A Comparitive Overview of Digital Audio Broadcasting (DAB) Systems
prepared by REC Networks
In this document, we will examine five different Digital Audio Broadcasting (DAB) formats and how they would work in the American broadcasting culture, especially it's impact on the future of community based low power broadcasting. At the end, based on this information, we will draw our own conclusions to how Digital Audio Broadcasting should be done in the United States.
HD RadioTM - iBiquity - In Band On Channel (IBOC)
The iBiquity system, trademarked "HD Radio", has been accepted as the interim digital broadcast standard in the United States for existing AM and FM broadcast stations. The system uses existing frequencies and most existing transmitters can be upgraded to support the HD Radio system. The system can operate as a "hybrid" operation where the digital information is carried along with the analog broadcast signal on the sidebands. This type of hybrid operation is opposed by those in the radio activist community because the digital subcarriers requires that the bandwidth of the station would be widened and would cause interference to stations on a first adjacent channel. Another apsect that was opposed by the radio activist movement was the licensing plan that was proposed by iBiquity that determines licensing fees by market size and station revenues. REC opposes the use of HD Radio on the AM broadcast band, especially at night due to the interference caused to first adjacent channels. Daytime AM service in rural areas that depend solely on stations from outside the service contour could also be affected. REC feels that iBiquity should work with the LPFM community to make a digital solution that could be reasonably accessible from a financial standpoint. REC feels that the LPFM community should keep HD Radio as a "fallback" in the event that efforts by some to stop IBOC digital is not successful. If LPFM adopts IBOC, then LPFM would also need to accept a second adjacent channel restriction between two LPFM stations as there is a potential that the sidebands of two LPFM stations would overlap causing interference. Currently, imposing a second adjacent channel restriction would impact less than 10 LPFM stations.
Uses existing AM and FM frequencies
Favors Individual Media
Power upgrades not required
Compatible with most existing receivers
First adjacent channel interference
Impacts to the rural AM service
Not compatible with C-QUAM AM Stereo
Eureka 147 DAB
Eureka 147 is the DAB standard in many parts of the world. In Europe, VHF frequencies between 174-240 MHz are used while in Canada, L-Band (1452-1491 MHz) frequencies are used. Eureka 147 uses a 1.5 MHz wide signal. Within that signal (called an "ensemble") the bandwidth is divided among multiple services. Services can be combined between different bandwidths, both mono and stereo and can include data transmission. Eureka 147 appears to be the format favored by the radio activist movement. Eureka 147 is more desireable in the European market because of the way that broadcasting is done out there. In virtually all countries, there is a state-run or public broadcaster that transmits on average, at least 4 different program services. All of these services on FM are broadcast from the same transmitter site, usually with equal power for all stations at the same site. In some countries including Canada and the UK, each broadcast region usually has one ensemble operated by the public broadcaster (BBC, CBC, etc.) and one or more ensemble operated by a private company. Licensed stations are then retransmitted through the ensemble. The landscape of American radio is completely different than those in Europe and even Canada. In American radio, we have major markets where one company (such as Clear Channel) may own several stations in a market and then there are many stations located in rural communities. In order to accomodate every owner, it will require a significant amount of bandwidth. The only ways Eureka 147 would work in the USA is if the ensembles can be made smaller and if spectrum can be found. The 174-240 MHz band in the USA is used by several services including TV channels 7-13, maritime services, amateur radio and military communications. The L-Band spectrum used by DAB in Canada is used in the USA for the Aeronautical Telemetry (ATM) service. REC feels that it would be difficult to clear the ATM band for Eureka DAB and that the size of the spectrum that would be compatible with Canada's DAB system is not enough to accomodate America's radio landscape. If a 1.5MHz Eureka system was ever authorized, the ensembles would be operated by group owners such as Clear Channel and it would be likely that non-commercial ensembles would be operated by NPR or the NPR affilliated entity. The average small LPFM operator would have to "find space" on an existing ensemble in order to get on the air. Eureka is not the answer.
Worldwide DAB standard
Flexible bitrates based on the need of the service
Equipment redily available
Designed for the Euro broadcast landscape
Bandwidth overkill for a small LPFM
Requires new transmitter sites
Microwave (L-Band) operates will require high powers (8kW)
Favors more concentrated media (Clear Channel)
ISDB-T Integrated Services Digital Broadcasting-Terrestrial
ISDB-T has been adopted as the digital broadacst standard in Japan. The system uses a 6, 7 or 8 MHz bandwidth channel to operate multiple services. Each service can use one or more segments of a channel. Each channel has 13 segments. The bandwidth is adjustable as to fit in a single TV channel (some countries have 7 and 8 MHz wide channels). This system is currently testing in the 190 MHz area and will eventually be used on UHF TV channels. Many of the arguments that we have about Eureka 147 would also apply here. This formate can allow several services from the same transmitter with equal power and once again would favor an operator that runs multiple services in a market. With each service requiring a full TV channel and in light in the recent reduction of core TV spectrum, ISDB-T would not work too well in the USA.
Can share spectrum with TV
Favors more concentrated media
Bandwidth overkill for a small LPFM
Requires new transmitter sites
Digital Radio Mondiale
Digital Radio Mondiale (DRM) has been adopted by the International Telecommunications Union (ITU) as a digital broadcasting standard for medium-wave (AM) and shortwave broadcasting. This system is similar to HD Radio as it will allow for a "simulcast" mode where analog and digital can operate on the same frequency. Unlike HD-Radio, DRM is non-proprietary technology. For AM broadcast, REC favors DRM over the HD Radio system because it is an established international standard and it appears that the transitional simulcast signal as well as the post-transition digital signal will take under 10kHz of bandwidth and we would not have the same first adjacent channel problems that HD Radio exhibits. REC does not know if this system can be adapted for VHF FM broadcasting but if it can operate digital hybrid without occupying more than 200 kHz of bandwidth, we would support it as an IBOC solution.
Accepted international standard
Favors Individual Media
Operates on existing frequencies
No increase in bandwidth
No increased adjacent channel interference
Currently only available for AM/SW
Digital Radio Mondiale
MMBS - Mobile Media Broadcasting System
The MMBS concept was developed by the Consumer Electronics Manufacturing Association as a digital broadcast standard to be used in the recently re-allocated upper 700 MHz UHF spectrum. No further technical information is available about this standard. "With multi-channel audio, MMBS would give mobile listeners an exciting immersion into the entertainment experience already a part of high-definition television and DVD," noted Ralph Justus, CEMA director of Technology and Standards.
CEMA News Release
First we must answer the question, is there a need for DAB in the United States? If the current state of XM and Sirius is any clue, there is not a big demand for subscription DAB. We don't really know what the demand for free-DAB will be. In countries that are using Eureka 147, DAB has been able to allow the public broadcasters such as the BBC to expand the choices of services available. Never could I have never imagined that the BBC would ever had a "Radio 7". Thanks to Eureka 147, they do. Broadcasters claim that the proliferation of CDs and MP3 players demand the need for DAB in America. REC feels that a better weapon for battling the CD player and MP3s is the Motorola Symphony Digital Radio chipset that can be used to enhance existing analog FM radio without any conversion to digital.
As we have touched on, one of the main advantages of going digital is the easy ability to multi-cast. The ability to operate more than one audio service on a single frequency has the support of large public broadcasters like NPR (but has been opposed by NAB). Since general coverage SCA (subcarrier audio) receivers are illegal, there is no way for an FM broadcast station (LPFM or full power) to multi-cast. If a digital solution is available that will allow a broadcast station to multi-cast but not go outside their bandwidth or seriously degrade the analog service, we would support it. Unfortunately, no such system exists yet.
Since the DAB cat has been let out of the bag and it appears that DAB deployment is emenant, we must look for a system that will be both compatible and accessible to both broadcasters (including small minority broadcasters and LPFM stations) and to listeners. So if a DAB system is deployed we must take some things into consideration.
REC feels that because of the current state of spectrum in the United States, we must look at either a system that occupies the same existing broadcast channel as the existing broadcast channel or a situation where TV broadcast spectrum is used. REC opposes any kind of IBOC system that involves placing sidebands outside of the original analog bandwidth as it will cause interference to first adjacent channel stations. To address this issue on AM broadcast radio, the DRM system is gaining worldwide acceptance for it's digital capabilities and it's ability to keep a signal within a 9 or 10 kHz channel. REC feels that DRM can be used on AM for both daytime and nighttime use.
For FM and LPFM broadcasting, we have not found a format that really meets the needs of the American radio landscape while being reasonably accessible to community and minority broadcasters. Eureka 147 meets the needs of other countries because of their broadcast landscape that is based on multicasted public radio services as well as national, regional and local commercial services. Only companies like Clear Channel have the services required to fill an ensemble for Eureka 147 use. In addition, there is simply not a big block of spectrum available for such as service. The L-Band that is being used in Canada is being used in the USA by a non-broadcast service and getting the spectrum cleared may not be possible. Especially if the spectrum is considered "government", it is not under the juristiction of the FCC and the NTIA (National Telecommunications Information Administration) would have to release the spectrum for non-government use. Many who follow ham radio issues know that the NTIA is currently blocking a secondary allocation of the 5 MHz band to amateur radio.
Unless another technology comes to the market, we may need to have to consider the iBiquity HD Radio system as a "fallback" if LPFM is to have any form of a digital future. This may have to include imposing a second adjacent channel restriction (of 6km) between two LPFM stations. Currently there is no restriction.
An alternate FM DAB technology must be spectrally efficient. It must not take up more bandwidth than what is allocated to the channel thereby not causing any adjacent channel interference. An alternate FM DAB technology must be open-source. It must be made available to any manufactuer who wishes to develop equipment around the technology and therefore making it free of individual license fees. An alternate FM DAB technology must be embraced by the manufacturers. If no one is making the radios, no one will listen.
For those who are following the creation of a Citizen's Broadcast Band (CBB) at 26 MHz may want to consider a DRM based digital system in this band.