NPR Labs says in its more than 260-page study of digital coverage and interference analysis it found the impact of a 10 percent digital power increase to analog service varied considerably.
At 10 percent IBOC transmission power, most stations would gain covered population, approximately equally analog indoor and portable and exceeding auto.
However, that gain would come at a steep price: an unqualified 10 percent increase in the IBOC power “is predicted to cause substantial interference to analog reception of a significant number of first- and second-adjacent” channels.
In addition, “While elevated sideband approaches are being contemplated by some in the radio industry,” states NPR Labs in the document, “the data developed in this study indicates substantial adverse analog interference tradeoffs would result at many stations from an unqualified increase to 10 percent sideband injection if all stations were transmitting at 10 percent injection.”
One station may have a projected potential dramatic decline in analog coverage if all nearby stations began transmitting in HD-R, while another may have virtually no adverse impact. One station may experience a huge gain in mobile HD coverage, and another almost none, according to the study, which adds that these factors are the inevitable result of overlaying a new, channel-incompatible service on a fundamentally uncorrelated series of local signal environments.
Because of these variables on the impact of adjacent analog channels, methodology must be developed to predict the potential for interference due to elevated IBOC transmission power and apply it as a regulatory control on such operation, concludes NPR Labs in the study.
These are some of the major top level conclusions in the study of digital coverage and interference analysis.
While NPR has presented some findings from the study — including a serious look at the likely results of elevated power levels — to broadcasters in public settings such as the spring NAB Show, in July it released the entire document on its Web site (www.nprlabs.org)
The Corporation for Public Broadcasting funded the 18-month study, concluding testing in March 2008.
In creating its coverage-prediction model, NPR Labs said unlike traditional maps based on nominal field strengths, such as the F(50,50) 60 dBu contour, it based its coverage maps for more than 800 public radio stations, on a set of what it called “signal link budgets” for each service type. This model, according to NPR Labs, represents a realistic coverage prediction, based on factors such as receiver performance, receive antenna efficiency, building penetration loss and signal availability under fading conditions.
In general, NPR concluded that raising the digital power level would substantially improve the reception of auto, indoor and portable HD-R receivers.
(click thumbnail)Of the 75 stations NPR Labs studied in detail as it applied its IBOC Coverage Prediction Model, results for WDUQ(DM), Pittsburgh were average in terms of predicted coverage and predicted receive interference. Note results assume all stations in a region are digital. Source: NPR LabsNPR Labs looked at how the IBOC power levels at 1 percent and increasing to 10 percent would affect 75 noncoms.
In addition, “While elevated sideband approaches are being contemplated by some in the radio industry,” states the labs in the document, “the data developed in this study indicates substantial adverse analog interference tradeoffs would result at many stations from an unqualified increase to 10 percent sideband injection if all stations were transmitting at 10 percent injection.”
It also takes a hard look at the costs of optimizing transmission equipment to accommodate any digital power increase for noncoms and concludes, “Initial projected system-wide estimates of the costs of deploying a combination of optimization strategies for indoor digital coverage parity could approach a doubling of transmission investments.”
It says there appears to be little difference in the potential impact of IBOC on analog service between the reserved and non-reserved bands.
In general, the labs found that at the current 1 percent IBOC power on all stations, auto, called “mobile” in the study, only slightly underperforms quality analog coverage (85 percent), while indoor (38 percent) and portable coverage is substantially smaller than analog.
At 10 percent IBOC transmission power, most stations would gain covered population approximately equaling analog indoor and portable and exceeding auto. However, they’d pay the price from digital interference to their analog signals.
At this power level, stations would lose an average of 26 percent of their FM auto population coverage because of IBOC interference, the lab concludes. “Interference would affect some stations severely,” 41 percent could lose a third or more of their covered population and 18 percent would lose more than half of their population.
And how would a 10 percent IBOC level affect indoor and portable reception?
The digital covered population totals would be 83 percent and 81 percent of analog coverage, respectively. Analog FM indoor and portable covered population totals are reduced by 22 percent and 6 percent respectively due to interference from the digital signal; 27 percent could lose one-third or more of their covered population and 16 percent could lose more than half of their population, said NPR Labs.
Transmission-based remedies worth exploring include the use of single-frequency network boosters to improve indoor digital coverage while controlling analog interference effects.
It’s apparent that a digital-only booster could fill in a shortfall in IBOC coverage, relative to analog service, without aggravating analog FM multipath, said the labs, however, there are some engineering hurdles to accomplish. For example, the cost of a full digital generation and amplification system currently costs tens of thousands of dollars. In addition, digital data framing and encoding at both transmitters much be precisely synchronized.
The data stream carrying audio and control signals must be relayed to the booster, which is likely to be located a large distance from the studio. These requirements make the current cost of a booster prohibitively high when limited numbers of listeners are served.
A low-cost booster system that meets the strict signal integrity and synchronization standards of IBOC DAB must be researched and developed, according to the study.
NPR Labs also says a system to raise the IBOC power in increments less than the proposed 10 percent should be developed and tested, as well as the use of separate directional antenna systems and asymmetrical IBOC sideband power.
“Because of the variable impact on analog coverage, as well as our field experiences with surprisingly good single sideband performance, unequal sideband power strategy could dramatically reduce the potentially serious negative analog impact of elevated sideband power, while also providing important coverage improvement at specific stations.” Both lab modeling of expected results with analog and digital receivers should be followed with field trials to demonstrate the potential viability of this approach, believes NPR.
NPR Labs studied the performance of 50 IBOC receivers and doesn’t expect improvements in HD-R receivers and antennas to be a significant remedy for the shortfall in indoor and portable reception. NPR Labs Senior Technologist John Kean, project manager for the study, also said this at the spring NAB Show, as well as noting the possibility that cost pressures could cause some manufacturers to reduce the quality of their front-end design and components used in HD-R radios. Other techniques, likely transmission-based, will be needed to improve service.
More testing of the impact of raising the IBOC power levels on HD-R receivers is needed, says NPR Labs, including testing of radio reading-service SCA receivers, to develop a national policy for sideband power increases.
‘Defacto’ car radio by 2012
NPR stresses in the study that the digital transition is a multi-year event with three phases. In 2008, we are still in the launch phase, which NPR expects to come to a close with the substantial completion of the initial 1 percent IBOC build-out that is on-track for 2011–2012 across public radio.
This timeline will coincide with the shift to HD Radio dominance in receiver manufacturing as the shift to “digital IF’s and high density DSP technology will be substantially complete by 2012 and result in nominal costs for OEM automakers to embrace HD Radio as the defacto radio offering of new vehicles in those model years.’
NPR bases that on information from CE automotive suppliers, which project 2012, four years from now, will be the breakthrough year with more than 10 million HD-R radio receivers in the field — a figure typically associated with achieving mass market penetration.
Phase two, predicted to occur somewhere between 2010 and 2013, will see commercial broadcasters complete their digital infrastructure. This optimization phase will see ubiquitous indoor signal replication.
The third and final all-digital phase could begin by 2020.
To date, NPR hasn’t received reports of an adverse economic impact from any loss of analog listening from IBOC operations. It has only received two reports of received interference beyond the protected contours to FM listening. Beyond the protected contours, translators are used to supplement weak signals.
In conclusion NPR believes, “The question of getting from here to there without substantial penalties to analog coverage is likely a matter of successive, calculated strategies, potentially trading off some increments in analog interference risk for more digital coverage, commensurate with increases in digital receiver penetration.”
NPR briefed members of the Media Bureau and Audio Division at the Federal Communications Commission on its findings in July.
The results of this study differ greatly than those of 18 organizations, mostly commercial radio groups, that have asked the FCC to bless a voluntary digital power increase of up to 10 percent from the current –20 dB up to –10 dB (search for “Digital Power Boost Is on the Table” at radioworld.com). The coalition said its tests results found there would be “minimal risk of harmful interference” to the existing analog service.