Digital Radio Mondiale (DRM) is set to revolutionise broadcasting on the long, medium and short wave bands. Since the very earliest days of broadcasting these wavebands have been filled with signals that are amplitude modulated. These transmissions are of low audio quality and particularly in recent years there has been a move away from these bands to find higher quality transmissions. Broadcasts in the VHF FM band have received far more listeners with the result that audience figures are dropping for AM broadcasting. Now DAB Digital Radio is available in many countries and this has set new standards in broadcasting. The next stage is to improve the transmissions on the long medium and short wave bands. As the requirements are very different to those experienced on the higher frequencies the DAB standard is not applicable and as a result a totally new system has been developed. Known as DRM it provides many of the improvements that are badly needed along with the flexibility to allow for future developments.
What is DRM?
DRM itself is a consortium of broadcasters, network operators, equipment manufacturers, broadcasting unions, regulatory bodies and other organisations representing 29 countries. It was founded in Guangzhou, China in 1998 and now has its headquarters in Geneva. Now with 82 members, the wide base of its membership has been part of the reason for its success. It has been able to draw on the experience of the membership to ensure that the resulting standard met the requirements, and it has also drawn on the experience gained by the Eureka project that was set up to develop DAB Digital Radio. As a result the new system has come to fruition remarkably swiftly. A preliminary system was designed and tested within a laboratory and this was later extended to include field trials on air to ensure that the new system would successfully meet all the requirements.
The DRM system
When the specification for DRM, Digital Radio Mondiale was being drawn up there were a number of key requirements that needed to meet. The main thrust of the development was to ensure that far greater audio quality could be achieved, but this needed to be achieved whilst keeping the transmissions in a form where they could operate alongside the existing AM transmissions. This meant having the ability for the transmissions to occupy a variety of different bandwidths dependent up the location and frequencies in use. In the Americas a 10 kHz channel spacing is used on the medium wave band whilst in Europe there is a 9 kHz spacing. On the short wave bands a 5 KHz channel spacing has been adopted. It is necessary for the new standard to be able to be compatible with these whilst offering the possibility of other bandwidth options for the future.
Data can also transmitted. Not only does this supply information required for decoding the signal but it also allows data to be transmitted in support of the programme. One particularly useful feature for the short wave bands is that a list of alternative frequencies is transmitted so that listeners can be transferred to better channels very easily as conditions change.
Another advantage of the new system is that it can support what is termed a single frequency network (SFN). This allows a single frequency to be re-used even within the coverage area of the first transmitter without mutual interference. Currently frequencies can only be re-used used outside the coverage area of the first transmitter to avoid interference problems. By using an SFN, far more efficient use can be made of the available channels. With spectrum bandwidth always in short supply, this is another important feature.
There are two main elements to the new transmission system. These are the audio coding and the RF modulation used.
The main audio encoding system employs two main techniques. The first is called Advanced Audio Coding (AAC). It is found that the ear does not perceive all the sounds that are heard. A strong sound on one frequency will mask out others close in frequency that may be weaker. AAC, therefore, analyses each section of the spectrum and only encodes those sounds that will be perceived.
However AAC on its own does not provide sufficient compression of the data to enable the transmissions to be contained within the narrow transmission bandwidths required. To provide the additional data compression required a scheme known as Spectral Band Replication (SBR) is employed. This analyses the sounds in the highest octave which are normally from sounds such as percussion instruments of those that are harmonically related to other sounds lower in frequency. It analyses them and sends data to the receiver that will enable them to be reconstituted later.
Data to provide the different functions on the transmission is organised into a number of channels that are then applied to the overall modulating signal. The main payload for the signal is known as the Main Service Channel (MSC) and this includes the audio signal data. Two subsidiary channels are also available. These are known as the Fast Access Channel (FAC) that provides the essential data required to fully decode the signal and the Service Description Channel (SDC).
The transmitted signal uses a form of modulation known as Coded Orthogonal Frequency Division Multiplex (COFDM). This form of modulation based around OFDM but with additional coding, and it is very resilient to many common forms of interference and fading. Its main drawback has been that it requires a significant level of signal processing to extract the data from the carriers and reassemble it in the correct fashion. However signal processing ICs are now sufficiently powerful and at a reasonable cost to make the use of this form of modulation viable. Interestingly COFDM is also used by DAB Digital Radio.
Note on OFDM:
Orthogonal Frequency Division Multiplex, OFDM is a form of signal format that uses a large number of close spaced carriers that are each modulated with low rate data stream. The close spaced signals would normally be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is shared across all the carriers and this provides resilience against selective fading from multi-path effects.
DRM, Digital Radio Mondiale has been trialled for some time with experimental transmissions from broadcasters including the BBC. Now scheduled transmissions are taking place from a number of broadcasters, with more commencing their transmissions all the time.