An 
11-page report, co-authored primarily by representatives of iBiquity, the NAB, and CBS, provides an overview of the methodology and preliminary results of a set of experimental all-digital HD broadcasts on WBCN-AM in Charlotte, North Carolina. It was the first test of the all-digital AM-HD system in more than ten years.
The authors believe the test broadcasts served as "an opportunity to begin developing a contemporary…record that would help educate the industry as to the capabilities of all-digital operation, develop all-digital operational parameters, and provide information which could be eventually submitted to the FCC for the purposes of obtaining permanent authorization for all-digital service."
Interestingly, the report suggests that the tipping-point for radio’s digital transition will be when HD receiver penetration reaches 85%. With a current penetration rate of 1.7%, this suggests an all-digital transition remains several years away – unless HD proponents are able to redefine what "receiver penetration" means, which is not outside the realm of possibility in coming years.
WBCN operates on the expanded AM band at 1660 kHz, with 10,000 watts of power during the day and 1,000 watts at night, utilizing a non-directional signal pattern. It has been broadcasting in hybrid analog/HD mode since June of 2007.
The tests took place over "three weekends in late November and December 2012," encompassing about 30 total hours of broadcasts. Reception was measured in both mobile and indoor environments: eight test-drive routes were plotted and indoor reception was measured on an Insignia Narrator receiver in 15 locations encompassing "a variety of building types."
In the car, WBCN’s all-digital signal could be heard relatively clearly during the day anywhere between 25 to 45 miles from the transmitter, and at night for 10 to 15 miles. Indoors, all-digital reception was possible at two-thirds of the test locations, all of which were within 13 miles (daytime) and 7 miles (nighttime) of the transmitter.
Inside Radio‘s report on the tests touts the results as promising, but it’s important to note that WBCN represents a best-case experimental platform. Expanded-band AM stations are less susceptible to interference from other stations (because there are fewer stations located on expanded-band channels), and WBCN’s antenna system is a straightforward design already optimized for HD broadcasts.
Under "Future Activities," it is noted that NAB Labs will coordinate further testing of the all-digital AM-HD protocol to "fully develop a performance record of operation in this mode." How extensive this record will be remains to be seen; in the interim, hopefully a more detailed analysis of the 2012 tests is forthcoming.
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