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By JIM ROWE The Future of Radio Broadcasting? There is no doubt that DRM digital radio would provide the best way of extending radio broadcasting over the whole of Australia – and further. Here’s how it works. D RM or Digital Radio Mondiale was developed and is promoted by the DRM Consortium, an international not-for-profit consortium which has over 100 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” simply means “world wide” in both 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 on the existing AM broadcasting channels within them. The other variant is DRM+,which uses frequencies from 47-108MHz – these include the old analog TV channels 1 and 2 as well as the FM broadcast band. 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 proposed DRM-based digital radio future? After all, we’ve now had digital radio broadcasting in Australia for the last eight years or so using the DAB+ system. But because DAB+ works in VHF Band III (174–240MHz), it has a relatively short range and as a result is really only suitable for the larger cities and their suburbs. Although DRM30 looks set to become the world standard for digital radio broadcasting below 30MHz, DRM+ might well end up competing with DAB+ in the VHF band. This is quite possible, because DRM+ is being promoted as a replacement for analog FM broadcasting in the 88–108MHz band. Receivers able to receive both DAB+ and DRM+ – as well as DRM30 , analog AM and FM – are starting to appear. But what’s the difference between DRM and DAB+ anyway? In fact, there are many technical similarities and not many differences. All are digital audio broadcasting systems which use OFDM – the technique of modulating digital information on an array of closely-spaced 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 and n) and ADSL broadband over copper telephone lines. DRM has now been updated to xHEAAC which is backward-compatible to HE AAC V2. The xHE AAC can produce excellent speech quality at a much lower bit rate. DAB+ is yet to upgrade. HE AAC is used for sound in MP4 or MPEG4 video. These compression systems reduce the amount of data required for transmission so that it will fit in the channel bandwidth Vive la différence! The differences between the two Fig.1: the main difference between DRM30 and DRM+, apart from frequency, is the transmission frame length – 400ms for DRM30 vs 100ms for DRM+. siliconchip.com.au September 2017  63 systems are rather more subtle. DAB+ uses 1,536 subcarriers transmitted in parallel, each with a bandwidth of 1kHz and spaced apart by the same figure. This gives a DAB+ subcarrier channel with a total bandwidth of 1.537MHz and can convey between 15 and 26 different high quality digital audio signals as well as their accompanying data. The program data is assembled into blocks, labeled and each program is sent sequentially until all have been sent and then the sequence is repeated continuously. 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. Since Australian AM radio stations have an RF bandwidth of 18kHz, this can also be used. For HF broadcasting the bandwidth could be 5, 10 or 20kHz depending on frequency availability. The greater the bandwidth, the higher the reliability or better quality. DRM30, DRM+ and DAB+ can all transmit stereo sound but HF DRM30 can give continent-wide stereo coverage. Modes, bandwidth and 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 Table 1. 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 The idea behind this is that 64-QAM can encode 64 points in its amplitude/ phase or “I/Q constellation”, allowing the subcarriers to carry five bits of information in each digital sample or ‘symbol’ – and hence a higher total bit rate. However, the 64 points in a 64QAM constellation are inevitably closer together in both amplitude and This GR-216 DRM30 receiver has been evaluated by Tecsun Radios (Aust) and they have confirmed that it receives DRM transmissions from New Zealand in Australia. This receiver also handles AM and FM reception. modulated onto the RF subcarriers using the quadrature amplitude modulation (QAM) system. DRM30 broadcasters have the option of choosing either 64-QAM or 16-QAM coding, while DRM+ broadcasters can use either 16QAM or 4-QAM. phase, making it more susceptible to data corruption, due to noise and interference. In 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 4 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.1. DRM’s three data channels Table 1: the choice of frequencies, modes and coding options depends to a large extent on the coverage distance desired. 64  Silicon Chip The next thing to understand about DRM is that each DRM broadcasting signal consists of three basic data channels. There’s the Main Service Channel (MSC), which generally carries the encoded digital audio data; then there’s the Fast Access Channel (FAC), which carries a set of data parameters allowing siliconchip.com.au Table 2: Australia is significantly lagging behind when it comes to DRM broadcasts – this table shows Radio New Zealand’s DRM schedule to the South Pacific. the receiving decoder to quickly confirm things like the modulation system being used in the DRM signals. Finally there’s the Service Description Channel (SDC), which carries advance information like audio and data coding parameters, program service labels, the current time and date, and so on. Fig.1 should give you a basic idea of the way these three data channels are grouped into the data stream transmitted in DRM30 and DRM+ digital broadcasting. The 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 pe- riod 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 the MSC channel is transmitted using all of the remaining subcarriers, in parallel with the FAC data for all of rest of the super frame. DRM status world wide While we haven’t heard much about DRM as yet in Australia, it’s now well established in the UK, many of the European countries, Canada, India and Russia – plus in New Zealand. Radio Australia did transmit DRM30 on shortwave to Papua New Guinea from Brandon (Qld) but that ended in March 2015. Radio New Zealand International This “Avion” AV-DR-1401DRM Digital Radio sells on Amazon in India for about AU$330. Touted as India’s first DRM, it will also receive AM and FM broadcasts. siliconchip.com.au broadcasts DRM30 on shortwave for about 20 hours per day, mainly to the Pacific Islands. Receivers capable of receiving DRM30 are still in fairly short supply in Australia, and a lot of the DRM reception to date seems to have been using PC-based SDRs (software defined radios) – see our articles in the November 2013 issue of SILICON CHIP (www.siliconchip.com.au/Article/ 5456 and www.siliconchip.com.au/ Article/5459). However some of the European manufacturers like Morphy Richards have been producing DRM30 receivers, and Indian firm Avion Electronics (India) lauched its AV-DR-1401 radio recently. Chinese firm Gospell Digital Technology has also announced its GR-216 DRM receiver. Other DRM receivers you’ll find on the web are the Himalaya DRM2009, the Technisat Multiradio and the Uniwave Di-Wave 100. Why DRM30 for Australia? DRM30 digital broadcasting is particularly suitable for Australia, because of its much larger range. For example a DRM30 broadcast transmitter operating in the ‘AM’ band will have a range virtually identical to that of our existing analog AM broadcasters. And a 250kW HF DRM30 transmitter located in the geographical centre of Australia (Kulgera, NT) could cover just about all of the continent and surrounding waters. A much lower power DRM30 transmitter located in the geographical centre of Tasmania (Liena) could similarly cover the whole of that state. So adopting DRM30 would be the best way to ensure that ALL Australians received good broadcast radio reception – even those living in or moving through remote areas. And this brings up another point: DRM30 operating at HF provides much better reception in moving vehicles than either FM or DAB+ – which operate in the VHF spectrum. Best of all, though, is that existing AM and shortwave transmitters could in most cases be converted for DRM30 broadcasting at very low cost. The question really is this: why is Australia dragging its heels and letting just about all the rest of the world move into the digital radio future with DRM30 – when we could join them with very little outlay? SC September 2017  65