One of the issues that has not been discussed in the aftermath of the KNX move to FM is the double-barreled deficiency of AM signals in most US metro areas: signal and noise.
While KNX does not have a deficient signal, most of its band-mates in the LA metro and everywhere else do.
In the Top 100 markets, there are 1748 stations within the Metro Survey Areas. However, less than 170 have a 5 mV/m signal over at least 80% of the metro day and night. That is less than an average of 2 "good" signals per market. Some have a couple more, some have none at all.
When FM started being broadly viable around 1970, most FMs upgraded to their maximum facility which, if a class B or C, covered pretty much the whole radio market day or night. Each market had ten or more such full signals on FM.
Since 1970, population has spread out more and more in most metro areas. The existing AMs have become less and less able to cover the full market.
And then there is the issue of noise produced by florescent lights, dimmers, computers, wall warts and all kinds of other devices. While 5 mV/m was a great signal decades ago, and even a 2 mV/m could be listenable, today the ITU says that AM needs 15 mV/m to overcome urban noise. Even if we consider 10 mV/m adequate in most places, that means that there are truly only a few dozen AM stations that actually cover their entire market day and night.
If you look at the Nielsen numbers, in the "average" market there is under 10% listening to AM, and most of that is by people over 55. That means that most under-55 AM listening is confined to things like having the only sports station on AM, having no ethnic programming on FM, or niche formats such as religious denomination based formats.
For all practical purposes, AM is dead, although a few wounded still wander around on the battlefield.
