Sham Shui Po

/ Shek Kip Mei 6312.94 0 6312.94

Tai Kok Tsui / Mong Kok 5153.59 0 5153.59

Tai Wai / Shatin 10783.97 0 10783.97

Choi Hung / San Po Kong /Kowloon City 22268.55 0 22268.55

Central / Sheung Wan / Sai YingPun 12526.87 0 12526.87

Aberdeen / Apleichau / WongChuk Hang 13821.16 0 13821.16

Happy Valley / Tai Hang 8358.05 0 8358.05

Wan Chai / Causeway Bay /North Point 6154.57 550 6704.57

Hung Hom / Tokwawan 6110.62 0 6110.62

Yau Ma Tei / Tsim Sha Tsui 2553.37 0 2553.37

Total 94043.69 550 94593.69

Percentage 99.42% 0.58% 100%

Distance (m)

Annex 2

Protection Level Test Result

Protection Kwai Chung Level Total Sample Pts.

Channel 1 3 461 14 3.04%Channel 2 2 501 69 13.77%Channel 3 4 467 2 0.43%

ProtectionMa On Shan Level Total Sample Pts.


ProtectionTai Koo Shing Level Total Sample Pts.


ProtectionChung Hum Kok Level Total Sample Pts.


EFC

EFC

Grade A Sample PointsEFC

EFC

Grade A Sample Points



Annex 3(a)

Indoor Reception Test Result

No. ofLocation / Building Sample No. of No. of

Points Points Percentage Points PercentageWan Tau Hom Estate / Wang Wai House 21 16 76% 21 100%

Tung Tau Estate / Chun Tung House 21 17 81% 21 100%

Ka Wai Chuen / Ka Yee Lau 18 12 67% 17 94%

Chun Seen Mei Chuen / Chi Chun Lau 21 18 86% 21 100%

Yue To Sang Building 4 2 50% 4 100%

40, Waterloo Road 11 5 45% 9 82%

CR Building 25 19 76% 24 96%

Wo Che Estate / Shun Wo House 21 20 95% 21 100%

Sha Kok Estate / Sand Martin House 20 20 100% 20 100%

Pak Tin Estate / Tsui Tin House 15 6 40% 12 80%

Cheung Sha Wan Estate / Block 12 12 12 100% 11 92%

Mei Lam Estate / Mei Fung House (Blk B) 15 15 100% 15 100%

Sun Chui Estate / Sun Hok House 21 13 62% 21 100%

Wu Chung House 14 3 21% 10 71%

North Point Gov't Office 24 9 38% 5 21%

Trade Dept. Tower 12 2 17% 9 75%

Sheung Wan Civic Centre 4 4 100% 4 100%

City Hall 6 6 100% 6 100%

Oi Man Estate / Tak Man House 21 21 100% 21 100%

Ho Man Tin Estate / Block 8 15 15 100% 15 100%

Lei Tung Estate / Tung Hing House 21 16 76% 21 100%

Wong Chuk Hang Estate / Block 5 15 11 73% 15 100%

Whompoa Garden Shop Malls / Site 2, 11 & 12 31 4 13% 15 48%

Lai Tak Chuen / Tsuen Wing House 21 20 95% 21 100%

North Point Estate / West Main Block 21 19 90% 20 95%

Total Number of Locations: 25

Total Number of Sample Points: 430

Average % of Reception over Total LocationsAverage % of Reception over Total Sample Points

DAB (Grade A) FM (Good or Fair)

DAB(Grade A)72%

71%

FM (Good or Fair)90%

88%

Annex 3(b)

Frequency Histogram of Indoor Field Strengths

Field Strength Percentage of(dBuV/m) Reception≧51 71.63%

≧48 80.23%

≧47 83.26%

≧46 87.21%

≧45 89.30%

0

5

10

15

20

25

30

40 45 50 55 60 65 70 75 80 85 90 95

Field Strength (dBuV/m)

Occ

urre

nce

report on the second phase of digital audio broadcasting

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