Digital radio – an option for South Africa?

May 24th, 2016, Published in Articles: EE Publishers, Articles: EngineerIT

 

Marius Venter, Head: Network Planning, SENTECH

Marius Venter, head: Network Planning, Sentech.

Radio technology has seen very little innovation and development since FM stereo was introduced in the 1960s. It was the sound revolution of the time but FM also caused the demise of the AM and shortwave services of the SABC and LM Radio.

In 2002 the Southern Africa Digital Broadcasting Association (SADIBA) issued a report in which it said “to remain commercially attractive, radio as a medium will have to deliver improved quality service, greater choice, interactivity and multi-media. Digital radio technologies must rise to the challenge and deliver the multimedia radio of the future.”

Little has happened since 2002; and it was in 2014 when the subject was extensively discussed at the SADIBA conference. The 2002 paper re-emerged and digital radio mondiale (DRM), as one of the technologies came into the limelight with international speakers and a demonstration of DRM by the BBC transmitting DRM from their  shortwave relay station on Ascension Island with CD clarity, no noise, no interference.

In March 2014 Radio Pulpit obtained a DRM trial licence from the Independent Communications Authority of South Africa (ICASA). Sentech partnered with Radio Pulpit and Broadcom to set-up a DRM30 MF transmitting station on 1440 kHz at Kameeldrif, east of Pretoria.

“Our objective was to confirm the potential benefits of the DRM30 technology as a radio broadcast platform,” said Sentech’s head of network planning, Marius Venter. “We also decided to evaluate actual coverage versus predicted coverage (for both ground and skywave propagation modes); evaluate two different low-profile AM antenna systems; obtain sufficient measurement data for analysis to assist in reaching a conclusion on the overall performance of the technology and to determine if, how and where the technology could be applied to benefit broadcasters as well as a study of available commercial radio receivers for both fixed and mobile conditions.”

DRM is an international consortium of broadcasters, manufacturers, research institutes and stakeholders that decided to work towards a world-standard for digital radio broadcasting in the frequency bands below 30 MHz. The base specification was delivered to the International Telecommunications Union in December 1999. Field trials were conducted and revisions to the specification were completed by 2001 when roll-out of the broadcast technology started.

DRM30 uses the existing AM broadcast frequency bands and is designed to fit in with the existing AM broadcast band plan, based on signals of 9 kHz or 10 kHz bandwidth. It also has modes requiring only 4.5 kHz or 5 kHz bandwidth, and modes that can take advantage of wider bandwidths – 18 kHz or 20 kHz – allowing DRM30 to operate alongside AM transmissions in every market of the world.

Since then the original development engineers have moved ahead and DRM+ emerged, the name applied to the DRM standard when used on VHF frequencies. Its spectrum usage parameters are determined from the internationally agreed norms in the FM band (88 to 108 MHz). It has an occupied bandwidth of 96 kHz and a frequency grid of 100 kHz.

In the three-part trial Sentech conducted RF measurements and provided professional test equipment and the low-profile KinStar antenna. Radio Pulpit was responsible for the transmitter and content server and Broadcom provided facilities (including equipment container, standby generator, installation services and the Broadcom antenna).

Comparison of measurements taken on nalogue MW, DRM30 (16 QAM) and DRM30 (64 QAM) indicated that the field strength level and radio propagation followed the same trend irrespective of AM or DRM modulation used. AM and DRM30 are equally affected by tollgates, bridges, high voltage overhead power lines and terrain but AM audio experienced  degradation in mountainous terrain, where some DRM measurements indicated slight improvements over the same terrain.

Some interesting results were seen between the use of the Broadcom Antenna which is a traditional vertical mast commonly used by AM stations and the low-profile Kinstar AM broadcast antenna which is much lower. More severe sky-wave multipath interference was experienced on the Broadcom antenna compared to the KinStar antenna.

The Kinstar antenna consists of vertical and horizontal radiating wires over a standard radial ground wire ground plane. The length and arrangements of the wires are designed by computer optimised methods to provide the best compromise between reduced antenna height, antenna gain at the horizon and frequency bandwidth. Its much lower profile yet comparable performance makes it the ideal antenna for use near residential areas and airports.

Venter said that their study found that the advantage of MF propagation qualities such as wide area coverage and sky-wave propagation, enhanced audio compression improving efficient utilisation of the digital channel and much improved audio quality with additional features such as Electronic Programme Guide, Journaline, a text and news information system, News Feeds and Slideshows makes DRM an attractive AM proposition.”

He said that there are other digital systems which should be investigated and compared with DRM. “Ultimately we need to make the right choice given the overcrowding of the FM broadcast band and the requirements to cater for highly populated city areas and sparsely populated rural areas.”

DRM has taken off in a big way in India which may ultimately solve the availability of DRM radio receivers at a reasonable cost. It is uncertain whether DRM will be adopted in South Africa due to other digital systems also being considered. What is certain is that the future of radio in South Africa is digital.

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