18 Nov The challenges of Analogue and Digital broadcasting in the shortwave bands
Shortwave knows no boundaries or international borders and despite its apparent decline over recent years, it continues to provide a lifeline service for audiences over large areas or in remote corners of the planet and territories where free access to news, information or education is tightly controlled. Now “digital shortwave” in the form of Digital Radio Mondiale, DRM, is here (and not just on the shortwave bands!). But what are the challenges of broadcasting on the traditional “AM” wavebands? Neale Bateman, representing Encompass Media Services on the DRM Steering Board, offers us more explanation:
The arrival of long, dark winter evenings in the northern hemisphere (quite the opposite of course, if you’re south of the equator!) is a stark reminder to radio listeners all over the world of why international broadcasters need to change their transmission frequencies as we move between the seasons.
Regardless of the type of modulation used for shortwave transmission (conventional AM, or DRM) the factors which influence the propagation of radio signals between transmitter and listener are the same. The earth’s atmosphere, or more specifically, the electrically charged layers of the ionosphere, some 80 to 1,000 km above the earth’s surface, plays an important role in how radio waves behave, depending on the time of day, season and solar activity. Fading, multi-path and interference are all familiar distractions to listeners of traditional AM broadcasts, while digital radio techniques, such as DRM, can deliver an improved level of robustness..
With a properly planned transmission using the best frequency available for the time of day and season, broadcasters can serve the population of an entire country or even a continent, beamed from a single transmitter thousands of miles away.
The regions of the ionosphere which influence how radio waves behave at various frequencies are known as the D, E and F layers, which contain a high concentration of ions and free electrons, sensitive to sunlight, UV and other energetic radiation from the sun. The D layer, closest to the earth, exists only during local daylight hours, when that portion of the planet is illuminated by the sun. At medium wavelengths (MW) layer D is highly absorbent, while higher frequencies (shorter wavelengths) pass straight through. At night, the D layer disappears and the higher E and F layers become dominant; this phenomenon is the reason why distant medium wave stations can be received at night, as their signals are no longer absorbed and instead are refracted by the E layer, often by hundreds of miles from their origin.
Unlike FM and medium wave transmissions which radiate horizontally along the ground, largely following the curvature of the earth and becoming increasingly attenuated with distance, shortwave signals can be beamed towards the sky. At any time of the day or night, the E and F layers can reflect these signals back to earth, illuminating large geographical areas several thousands of miles away. The same principle applies, regardless of whether traditional analogue or digital (DRM) mode is used, making DRM digital radio on the SW bands a compelling way of delivering high-quality audio to mass audiences over very wide areas.
But this very useful property of shortwave propagation also comes with a drawback: the same frequency cannot be used all day long, or all year round because of seasonal change in the number of daylight hours at any given location. Our sun also undergoes a much longer period of change, known as the sunspot cycle, which sees a peak in solar activity approximately every 11 years.
As a general rule, higher frequencies work best during daylight hours and summertime (in the northern hemisphere), while lower frequencies propagate better in darkness (before dawn and during the long winter evenings), especially when the sunspot cycle is at its lowest ebb. The problem, of course, is that the same rules apply to everyone. So in periods of low solar activity, the lower frequency bands are crowded with every broadcaster trying to use the best possible frequency for their service.
This is one of the many circumstances when digital transmission systems such as DRM score an advantage over conventional AM. In any DRM transmission, some of the data packets which form the DRM stream are used for forward error correction (FEC); a technique known to digital engineers for many years but very cleverly used by DRM to improve the robustness of the received signal. The process introduces a short delay in the audio, but it means the receiver can re-assemble the received packets as best it can, even if some are lost due to fading or interference. In DRM, unlike DAB, the amount of data allocated to error correction is dynamic, and can be controlled by the broadcaster depending on the length and nature of the transmission path. Using more data for error correction inevitably means less for the actual programme content, but the result is improved resilience against the traditional deficiencies of shortwave reception.
Another major advantage of DRM is also great news for broadcasters: the amount of transmitted power (and therefore energy consumption) needed to adequately cover an area similar to that which can be achieved by a conventional analogue AM signal is significantly less. Energy savings of around 60% are typical, compared with the SW or MW transmission power needed for approximately similar coverage.
Other benefits of DRM over analogue – for both the broadcaster and listener – is that up to 3 separate programme channels can be transmitted in a single DRM carrier, at no extra cost. Significant too, is that a DRM channel can co-exist quite happily on the AM (and FM) bands with no adverse effect on analogue stations. That’s a major consideration to broadcasters and regulators alike when planning a digital upgrade or migration path.
But back to the long winter nights and the reasons why frequency changes need to be made in summer and winter to match the varying propagation conditions. This is particularly complex when broadcasts from international radio stations traverse the day/night horizon between transmitter and listener. Within the auspices of the International Telecommunications Union (ITU), mandated by the United Nations, the High Frequency Co-ordination Committee (HFCC) manages and co-ordinates global databases of international shortwave broadcasting. The HFCC meets several times a year to co-ordinate the frequencies used by all of the world’s major broadcasters.
The output from the HFCC is two seasonal frequency schedules – summer and winter – known as the ‘A’ and ‘B’ seasons. The changeover between seasons is internationally agreed upon to occur on the last Sunday in March (start of ‘A’ season) and the last Sunday in October (start of ‘B’ season). The new ‘B20’ season commenced on Sunday 25th October, and the frequencies agreed for all shortwave transmissions will continue until the start of the ‘A21’ season on Sunday 28th March 2021… and the whole planning processes begins again!