Two recent initiatives are introducing a new type of subscription digital audio broadcasting transmitted via satellite.
IEEE, PhD Carlos Pantsios
Digital Radio continues its development in the world with a firm step. Both the IBOC (HD Radio) standard, the DAB (DAB+) and the Digital Radio Mondiale have made remarkable progress in recent years. Likewise, considerable progress has been made in topics related to these technologies for the future of digital radio, IP and hybrid radio and, above all, digital radio in cars.
Broadcasting radio is an audio service, so consumers naturally compare it to the most prevalent audio quality technology, which is the compact disc (CD).
A good FM signal picked up by a high-quality stationary receiver compares favorably with CD sound. However, by placing an FM receiver in a car, with all the imperfections that the signal can experience on the transmission channel, the sound quality quickly drops. Among these imperfections are: ambient noise and RF signal reflections, which produce multi-path fading on the receiving side. The ruggedness of digital radio solves this, delivering CD-quality sound in virtually any broadcast radio signal reception condition.
Today, with most radio signal consumption effected in moving cars, signal degradation occurs continuously over time, complicating the problem of correct FM signal capture even further. To almost completely eliminate these problems, digital radio systems, such as DAB+ and IBOC (H Radio), have incorporated a number of very powerful and ingenious digital signal processing techniques, such as the coded "Orthogonal Frequency Division Multiplex" (COFDM).
In order to boost the mobile digital radio market in many regions of the world, seeking to maximize the quality of the in-car user experience, the European Economic Community's DAB+ System has made great strides – with all major markets increasing the proportion of new automotive units equipped with standard digital radio receivers. As more countries decide to install digital radio, attention has focused on converting millions of cars, already on highways, to the digital system, allowing drivers to continue listening to the digital radio broadcast when crossing borders between different countries.
As the number of digital radio cars in the world increases rapidly, it is essential that the user experience exceeds their expectations, and to this end the WorldDAB group has been working closely with the automotive industry to improve that user experience. The next step in this effort is to incorporate IP and voice control, which have already been adopted by most car brands.
In the meantime, work is being done on the type of receiving antenna and the sensitivity of digital radio receiver equipment to provide a quality link between network operators and car manufacturers.
WorldDAB has created a working group that integrated broadcasters, antenna and device manufacturers, as well as experts from the automotive industry, achieving an important collaboration for digital radio in the car. The group works for the substantial improvement of features, antennas, installation guides, marketing and other areas, in order to achieve the best possible user experience.
The group thus formed focuses on key areas, including technology development and innovation, enhanced performance, best-case marketing initiatives, as well as installation and training programs.
Meanwhile, in the United States of America, radio broadcasting, like digital terrestrial television did much earlier, is experiencing a very noticeable digital leap forward.
Two recent initiatives are introducing a new type of subscription digital audio broadcasting transmitted via satellite. It is known as the "Satellite Digital Audio Radio Services (SDARS"), and where users will be able to tune in to the same radio stations anywhere in the United States.
SDARS differs from the so-called digital music service in that the direct broadcasting satellite or cable system operators supply the digitized and compressed audio over their networks, both because of their programming and because SDARS can be received in a moving car, where today, most of the time the radio is listened to; something that is not possible in conventional audio services.
Meanwhile, free-over-the-air broadcasters are yet to decide on digital audio broadcasting technologies for both the AM and FM bands.
While better quality of service, more signal robustness and new services attract broadcasters, a less tangible reason to make the transition to the digital world is related to consumer perception. Terrestrial radio is one of the few analogue communications services that still exists in an increasingly digital world.
It has been 10 years until two U.S. SDARS systems recently decided to start commercial operations. These two innovative digital radio systems are operated by Sirius Satellite Radio Inc., based in New York City, and XM Satellite Radio Inc., located in Washington, D.C. They share numerous technical similarities, such as the approximate number of separate radio channels, as well as some differences, such as, configuration of different satellite orbits.
Importantly, local radio stations supported by sponsors that broadcast many formats of programs with limited audiences can never be economically viable. SDARS operators maintain that even narrowly specialized programming will only be possible through a subscription service. (Sirius planned to work commercial-free; XM planned to include commercials in its news/commentary programs.)
Coverage is clearly the most powerful element of satellite broadcasting. Radio markets (mainly rural), currently under-covered by radio services, with only a very small number of stations, will certainly accept the possibility of selecting digital satellite radio.
Technically speaking, the Sirius and XM systems are unique in many ways. They were designed to serve, first and foremost, mobile users and focused on a consumer market, where individual receivers should cost less than satellite receivers. These systems cannot rely on high-gain antennas with expensive tracking devices to keep the antenna aligned with the satellite, as is the case with other satellite systems such as Inmarsat, which serves maritime and other markets.
Another serious problem in mobile reception in cars, in particular, is the intermittent reception of signal blocked in its reception path by buildings, trees and other obstacles while the car is moving. The magnitude of the problem is a function of the latitude of the radio receiver since this establishes the angular elevation of the satellite above the horizon. The farther north the receiver is located, the lower in the sky the satellite appears and the more likely signal jamming will be. For both the Sirius and XM systems, the type of antenna used in reception is low gain and with a hemispherical reception lobe.
XM uses two satellites in geostationary orbit at 85 degrees W and 115 degrees W with perfect coverage over the United States and with transmission powers never before used in the entertainment industry.
SIRIUS uses a newer solution with three satellites in highly elliptical orbits, ensuring that two of the three satellites are always visible to all receivers in the United States. This maintains a higher elevation above the horizon, compared to
geostationary orbit satellites, achieving a lower possibility of signal blocking.
The SDARS service uses three diversity (redundancy) techniques: in space, frequency and time, ensuring stable and quality mobile reception.
In Europe, with few radio stations in the broadcasting band, many countries supported national radio networks. This led to a network of terrestrial transmitters known as single-frequency networks, which transmit all the same material over the same channel, offering virtually unlimited coverage over a very large geographical region. This was only possible thanks to the use of the COFDM modulation technique, which was also key to many other technologies. The system works for both fixed and mobile receivers and mostly under the DAB/DAB+ standard.
