NRSC Endorses AM IBOC for Daytime Only
Mar 21, 2002 12:00 PM, by John Battison, technical editor, RF
Washington, DC - Mar 21, 2002 - The NRSC's AM IBOC working group will potentially recommend to the FCC that Ibiquity's AM IBOC is suitable for use during daytime hours only. The committee's results show that AM IBOC during the day yields performance comparable to that of FM IBOC. Nighttime operation of the AM IBOC system has outstanding issues due to first- and second-adjacent channel problems. Ibiquity has already stated that it does not expect AM IBOC to work via sky wave signals because of the three subcarriers that occupy the 15kHz mask. This means that AM IBOC will rely solely on ground wave propagation.
With ground wave coverage, the AM IBOC system provides satisfactory results to the 2mV/m contour and sometimes as far out as the 0.6mV/m contour. Stereo AM IBOC sounds like good FM stereo. As the AM IBOC signal degrades, it "glides" to sound like FM mono and then deteriorates from there. This may come as a big surprise to some if there is no nighttime operation. One has to ask if it will be worthwhile for stations to use the system only during the day.
Overall, the AM IOC signal is very robust, but it suffers from the usual AM problems and may limited by first- and second-adjacent problems. The NRSC committee will present its findings on April 6 at NAB for approval by the NRSC.