For every radio station that signs on with a new IBOC digital hybrid signal, you can pretty well bet there's at least one person who's likely to spend a lot of time behind the wheel of a vehicle with an HD Radio, driving criss-cross patterns across the market in an effort to empirically determine how well the digital signal matches up with the pre-existing analog coverage. Until now, that person would almost certainly have been an engineer. But with HD Radio products beginning to find their way to consumers and station owners looking for a return on their digital investments, you can bet this scenario is about to change in a big way. As HD Radio integrates with the existing business model, station owners, programmers and clients will all want to know where their new digital signal will be heard.

Reliable coverage modeling

As a singular entity, probably no radio group has invested more time and effort in trying to fully understand and develop the potential of IBOC digital radio than National Public Radio. Through its engineering department and NPR Labs division, this organization has made a substantial investment in HD Radio, so it's no surprise they began looking for an accurate way to calculate digital service areas early on during the technology's deployment.

The test setup used by NPR Labs.
Click image to enlarge.

From the beginning of the HD Radio rollout, most engineers assumed that an FM digital hybrid signal with the specified 20dB D/A ratio fed to a common antenna would provide digital service contours likely to mimic those of the station's analog coverage, albeit with a slight reduction in total service area. Thus, as with analog FM signals, field strength would be the principal predictor of digital service coverage. But the roles fading, multipath and interference might play in disrupting digital reception was not clearly understood or appreciated. As anecdotal reports of unexpected variance in digital coverage began to surface among NPR member stations, a need to better understand reception issues became apparent.

With this in mind, attempts to evaluate IBOC digital coverage at multiple sites began as early as 2004, using a portable test rack consisting of an HD Radio car receiver, field strength monitoring equipment and a GPS-equipped laptop computer to record data while radials form various IBOC transmitter locations.

In a paper delivered at the 2006 NAB Broadcast Engineering Conference, an NPR Labs study of digital coverage involving 26 different IBOC hybrid FM signals clearly demonstrated that field strength by itself was not a an accurate predictor of digital service area. Wide variation existed in the amount of digital signal required for reliable acquisition among the stations sampled.

Clearly, existing analog coverage models were not going to work for HD Radio, and further research was needed.