18 Mar DRM for Shortwave – How it Physically Works
Author: Neale Bateman, Radio Engineering Consultant. This article first appeared in IEEE Q1/2021 publication)
The long, dark winter evenings in the northern hemisphere (or if you’re south of the equator, the long hot summer days!) are a stark reminder to radio listeners all over the world of why international broadcasters need to change their transmission frequencies as the seasons progress.
Regardless of the type of modulation used for shortwave transmission (conventional AM, narrow-band FM or DRM) the factors which influence the behaviour of the medium through which radio signals need to travel to reach the listener, are the same. That medium of course is the atmosphere, or more specifically, the electrically charged layers of the ionosphere, some 80 to 1,000 km above the earth’s surface – which depending on the time of day, season, and solar activity, significantly affect the effectiveness of radio transmissions in the medium and shortwave bands. Fading, multi-path and interference are all familiar distractions to listeners of traditional AM broadcasts while digital radio techniques, such as Digital Radio Mondiale (DRM), can deliver an improved level of robustness.
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 continent, beamed from a single transmitter thousands of miles away.
The region of the ionosphere which influence how radio waves behave at various frequencies are known as the D, E, F1 and F2 layers, which contain a high concentration of ions and free electrons, sensitive to sunlight, UV, and other energetic particles 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, it 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; therefore, 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.
Unlike FM and medium wave transmissions, which radiate horizontally along the ground and largely follow the curvature of the earth, shortwave signals can be beamed towards the sky. At any time of the day or night, the E and F layers play a role in reflecting 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 proposition to deliver high-quality audio to mass audiences over very wide areas.
But the useful property of shortwave signals to travel long distances also comes with a drawback: the same frequency cannot be used all day long, or all year round. This is because the diurnal and seasonal changes in the number of daylight hours. The sun itself also undergoes a much longer period of change, known as the sunspot cycle. This variation in solar activity peaks around every 11 years. So, frequency planners can make long term predictions of which frequencies can be used with a reasonable degree of accuracy.
As a 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 when digital transmission techniques 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. This all takes time and introduces several milliseconds of delay into the decoding process. This 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.
The DRM multiplex can also contain other embedded data which contributes to enhancing the listener’s experience and negates the need to manually search for the best frequency to receive the desired programme content. AFS (Alternative Frequency Switching or Signalling) allows the broadcaster to transmit information relating to either a particular service within the 3 programme DRM “multiplex”, or the entire MUX. For example, where the same programme is simultaneously available on other frequencies in that area or region, the receiver can select the stronger or most robust signal without any action required by the listener. Or it can point the receiver to other delivery platforms where the same programme ensemble is also available, e.g., FM, etc. according to the Service ID. AFS can also be used in mobile applications to automatically retune the receiver as it moves between coverage areas – particularly useful with in-car DRM receivers to provide seamless switching in regions where DRM is deployed nationally on the medium waveband.
AFS data is contained within the Service Descriptor Channel (SDC) of the DRM MUX and carries further information about the multiplex itself, including a 16-character label, date and time, and other service-related data such as regional ID, frequency schedules, support for announcement switching (e.g. Emergency Warning applications) and conditional access if implemented. The SDC frame is broadcast every 1.2 seconds, and its content is important to the correct operation of AFS so that the receiver can continuously analyse, compare and select the best option at any moment in time.
But back to the long winter nights (or hot summer days!) and the reasons why frequency changes need to be made in summer and winter to match the varying propagation conditions. The situation is more complex when broadcasts from international radio stations traverse the day/night horizon between transmitter and listener, or between summer and winter conditions on long north/south paths, or both! 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 a global database of international shortwave broadcasting. The HFCC meets several times a year to co-ordinate the frequencies used by all 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 to occur on the last Sunday in March (start of the ‘A’ season) and the last Sunday in October (start of the ‘B’ season). The current ‘B20’ season commenced on Sunday 25th October 2020, and the vast majority of frequency assignments for shortwave transmissions will continue until the start of the ‘A21’ season on Sunday 28th March 2021. It’s a full- time occupation for frequency managers all over the world, and planning is already well underway for next winter’s schedule (B21), even before the summer begins!
Author: Neale Bateman, Radio Engineering Consultant: firstname.lastname@example.org