Today on my commute home, I was pleased to discover that the high frequency distortion I've been hearing on WFMT on audio peaks (on the analog FM signal) has finally gone away. It sounds like they might have reduced the peak deviation ever so slightly. Ironically, I had recently started hearing this same problem on several other Chicago area stations that are running HD. But not all, and not on analog-only stations.
I have a hunch that the problem involves mixing between the high frequency sidebands of the FM signal and the HD sidebands. It is exacerbated by the relatively severe FM pre-emphasis curve. Of course, that issue has been around for a long time, but until now it probably was not as critical because there wasn't so much other high frequency energy present. The distortion is probably generated in the receiver, but I've heard it on a number of different tuners, so it may be something to pay attention to.
Incidentally, I wonder if excessive deviation could cause problems for the HD signal as well. In summary, stations running HD systems may need to be especially careful about peak deviation levels.
I have a hunch that the problem involves mixing between the high frequency sidebands of the FM signal and the HD sidebands. It is exacerbated by the relatively severe FM pre-emphasis curve. Of course, that issue has been around for a long time, but until now it probably was not as critical because there wasn't so much other high frequency energy present. The distortion is probably generated in the receiver, but I've heard it on a number of different tuners, so it may be something to pay attention to.
Incidentally, I wonder if excessive deviation could cause problems for the HD signal as well. In summary, stations running HD systems may need to be especially careful about peak deviation levels.