An extensive study by NPR Labs points to significant trade-offs between the audience reach of digital HD Radio and the amount of interference to analog FM.
Even though its transmission power is just 1 percent of analog FM’s, its range for listeners in cars comes close to equaling analog, the CPB-funded study found. But HD reaches areas including little more than one-third as many indoor listeners.
To extend its range, the National Association of Broadcasters in January joined other industry groups advocating an optional power boost for HD Radio, permitting stations to emit digital signals at 10 percent of analog transmitters’ power. The FCC has not yet begun a proceeding to consider the change.
That could give HD Radio more listeners, but it would have a bigger negative effect on listening to analog radio, which is still far more popular, according to the NPR study. It estimates that 41 percent of public radio stations would lose one-third or more of the car radios their analog signals can reach.
With the power increase, digital’s interference with analog would nearly double, affecting 26 percent of listeners in vehicles (table below).
|HD Radio coverage
v. analog interference
|Present power levels||Proposed power increase|
|Coverage: Population reached by HD Radio, compared with analog FM||In vehicles||85%||117%|
|Inteference: Population whose analog reception is affected by HD Radio||In vehicles||14%||26%|
|Source: NPR Labs report to CPB: Digital Radio Coverage & Interference Analysis.|
Boosting the digital signal tenfold also could add new costs, because many of the new digital transmitters and other equipment would have to be completely replaced, said Mike Starling, NPR chief technology officer and head of NPR Labs.
NPR stands behind HD Radio as an important advance, Starling wrote in a memo introducing the report. “However,” he said, “we cannot responsibly support boosting HD Radio power [to] 10 percent en masse to the detriment of existing FM analog signals.”
NPR Labs and the Association of Public Radio Engineers will present findings of the report for station engineers at a day-long seminar Sept. 16 in Austin, Texas, preceding the National Association of Broadcasters Radio Show. Starling will also appear with advocates of the power increase on a panel Sept. 19 during the NAB conference.
Some station engineers heard a summary of the report in an NPR audio interconnection Aug. 5. Copies of the 280-page report and its executive summary are posted at nprlabs.org.
There have not been many complaints about digital interference to analog among FM stations so far. As of March, NPR said, only three had been filed in a year and a half.
But interference may go unreported because digital interference sounds like a hiss or a buzz, and analog radio listeners can’t make out faint voices to identify the source, Starling told Current.
Interference may increase, however, as broadcasters add more HD Radio signals in the FM channels. In less than two years, the number of pubradio stations with HD Radio signals grew from 115 to more than 350, the report said.
The problem varies greatly from city to city. Interference is worst in areas with crowded FM bands, as in the Northeast. Because HD Radio’s digital signal is carried in sidebands on the edges of the FM channel, it may be cheek by jowl with an adjacent signal, and FCC rules provide no protection, the report said.
The commission should develop new rules for allocating channels that protect existing stations, says John Kean, senior technologist with NPR Labs, the study director.
The study took nearly two years—testing actual receiver performance, developing a formula for predicting analog and digital coverage, doing field tests at 10 stations, and, for the first time, mapping coverage under several scenarios for 850 public radio stations.
To minimize interference, the report suggests, broadcasters could employ separate directional antennas for the digital portion of the signal or perhaps the new Single Frequency Network boosters, now in development, that would repeat only the digital signal. Where a station on an adjacent channel could suffer interference, the HD Radio signal could be put in the FM channel’s opposite sideband—on the edge of the channel away from the other station’s signal.