I'm always interested in the Neilsen PPM results for our local radio stations. Often, when I learn the numbers, I'll even write about it. I just assumed PPM (Portable People Meter) dependably measured stations encountered, but apparently it has "blind spots".
This article was enlightening for this radio fan. Depending on the station's encoding, the PPM may or may not pick it up. In fact, if the station is primarily talk, it's often missed.
Voltair is a new service marketed to radio stations that promises "advanced audio signal processing to enhance the detectability of the watermark codes within the context of your programming objectives.