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Signal Distortion Not Just a Station Problem

Most FM receivers can't handle high modulation

When it’s time for your station brain trust to set, suggest, or adjust modulation for your station, you might want to bring to that meeting a copy of “The Curious Behavior of FM Receivers During Overmodulation,”a paper presented at the Broadcast Engineering Conference at the 2015 NAB Show.

Jeff Keith, senior Project Development Engineer for Wheatstone Corp.’s Audio Processing Products, studied the effects of overmodulation on the station side and on 15 FM consumer receivers (portable, tabletop and automotive), specifically intermodulation distortion. “I tried to stick to the radios listeners actually use.”

The receiver chips in those radios introduce their own set of problems, which are made worse by overmodulation. “Most of these receivers that are built today really don’t do a good job demodulating even a normally-modulated FM signal,” says Keith, who suggests that 120% modulation “is probably safe for the majority of radios. “More than that and I think you’re rolling the dice and really doing a disservice to some percentage of your listeners. Higher modulation than that we know is going to be problematic for some radios; we don’t know which ones; the listeners don’t know, and those listeners certainly don’t know that it’s not the station that’s distorted.”