SARAO News

When the residents of 15 Karoo towns turned on their televisions on 15 February 2017 this year, they had reason to celebrate the end of the ‘snowstorm’ that was usually present on their screens.

The end of the ‘snow’ signalled an important milestone in South Africa’s migration to digital television. It was the culmination of an extensive process of negotiations and planning that involved SKA and the highest levels of government, and which will have far reaching effects on the data that is produced by radio astronomers at the SKA Karoo site.

Digital migration is part of the story, but the rest is about trying to improve the radio frequency environment for the SKA, and working with policy makers to find an acceptable policy and technical solution for scientists and the people who live in the area of the SKA.  

Radio spectrum is an extremely valuable and finite commodity, so any technologies or opportunities that enable the efficient use of that resource has clear benefits for radio astronomy.

SKA’s input into South Africa’s digital migration project was a natural extension of the work being done at the Karoo site, both from a scientific and community development point of view.

There was a need for a protective regime for the SKA telescope, so the call for digital migration in 2007 provided an opportunity for SKA to re-open discussions with the relevant government departments. It was vital to develop effective ways of working together to ensure compliance with the Astronomy Geographic Advantage (AGA) Act of 2007 which protects the SKA from harmful frequency interference. The existence of analogue television transmissions in the area was responsible for most of the unwanted radio noise. 

A working group made up of representatives from a number of government departments and agencies was established. The team included input from the Department of Communications, the Independent Communications Authority of South Africa (ICASA), SENTECH, Universal Service and Access Agency of South Africa (USAASA), South African Postal Office (SAPO), the South African Broadcasting Corporation (SABC), and all broadcasting digital migration stakeholders.

The SKA worked with all the stakeholders to make sure that the changeover happened quickly and efficiently and to ensure that the result had benefits for the broader community as well as radio astronomy.  

The Director of Media Liaison in the Ministry of Communications, Mish Molakeng, acknowledged recently that the successful Karoo switchover was only possible because of the input into the multi-stakeholder initiative by SKA, which he credited with fast-forwarding the project.

Because of the SKA’s policy intervention and digital migration programme, appropriate technology choices were made, with the result that the radio frequency environment for the SKA has been optimised.

The switch over was part of a process which began in 2006 when a digital broadcasting plan was agreed for the frequency bands 174–230 MHz and 470–862 MHz at the International Telecommunication Union (ITU) Regional Radiocommunication Conference (RRC-06) in Geneva.

The ITU is a specialised UN agency that coordinates the global use of radio spectrum, works to improve telecommunication infrastructure and helps to determine global technical standards.

According to this plan (known as the GE06), 116 countries, mainly in Africa and Europe, would switch over to digital. Completion targets for the different frequencies were between 17 June 2015 and 17 June 2020.

South Africa, as a member of the ITU, was a signatory to the new protocol, but implementation was slow. By 2015 the country had lagged far behind its self-imposed deadline. A number of factors had caused the delay, including a lack of technical skills. In 2010 it was estimated that there was a skills shortage in the communications sector of 70 000 people.

The ITU also allocates frequency bands for use in radio astronomy, but these impose restrictions on research, especially as the signals from the most distant objects shift in frequency as the Universe expands. In order for the scientific capability of the SKA to be realised, observations need to be carried out outside the specific frequencies currently allocated by the ITU for radio astronomy.

The broadcasting band is a huge part of the radio spectrum – occupying a frequency from 470 to 854 MHz, the spectrum which is also vital for the SKA.

Radio telescopes need to use as much spectrum as possible for scientific observations. As an example, one of the areas of research for SKA is into hydrogen gas, which releases a signal which occurs just above GSM (900 MHz) but it can also be detected right across the broadcasting band.

Any optimisation of that spectrum, consolidation of man-made transmissions or the removal of terrestrial signals in that band has a huge impact on the scientific return of the SKA. The analogue television signal switch off opened access to an estimated digital dividend of 300 MHz in the broadcast band.

It was clear that the old way of broadcasting would not reap all the benefits or opportunities offered by digital broadcasting, so simply replacing analogue transmitters with digital terrestrial transmitters would have been a lost opportunity.

Traditionally, analogue terrestrial broadcast networks were planned as multi-frequency networks. This has the effect that certain TV channels are not used in certain geographical areas so as to avoid interference with terrestrial broadcast services in adjacent regions.

In the Northern Cape alone, there were over 700 analogue transmitters. The modulation techniques used were inefficient, with the result that valuable spectrum was taken up by attempts to coordinate the signals transmitted so as not to interfere with each other.

With traditional analogue broadcast, each analogue transmitter broadcasts a single channel, which means that nothing else can be broadcast in that part of the spectrum. Digital migration opens the broadcasting band to multiple channels using the same part of the spectrum.

With digital broadcasting, sound, video and text are processed electronically and converted into digital format. This format is then transmitted and reconverted by appropriate receivers or set-top boxes into sound, text and video for display on TV set.

Viewers in the Northern Cape experienced ‘snow’ as well as limits to the number of services available. In addition, one of the implications of the ITU Protocol was that analogue channels would no longer enjoy protection, meaning that increased interference to broadcasts was a real possibility.

Various options were explored. One was to look at how television could be transmitted via satellite rather than the terrestrial infrastructure which are very costly in sparsely populated areas like the Northern Cape. Due to the cost efficiency inherent in this approach, the government made provision in the budget for set top boxes as part of the digital migration. Direct to Home satellite set-top box decoders are made available in sparsely populated areas like the Northern Cape Province, while Digital Terrestrial set-top box decoders are provided to communities in other parts of the country.  

In most parts of the world where the digital migration has already taken place, the analogue signals have just been replaced by digital terrestrial signals. The Northern Cape is unusual in that it has leapfrogged directly to satellite, which makes sense in an area which has a low population density and where transmitting terrestrial signals is costly. And, of course, the satellite transmission uses a much less important part of the spectrum in terms of the needs of the scientists at SKA.

The process is ongoing, but currently the towns of Brandvlei, Carnarvon, Fraserburg, Kakamas, Kenhart, Kiemos, Loeriesfontein, Nieuwoudtville, Pofadder, Sutherland, Van Wyksvlei, Vosburg, and Williston have been switched over.