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 little has happened since FM took over local broadcasting. It caused the demise of AM stations and the shortwave services of the SABC and LM radio.
The Southern Africa Digital Broadcasting Association (SADIBA) issued a report in 2002 in which it said “to remain commercial 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.”
In the document SADIBA made recommendations on the critical aspects to be considered in order to allow for the introduction of digital radio in South Africa.
Little seems to have happened since 2002 until last month when the subject was extensively discussed at the SADIBA Conference where the 2002 paper re-emerged and digital radio mondiale (DRM), 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.
Discussing the advantages of DRM, Ruxandra Obreja, head of digital radio development at the BBC world service and chairman of the DRM Consortium said that DRM and DRM+ have proved to be the obvious choice for digital radio. But not everyone would agree with that.
Let us consider some of the various digital radio technologies available.
IDAB is based on in-band-on channel (IBOC) technology which looks at inserting the digital signal within the existing FM and AM channels without affecting other FM or AM transmissions.
FM IBOC is designed to operate in a 200 kHz FM channel allocation. It would have been very impractical to introduce FM IBOC into South Africa without re-engineering the current FM frequency plan based on 100 kHz channel.
According to the 2002 SADIBA paper the most established of all the digital radio technologies is the Eureka 147 system.
The technology is based on an open standard defined in a range of the European Telecommunications Standards Institute (ETSI) documents. But it requires different frequency bands which in itself is problematic given the scarcity of spectrum oin the UHF bands which are demanded by satellite communication channels (VSat) and wireless broadband.
The so-called digital dividend will also not provide the required spectrum as that process will take a long time to materialise.
Obreja believes that DRM and DRM+ is the obvious solution. DRM, the acronym for digital radio mondiale, is promoted by a consortium of broadcasters, manufactures’ research institutes and stakeholders that have endeavoured to work toward a world-standard for digital broadcasting in the frequencies initial below 30 MHz, operating on the AM and shortwave broadcasting bands. Since the original development engineers have moved ahead and DRM+ emerged, the name applied to the DRM standard when used on the VHF frequencies.
“The initiative to extend DRM began with a vote at the 2005 General Assembly to begin the design, verification and testing of the parameters needed to allow DRM to operate in the VHF broadcasting bands between; primarily band I and band II,” said Obteja.
The design process began shortly afterwards and key decisions were made to ensure that the extension completely shared the successful design philosophy of DRM – it is “DRM but at higher frequencies”.
Its spectrum usage parameters are determined from the internationally agreed norms in the FM band (88 to 108 MHz). Therefore it has an occupied bandwidth of 96 kHz and a frequency grid of 100 kHz.
DRM+ provides bit rates from 35 kbps to 185 kbps at SNRs from 2 dB to 14dB and, like DRM, permits up to four services. It is therefore a flexible solution allowing single or small numbers of audio services to be broadcast together.
During the process of gaining endorsement from the ITU for DRM’s extension to the VHF bands, test results from various field trials conducted around the world were submitted. One of the interesting trials was carried out in December 2011 by Vatican Radio carrying out broadcasting tests of DRM+1 in the VHF band II at 103, MHz.
The aim of the tests was to verify the performance of DRM+ in a difficult interference scenario such as the FM VHF band II in Rome and to check the compatibility of the digital technology with existing antenna arrays having complex RF coupling systems such as the one located in the Vatican.
The frequency used was assigned to the Vatican in the GE84 Agreement and was chosen for two main reasons: it is not used during a few timeslots in the morning and it suffers from some strong interference coming from stations operating at 103,7 MHz and 104,00 MHz located close to Rome.
The tests were carried out taking into account the normal programme schedule. During the tests the digital transmitter was connected to the antenna feeder via a changeover, leaving the analogue transmitter in stand-by.
The antenna array is a complex system: four FM transmitters at different power levels share the same antenna with elliptical polarisation and omni-directional horizontal radiation pattern. The results were great.
Acceptable stereo coverage under mobile reception conditions has been verified in areas where predicted field strength is comparable with 44 dBmV/m and interference is negligible.
Using the most robust configuration for DRM+, it was possible to achieve better coverage in full stereo than an analogue FM signal; the overall subjective listening experience was better than that of FM interfered with by splashes coming from adjacent stations.
With South Africa’s poor performance in changing from analogue to digital TV, it may be some time before government will applies its mind to take a decision on digital sound radio.
The first step have however been taken by commercial enterprises. Pulpit Radio is conducting a DRM pilot from their transmitting station at Kameeldrift near Pretoria.
The 50 kW transmitter was installed by Broadcom International and made history with the first DRM audio broadcast in the Southern African region on 1440 kHz AM.
“The results were very good. The station was received in Botswane some 400 km away with CD quality audio,” Obreja said.
One of the issue is however the availability of receivers but Ruxandra Obreja said that experience from elsewhere where DRM was introduced local industry began manufacturing. “This will be a great opportunity to grow South Africa’s electronic manufacturing industry.”
There is software available to decode the DRM signals using a sound card and a dongle is under development that can be used on a laptop or even other devices that have a USB port.
With DRM, the use of medium and shortwave will open up many new radio channels. Each DRM channel can carry three radio programmes and one data channel requiring very narrow bandwidth of less than 5 kHz. Another advantage is that the system is also more energy efficient.
Source: EE Publishers