Raising FM-HD Power Levels Will Cause Increased Interference

And not just to adjacent stations – iBiquity’s proposed 10-fold power hike for FM digital sidebands will cause what one commentator has called “honkin’ interference” to an HD parent station’s analog signal. Although the suspicions of just what increasing the signal strength of FM-HD sidebands would do to analog FM radio coverage have been well-discussed in the engineering community for nine months now, the new report from NPR Labs confirms the worst.
The “monumental 18-month study,” involving extensive laboratory and field-testing of increased FM-HD sideband power finds that increased digital interference is simply unavoidable. While the tests do show that increasing FM-HD sideband power by a factor of 10 will make digital service coverage equivalent to (or, in some cases, slightly exceed) the coverage of a station’s analog signal, the modification comes at a price:

Mobile analog FM covered population would be reduced an average of 26% for the sample stations. Interference would affect some stations severely in portions of their analog mobile service area: 41% could lose one-third or more of their [analog] covered population and 18% would lose more than half of their population. …
Analog FM indoor and portable covered population totals are reduced by 22% and 6%, respectively. Interference would affect some stations severely in portions of their analog indoor service area: 27% could lose one-third or more of their covered population and 16% could lose more than half of their population.


Station impacts from IBOC DAB to analog FM vary widely from station to station, primarily due to the fact that the IBOC DAB digital sidebands are actually co-channel to neighboring stations on first-adjacent channels; the FCC’s first-adjacent allocation rules for analog FM cannot adequately protect against some close-spaced conditions [emphasis mine].

In addition, “Improvements in IBOC DAB receivers and antennas are not currently expected to be a significant remedy for the shortfall in indoor and portable reception. Other techniques, likely transmission-based, will be needed to improve service.” Does this mean HD-enabled stations will be permitted to increase their analog power levels to compensate? Talk about making a congested band even worse….
In summary, “Unqualified 10% IBOC [FM] transmission power is predicted to cause substantial interference to analog reception of a significant number of first-and second-adjacent channel stations,” in addition to the aforementioned analog/digital host-station cross-talk problems.
Interestingly, these tests were not conducted under the auspices of the National Radio Systems Committee (NRSC) – a broadcast standard-setting body which has been intimately involved in HD Radio protocol development every step along the way – except for this one. NRSC members are publicly questioning the integrity of the power-hike recommendation, and urge more research to be done before the FCC moves on this idea.
On top of all of this, there are increasing reports of trouble involving HD-reception in vehicles, and serious questions as to whether the FCC has the jurisdiction to force receiver manufacturers to incorporate HD reception technology into newer radios. (consumer adoption trends place satellite and wi-fi-enabled Internet radio receivers well-ahead of HD).
According to the FCC’s own figures, about 1,750 stations are now broadcasting in some form of HD technology – that’s just about 10% of all licensed stations in the nation. Will the agency destroy the village to save it?