The NRSC also adopted a guideline for recommended bandwidth for AM transmission. NRSC-G100 provides guidance for operators who would like to decrease first adjacent channel interference by operating with bandwidth narrower than the 10 kHz limit. Using the full 10 kHz bandwidth is lost on most receivers, as nearly all receiver IF filters limit audio response to much less than 10 kHz. Benefits from this technique are limited to cases where the licensees mutually agree to bandwidth reduction of stations that interfere with each other.

The results showed that mutually reducing transmission bandwidth down to 5 kHz  is a big plus for speech.  There was either a benefit, or no change from the 10 kHz results when reducing the transmitted bandwidth to 7 kHz irrespective of format or first adjacent channel interference level.

The study was done with a selection of receivers, and used blind A-B testing to determine listener preference.  This methodology is superior when the sources are a selection of several transmission bandwidths.

This study assumed symmetry:  both the desired signal and the undesired signal were limited to the same bandwidth.  This implies a standard to be adopted by all stations within the AM band, which may or may not be a factor for stations that receive interference from stations in other countries.

Since stations broadcasting speech receive a big plus, there will probably be pressure by the “talkers” to adopt the narrower 5 kHz limitation.   This is also the bandwidth limitation required for medium wave HD radio’s analog signal.

The study did not include the effect of HD radio interference to first adjacent channel stations, and whether an analog station has any benefit from wider transmission bandwidth when an HD signal is the interferor.  In the end, HD radio’s medium wave “Digital Only” signal will be an excellent neighbor to other “digital only” and Analog stations, but I expect few stations will be moving to this mode in the next year or so.