The Curious Behavior of Radios

CarRadio LargeLouder is better! Crank it up! Well, not so fast...

Ever wonder what your listeners' FM radios sound like when your station is knee deep in the loudness race and the modulation monitor is always pegged? Our audio processing development guru, Jeff Keith, wondered about that too.

So, during one quiet week at the Wheat processing lab, he decided to find out. He selected 15 radio receivers that most represented the majority of radios out there in use, and got out his trusty modulation analyzers, signal generators and other assorted test gear. He ran audio sweeps of de-modulated and de-emphasized FM audio and plotted SMPTE IM distortion of the receiver’s audio output as modulation was raised, among other tests. His main goal was to discover distortion trends in radios during 110% or more modulation. Here are a few of his findings, the details of which will be presented during the upcoming NAB Broadcast Engineering Conference (BEC).

  • The more recent the radio model, the more intolerant of high modulation it is likely to be.
  • Newer AM/FM/HD radio IC chips detect high deviation (over-modulation) and often, in an attempt to fix the problem, create unpleasant audio effects.
  • Many consumer receivers have restrictive intermediate frequency (IF) bandwidths, which can mean perceptibly distorted audio even when tuned to a normally modulated station. The IF bandwidth of one radio measured was barely 100kHz wide at the 3dB point.
  • Half of the receivers tested added significant IM distortion at modulation levels as low as 120%.

Jeff Keith’s paper “The Curious Behavior of Consumer FM Receivers During Hyper-modulation” will be published in the 2015 NAB Broadcast Engineering Conference (BEC) Proceedings and presented during the NAB Engineering Conference, Sunday, April 12.

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