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Fig.1 (above): the blue areas of this map show the countries that are currently broadcasting regular DRM services, while those countries that are either conducting trials or have decided to become involved in DRM are shown in yellow. DRM Digital Radio What It’s All About . . . You have heard about DAB+ digital radio which has about the same line-of-sight range as FM transmissions. Now there’s DRM which stands for “Digital Radio Mondiale”. It’s the new international standard for long-distance digital radio broadcasting on the long wave, medium wave, shortwave and VHF bands. In this article, we explain the basics of DRM and how it works. And elsewhere in this issue we tell you how to receive and decode DRM signals using your PC with our SiDRADIO. By JIM ROWE 20  Silicon Chip siliconchip.com.au DRM (Digital Radio Mondiale) TRANSMISSION MODES, OPTIONS & CHARACTERISTICS VARIANT TYPICAL USES MODE A LF & MF GROUND-WAVE, 26MHz BAND LINE-OF-SIGHT B MF & HF TRANSMISSION ON SKY-WAVE DRM30 C D DRM+ E DIFFICULT SKY-WAVE CHANNELS ON HF NVIS SKY-WAVE (HIGHEST DOPPLER & DELAY SPREAD) VHF TRANSMISSIONS IN BANDS ABOVE 30MHz SIGNAL BANDWIDTH OPTIONS (kHz) MSC QAM CODING OPTIONS MAX. ROBUSTNESS (4.5, 5) 16-QAM 13.1 kb/s 16.4 kb/s 9 64-QAM 19.7 kb/s 30.9 kb/s 10 16-QAM 14.8 kb/s 18.4 kb/s (18, 20) (4.5, 5) 64-QAM 22.1 kb/s 34.8 kb/s 16-QAM 10.2 kb/s 12.8 kb/s 9 64-QAM 15.3 kb/s 24.1 kb/s 10 16-QAM 11.6 kb/s 14.6 kb/s (18, 20) 64-QAM 17.5 kb/s 27.4 kb/s 16-QAM 9.2 kb/s 11.5 kb/s 64-QAM 13.8 kb/s 21.6 kb/s 16-QAM 19.3 kb/s 24.1 kb/s 64-QAM 28.9 kb/s 45.5 kb/s 16-QAM 6.1 kb/s 7.6 kb/s 64-QAM 9.1 kb/s 14.4 kb/s 16.2 kb/s 10 20 10 20 100 APPROXIMATE AVAILABLE BIT RATE MIN. ROBUSTNESS 16-QAM 13 kb/s 64-QAM 19.5 kb/s 30.6 kb/s 16-QAM 99.4 kb/s 186.3 kb/s 4-QAM 37.2 kb/s 74.5 kb/s Fig.2: DRM transmission modes, coding options and available bit rates. The ‘main service channel’ or ‘MSC’ (ie, the digital audio channel itself) of both DRM30 and DRM+ signals is generally modulated onto the RF subcarriers using quadrature amplitude modulation (QAM). D RM HAS BEEN developed and is being promoted by the DRM Consortium, an international not-forprofit group which has over 93 member organisations in 39 countries. Many of the members are broadcasters but there are also many transmitter and receiver manufacturers, as well as broadcasting standards bodies. The aim of the Consortium is to support and spread a digital broadcasting system suitable for use in all of the frequency bands up to VHF band III. You can find more about the DRM Consortium at www.drm.org By the way, ‘mondiale’ means ‘world wide’ in French and Italian. There are two main variants of DRM. First, there is DRM30, intended specifically for use on the traditional low, medium and high-frequency (shortwave) bands below 30MHz and the existing AM broadcasting channels within them. The other variant is DRM+, intended for use at VHF and in particular for FM broadcast Band II (87.5-108MHz). Both variants use orthogonal frequency division multiplexing (OFDM) for reliable transmission and digital compression/coding for high spectrum efficiency. They can also carry digital data services along with the audio signals, such as station names, time, date and program information. DRM30, DRM+ and DAB+ So where does DAB+ fit into this siliconchip.com.au proposed DRM future? After all, we’ve now had digital radio broadcasting in Australia for the last four years or so using the DAB+ system but it’s been confined to the larger cities and their suburbs. There is no definitive answer as yet. It looks as if DRM30 is likely to become the world standard for digital radio broadcasting below 30MHz but DRM+ might well end up competing with DAB+ in the VHF and UHF bands. This is quite possible, because DRM+ is being promoted as a replacement for analog FM broadcasting in the 88-108MHz band while DAB+ is now firmly established in the 174-240MHz band (Band III). We’ll just have to wait and see what happens. One possibility is that receivers able to receive both DAB+ and DRM+ may become popular. So what’s the difference between DRM and DAB+? In fact, there are many similarities and not many differences. Both are digital audio broadcasting systems which use OFDM – the technique of modulating digital information on an array of closelyspaced RF subcarriers, instead of a single main carrier. This is exactly the same kind of modulation used in DVB-T television, wireless LANs (IEEE 802.11a, g & n) and ADSL broadband over copper telephone lines. Each carrier is 90° out of phase (ie, orthogonal) with its neighbours on either side, to reduce mu- tual interference. And both DRM and DAB+ use a digital signal processing (DSP) coding/compression algorithm known as MPEG-4 High Efficiency Advanced Audio Coding v2 (HE-AAC v2) to process the digital audio samples for modulation of the multiple-frequency OFDM subcarriers. The differences between the two systems are rather more subtle. DAB+ appears to use 1536 subcarriers trans­ mitted in parallel, each with a bandwidth of 1kHz and spaced apart by the same figure. This gives a DAB+ subcarrier ‘block’ a total bandwidth of 1.537MHz. However, since this block can convey as many as 16 different high quality digital audio signals as well as their accompanying data, DAB+ signals tend to be grouped together in ‘multiplexes’ whereby the separate broadcasting signals are effectively mixed together into a single DAB+ subcarrier block for transmission. The individual signals are separated again in the receiver. In contrast with this DAB+ multiplexing system, DRM30 has been designed specifically for use in the AM bands below 30MHz. As a result, its individual broadcasting signals are generally encoded so that each one fits neatly into the 9kHz or 10kHz channels traditionally used in this part of the spectrum. DRM30 is also capable of encoding into 18kHz or 20kHz channels, for higher quality or greater reliability. November 2013  21 MSC SDC MODES A – D (DRM30) FAC FAC FAC TRANSMISSION FRAME (400ms) TRANSMISSION SUPER-FRAME (1200ms) MSC SDC MODE E (DRM+) FAC FAC FAC FAC TRANSMISSION FRAME (100ms) TRANSMISSION SUPER-FRAME (400ms) Fig.3: how the three data channels are grouped into the data stream transmitted in DRM30 and DRM+ digital broadcasting. DRM30 modes group the data into 1200ms-long ‘super frames’ consisting of three frames 400ms long, while DRM+ groups the data into 400ms-long super frames each consisting of four frames 100ms long. Similarly, DRM+ is designed to encode single mono, stereo or surround sound signals into a channel 100kHz wide, making it compatible with the FM channel structure used in the Band II VHF spectrum. Modes, bandwidth & QAM options To achieve the desired level of performance on the bands below 30MHz, DRM30 broadcasters use four different encoding modes designated “A”, “B”, “C” and “D”, while DRM+ broadcasters use a fifth encoding mode designated (you guessed it!) “E”. Each of these modes is designed to achieve the best performance in a different broadcasting application, as you can see in the table of Fig.2. You’ll also note from this table that the ‘main service channel’ or ‘MSC’ (ie, the digital audio channel itself) of both DRM30 and DRM+ signals is generally modulated onto the RF subcarriers using the quadrature amplitude modulation (QAM) system. DRM30 broadcasters have the option of choosing either 64-QAM or 16QAM coding, while DRM+ broadcasters can use either 16-QAM or 4-QAM. The idea behind this is that 64-QAM CURRENT DRM30 TRANSMISSIONS IN THE SOUTH PACIFIC TIME (UTC) TIME (EAST) FREQUENCY BROADCASTER TARGET AREA 04:59 – 06:50 14:59 – 16:50 11675 kHz RADIO NEW ZEALAND PACIFIC 06:51 – 07:58 16:51 – 17:58 9890 kHz RADIO NEW ZEALAND TONGA 07:59 – 10:58 17:59 – 20:58 9890 kHz RADIO NEW ZEALAND PACIFIC 10:59 – 12:00 20:59 – 22:00 9890 kHz RADIO NEW ZEALAND PACIFIC 15:51 – 17:45 01:51 – 03:45 7330 kHz RADIO NEW ZEALAND COOK ISLANDS 17:46 – 18:35 03:46 – 04:35 7330 kHz RADIO NEW ZEALAND COOK IS, SAMOA, TONGA 18:36 – 18:50 04:36 – 04:50 9630 kHz RADIO NEW ZEALAND COOK IS, NIUE, SAMOA, TONGA 18:51 – 19:35 04:51 – 05:35 9630 kHz RADIO NEW ZEALAND 19:36 – 20:50 05:36 – 06:50 15720 kHz RADIO NEW ZEALAND SAMOA, NIUE, TONGA 20:51 – 21:50 06:51 – 07:50 17675 kHz RADIO NEW ZEALAND SOLOMON IS, SAMOA, NIUE, TONGA 21:51 – 04:58 07:51 – 14:58 11675 kHz RADIO NEW ZEALAND PACIFIC 01:00 – 03:00 11:00 – 13:00 19000 kHz RADIO AUSTRALIA PACIFIC 07:00 – 09:00 17:00 – 19:90 7410 kHz RADIO AUSTRALIA SW PACIFIC SAMOA, NIUE, TONGA 09:00 – 11:00 19:00 – 21:00 9475 kHz RADIO AUSTRALIA SW PACIFIC 11:00 – 13:00 21:00 – 23:00 6080 kHz RADIO AUSTRALIA WEST PACIFIC, PNG 13:00 – 15:00 23:00 – 01:00 9890 kHz RADIO AUSTRALIA PACIFIC 15:00 – 17:00 01:00 – 03:00 5940 kHz RADIO AUSTRALIA SE ASIA 17:00 – 19:00 03:00 – 05:00 9475 kHz RADIO AUSTRALIA SE ASIA 14:00 – 18:00 24:00 – 04:00 5845 kHz BBC WORLD SERVICE SE ASIA Fig.4: current DRM transmission times & frequencies in the South Pacific area. With the exception of a BBC World Service transmission, they all come from Radio Australia and Radio New Zealand. 22  Silicon Chip can encode 64 points in its amplitude/ phase or ‘I/Q’ ‘constellation’, allowing the subcarriers to carry six bits of information in each digital sample or ‘symbol’ and hence a higher total bit rate. However, the 64 points in a 64-QAM constellation are inevitably closer together in both amplitude and phase, making it more susceptible to data corruption due to noise and interference. By contrast, 16-QAM has only 16 points in its amplitude/phase constellation, so the individual points are further apart – making it more suitable for noisy conditions, even though it can encode only four bits of information in each digital symbol (and hence a lower overall bit rate). The 4-QAM option available for DRM+ takes this trade-off even further, allowing it to encode only two bits per digital symbol and hence a lower overall bit rate again. But that’s not really too much of a problem when DRM+ signals are encoded into a 100kHz wide channel, as you can see from Fig.2. DRM’s three data channels Each DRM broadcasting signal consists of three basic data channels: (1) the Main Service Channel or ‘MSC’, which generally carries the encoded digital audio data; (2) the Fast Access Channel or ‘FAC’, which carries a set of data parameters allowing the receiving decoder to quickly confirm things like the modulation system being used in the DRM signals; and (3) the Service Description Channel or ‘SDC’, which carries ‘advance’ information like audio and data coding parameters, program service labels, the current time and date and so on. Fig.3 shows how the three data channels are grouped into the data stream transmitted in DRM30 and DRM+ digital broadcasting. DRM30 modes group the data into 1200ms-long ‘super frames’ consisting of three frames 400ms long, while DRM+ groups the data into 400ms-long super frames each consisting of four frames 100ms long. In both cases, the SDC data is transmitted across all subcarriers for a period of two symbols at the start of each super frame. For the rest of each super frame, the FAC data is transmitted using a specific sub-group of subcarriers during each transmission frame, while the coded audio data in siliconchip.com.au there are only a few DRM broadcast signals in our vicinity (ie, the South Pacific). In fact there are no DRM+ signals at all and only a few DRM30 signals – mainly those being broadcast by Radio Australia from Shepparton in Victoria and Radio New Zealand International (RNZI) in Rangitaiki, in the North Island. It’s true that the BBC directs a DRM30 broadcast into the South-East Asian area for a couple of hours each day (12pm – 2:00am EAST), from their transmitter in Thailand. However you may not be able to find this signal (5845kHz) unless you have a really good HF antenna – a very high longwire antenna, for example. You’ll find a list of current DRM30 broadcasts by Radio Australia and RNZI in Fig.4, which gives times in UTC and EAST (Eastern Australia Standard Time), together with the frequencies in use. How to identify DRM signals Fig.5: a DRM signal at 15,720kHz, as depicted in the waterfall display of the software program SDR# (running on the SiDRADIO described in the October & November 2013 issues). The DRM signal appears as a rectangular block. the MSC channel is transmitted using all the remaining subcarriers, in parallel with the FAC data for the rest of the super frame. DRM status world wide While we haven’t heard much about DRM in Australia, it’s now well established in the UK, many European countries, Canada, India and Russia. It is also in Australia and New Zealand, although you’d be excused for not being aware of this. Radio Australia broadcasts DRM30 on shortwave for three hours per day, while Radio New Zealand International broadcasts DRM30 for 20 hours per day (mainly to the Pacific Islands). To get a better idea of the current state of DRM broadcasting world wide, refer to the map in Fig.1. The countries which currently broadcast regular DRM services are shown in blue, while those which are either conducting trials or have decided to become involved in DRM broadcasting are shown in yellow. As you can see, DRM is already well established. In fact the Digital Radio Mondiale Consortium claims that there are now over 120 regular DRM services in over 24 different countries and that ‘half siliconchip.com.au the world’s population’ is now in a position to receive DRM (if they had receivers, that is). Actually, receivers capable of receiving DRM are in short supply and most DRM reception to date seems to have been achieved using PC-based SDRs. However, European manufacturers like Morphy Richards have been producing DRM30 receivers and the Chinese firm Chengdu NewStar Electronics is cranking up production of its DR111 DRM30 receiver. No doubt many other firms in China will follow suit. DRM30 signals in our vicinity As mentioned earlier, currently Sorting Out The Jargon Don’t get DRM (Digital Radio Mondiale) confused with DMR. DMR stands for Digital Mobile Radio, which is a protocol for narrowband mobile communications. Also note that the term DRM is also universally used as the acronym for Digital Rights Management, a class of technologies used to fight copyright infringement of digital content. So how can you identify a DRM signal when you’re searching for one? If you’re using a conventional analog shortwave receiver, it will sound a bit like a ‘white noise’ signal – just a lot of hissing, whether you’re searching in AM or SSB/USB mode. On the other hand if you’re using an SDR with a spectrum display (such as SDR# running on units like the SiDRADIO), a DRM signal will look like a rectangular ‘block’, as shown in the top area of Fig.5. The DRM signal shown here is not very strong and as a result, its top surface ‘dances around’ with variations in the individual subcarriers. If the signal were stronger, the top surface of the block would be smoother. Note that the ‘width’ of the block will depend on the DRM signal’s bandwidth. Most DRM30 signals seem to be 10kHz wide, ie, they occupy 10kHz of the spectrum. Waterfall plot The other thing to note is the waterfall plot in the lower area of Fig.5. As you can see, a DRM signal tends to display as a wide vertical ‘band’, quite different from the much narrower and varying width ‘sound track’ display produced by an analog AM signal. The band will be fairly solid if the DRM signal is reasonably strong but will tend to have diagonal bands if the SC signal is weaker. November 2013  23