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Digital Radio Part 4: Signal Formats & DAB+/DRM Comparison In this final instalment we discuss the signal formats and give a comparison of DAB+ and DRM. Also included is a brief discussion of antennas suitable for DAB+ reception. D igital TV & radio systems have two reception conditions. They either provide good sound and picture quality or there is no reception at all. Perhaps we should qualify this by saying that when you are on the brink of signal failure, digital TV can be plagued with partial pixellation of the picture and loud clicking in the sound as it drops in and out. So under a variety of reception conditions, there is no gradual reduction of sound and picture quality. To produce the designed quality there must be adequate signal and no noise, particularly impulse noise. Impulse noise is generated by lightning strikes, electrical switching, arcing power line insulators, arcing in brush motors and unsuppressed ignition systems in petrol motors. So for digital radio it is important to get reliable reception otherwise the sound will have annoying gaps in it. Errors in digital radio systems The effects of errors are: • Corrected Errors are not detectable by the listener. 8% of the transmitted signal is mathematically related A single Audio Superframe. Five blocks of synchronisation, transmitter identification and Fast Information Channel data, shown in black, white and grey have been inserted. to the data being sent. So the additional error correction data can be used to correct errors in the main signal. • Detected Cyclic Redundancy Check enables the detection but not the correction of the main signal. So when an error is detected the errors are concealed as described below. • Interleaving is shuffling the data in time. The advantage of interleaving means that a burst of noise is distributed over a much greater range of bytes so that error correction and detection is more likely to be effective. • Differential Grey Code Absolute numbers are not transmitted, the amount of change is transmitted instead, so if one value is 7 and the next value is 0 the number 7 is converted into grey code. For any number change in Grey Code only one-bit changes in the byte. So if the Grey decode detects more than one bit change in a byte, that byte is detected as an error. For example, a change between 7 to 0: Decimal 7, binary 111, Grey Code 100 Decimal 0, binary 000, Grey Code 000 Error Concealment Three Audio Units shown in three rows. This is a single Audio Superframe, which contains 120ms of audio 58  Silicon Chip The decoder will use the previous two audio units to guess the values for a corrupted audio unit. If subsequent siliconchip.com.au MEDIA RELEASE 1 May 2009 Perth first to switch on digital rad io Commercial radio stations in Perth – Mix 94.5, 92.9, Nova 93.7, 6PR, 96 fm, 6ix, Radar, Pink Radio and Novanation create rad io history on Monday, 4 May 2009 when they begin broadcasting the first permanent DAB+ dig ital radio services in Australia. by Alan Hughes audio units are also in error the signal will be faded out. A fade in is performed by the decoder when the data error rate drops. Errors in the Parametric Stereo signal are camouflaged by assuming that the sound source is stationary. Data organisation An Audio Unit Data Block consists (in time order) of the following: • Header: two bytes of error correction, one byte of program characteristics, 14 bytes to identify which audio unit it is in an audio superframe and a CRC check. Cyan, purple, yellow & blue. • Program Associated Data: consisting of two bytes of fixed length and some variable length data. Shown hatched • Scaling signal: it tells the decoder to change the loudness in various sized steps. • The filters with the loudest signal which is not masked by another loud adjacent frequency. The difference in amplitude is added to the data. • A Spectral Band Replication signal. • A Parametric Stereo signal to indicate the direction of the sound and the nature of the reverberation. • 3 x 10 bits of Reed-Solomon error correction data Shown pale blue. The advantage of having five blocks of synchronisation is that when the receiver is searching for a program it will spend less time waiting for the transmitter identification and can synchronise faster during tuning and after a noise burst. Each program has an Audio Superframe in a sequence up to nine programs. Some Audio Superframes can be replaced with data in a block the same size if the broadcasters wish to do so. Comparison of DAB+ and DRM We’ve been concentrating on the DAB+ digital radio standard because it is the system now being introduced to siliconchip.com.au Joan Warner, chief executive officer of Commercial Radio Australia the ind ustry body that has driven the move to digital rad io said Monday was a milestone for the industry in Australia and is the biggest innovation in radio sinc e the introduction of FM in the 1970’s. “The switch on of digital radio is a culmination of seven years work with the Fed eral Government, the Australian Communications and Media Authority (ACMA), commercial broadcaste rs, the ABC and SBS, together with retailers and manufacturers of digital radios to ensure a comprehensive and coordinated switch on of a com pelling new way of listening to radio,” said Ms Warner. “The Australian radio industry has invested in and created its digital future and will compete with other digital technologies and continue to maintain radio’s relevance in listener’s lives,” said Ms Warner Ms Warner said for the first we ek to 10 days the DAB+ broadcasts will be in inte rference test mode which means that the power ma y be lower at night while any interference is assess ed. Commercial digital radio service s are expected to be switched on in each city from the dates below barring any weather delays. For the first 10-14 days services might be on low power at night as any potential interference is add ressed. Perth Melbourne Adelaide Brisbane Sydney - 4 May - 11 May - 15 May - 25 May - 30 May ABC and SBS are expected to commence digital services throughout June/July. For further information on digital radio visit: www.digitalradioplus.com.au June 2009  59 Notes & Errata The text on page 12 of the April 2009 issue on COFDM multiple carriers refers to column data being “fed into an analog to digital converter (DAC).” It should read Digital to Analog Converter (DAC). Also Fig.2 on page 12 of the same issue is a generic functional diagram. As a result the DAC is shown feeding the Inverse Fast Fourier Transfer. In reality, the IFFT is done digitally, so the DAC will be fed from the IFFT instead of feeding it. Fig 3. A 4-QAM only allows four conditions shown as purple dots. Note all of the 4 conditions have the highest power signal from the tower. This gives the best signal reliability but at the expense of a low data rate of transmission. The purple dots are also part of 16 and 64 QAM. 64-QAM’s maximum radiated power is the same as 4-QAM but the minimum radiated power is 18 dB lower. So you transmit much more data but it is more likely to be affected by noise. And Fig.4 on page 13 shows ADCs following the IFFT. In fact, the chip manufacturer controls the location of the ADCs. QAM demodulation can be done digitally or in analog then digitised. So a single high speed ADC can digitise the IF signal or the QAM demodulation can be performed prior to the Fast Fourier Transform. Finally, a DAB+ OFDM makes the signal on each carrier 1536 times longer than when a single carrier is used. Since the pulse is so much longer it can be sampled more than once. This is a similar technique to eliminating contact bounce on computer keyboards. A change of state is detected and is then rechecked on the next sample period to ensure the first sample was not an error. Once the consecutive samples are identical, any further samples can be ignored until the next change of state. Hence delayed signals can be ignored. 60  Silicon Chip Parameter Coverage area Number of programs per channel Operating Frequencies Possible channels Repeaters DAB+ Modes DRM Robustness modes Audio Sample rates Sound Quality Parametric Stereo Maximum bit rates Audio Superframe DAB+ DRM Region (<100km radius) Terrain affected <9 174 – 240MHz (Band 3) 1,450 – 1,500MHz Local to >2,000km Regardless of terrain 1 or <4 speech 0.5265 – 1.6065MHz, 2.3 – 2.495MHz (MF) 5.9 – 26.1MHz (HF) 22 (B3) + 22 (L) Depends on area and terrain Four modes: Single Frequency Networks, Cable and two frequency bands 24ksample/s SBR Full audio frequency range using SBR Yes 1152kb/s thus allowing multiple channels 3 Audio Units 69 MF, 221 HF Not required Four levels up to high speed moving receiver and long periods allowed for reflections. 24ksample/s SBR Full audio frequency range using SBR Yes 64 - 72kb/s (18 20kHz channel width) 10 Audio Units The main differences between DAB+ and DRM are the frequencies used and therefore the coverage. DRM operates on much lower frequencies which offer significantly greater range than DAB+. This can be both a blessing and a curse! Australia. However, it’s not the only one. You may have heard of DRM (Digital Radio Mondiale) which is a system used in several overseas countries. The table below shows the similarities and differences. The DAB+ and DRM transmission systems are very similar except for the frequency bands used. The main advantage of DRM is that there will be reception regardless of location, not only for fixed installations but also in moving vehicles. This would allow high quality sound regardless of location. DRM could also be used by Radio Australia for international transmissions. Australia has 45 transmitter sites with a FM transmitter with a radiated power of 20kW. These sites each cover a regional area and are ideal for DAB+. Four high-powered DRM sites could cover all of Australia with state-based programs for national broadcasters. There are at least 246 sites which could be replaced by the four sites. Radio New Zealand International has been broadcasting DRM since 2006 and because of the low frequency, these signals are often receivable in Australia (particularly the east coast). The website www.rnzi.com/index. php shows the frequency schedule. They are using a 10kHz-wide channel and ruggedness mode B. The audio is 64-level QAM. The sound is mono. Radio Australia has been experimenting at their Brandon, Qld site with a low-powered HF DRM signal. Standards Australian Standard 4943.1-2009 DAB+: ETSI TS 102 563 DAB: ETSI EN 300 401 DRM: ETSI ES 201 980” Antennas for digital radio On portable and clock radios, etc, DAB+ channels 5A – 13F will use the same antennas as currently used – a telescopic rod, a wire or an antenna made up of the headphone leads. The optimum lengths for FM reception are 767mm; for DAB+ Ch 5A – 13F, 362mm and for DAB+ Ch LA – LW, 204mm. The coverage area of DAB+ channels 5A – 13F is similar to that of digital TV channels 6 – 12 (ie, 175 to 224MHz; VHF Band 3). However, in weak signal areas a vertically-polarised Band 3 Yagi-Uda antenna (ie, mounted so that the elements are vertical) should give reliable reception. A Yagi should be mounted so that its siliconchip.com.au 650 mm 350mm 794mm TRANSMITTER IN THIS DIRECTION INSULATED BLOCK 75 COAX CABLE TO THE RECEIVER 300: 75 BALUN 650 mm 361mm 350mm 794mm 361mm A Yagi antenna is a good choice for DAB+ reception.A  3-element simple beam  suitable for digital TV should  be adequate in most areas.  If you want to try  making one yourself (and it’s not  that hard!), follow these dimensions. TOTAL DIPOLE LENGTH 722mm              MAST boom is aimed at the transmitter, with the longest element away from the transmitter. Do not connect these antennas to the downlead used for TV reception. This is likely to produce broken up pictures and sound in digital TV programs and may cause patterning to analog TV reception. So you need to use a separate down lead to the DAB+ radio receiver – almost certainly 75Ω coaxial cable. As with all coax, you get what you pay for – and if you’re in a low signal area, you’re going to need high quality, low-loss type. MATV systems will need a separate channel amplifier to control the signal level and filter out TV signals picked up by the above antenna. What’s available? As far as we can tell, there are currently no local antenna models available to receive the whole DAB+ band, including channels 13A – 13F; however some manufacturers have indicated they will probably gear up when demand picks up. We have found one imported antenna, the Spanish-made Ikusi DAB030, which does cover the whole DAB band. It’s available through Ikusi Australia distributors. Contact Ikusi on (03) 9720 8000 or visit their website (see below). Some suitable antennas for the lower section of the band include: Hills DY4 – DY14 Matchmaster 03-DR3004 – 03-DR3018 Fracarro BLV4F, BLV6F           Links www.digitalradioplus.com.au http://worlddab.org http://DRM.org http://infostore.saiglobal.com http://pda.etsi.org/pda/ www.ikusi.com SC The Ikusi DAB030, shown at right, covers the whole DAB+ band from 175 to 240MHz. As far as we can tell, it’s the only one which currently does so. siliconchip.com.au                 June 2009  61