"Another Editorial Rant about HD Radio"

"AM HD Radio critics have hailed how the AM HD carriers do not fit
within the NRSC-2 passband and permissible FCC emission mask. If true,
this would be illegal. However, iBiquity was careful to design these
carriers to fit within the NRSC-2 mask. They took advantage of a
'legal loophole,' in that the NRSC-2 mask was never designed for
continuous energy to be contained within the two outer (lower and
upper) sidebands of any given AM radio station... This hissing sound
steps onto the station's first and second adjacent neighbors."

http://meduci.com/

"Statement of Jeff Littlejohn Senior VP Engineering Services Clear
Channel Communications Regarding AM IBOC Field observations Presented
to the National Radio Systems Committee"

"The current AM allocation rules require Co-Channel stations to
provide 20:1 protections to each other and first adjacent channel
stations to provide 2:1 protection to each other. While this works
fine in an all-analog environment, it does not seem to be sufficient
in the presence of IBOC. The energy above 10 KHz from the proposed
Hybrid IBOC signal significantly exceeds the energy present in the
current analog AM signal. For this reason, the amount of energy
provided to a first adjacent station is significantly more detrimental
than our current allocation rules allow for."

http://www.am-dx.com/clearchannelrprt.pdf