I recently bought an inexpensive USB software-defined radio dongle, and have been poking around my local spectrum:
The above image is one of the local FM radio stations (88.7 Mhz, to be specific). I am wondering what the large, rectangular side-bands are. They don't seem to be involved in the actual radio content, as this is the only station that has them, and all the other stations seem to be received fine.
It's also not the stereo component of the signal, as again, stereo demodulation for other stations that lack these sidebands works without issue.
Each sideband is 125 Khz out from the center of the signal (i.e. the inner edges of the rectangular sidebands are 250 Khz apart).
Best Answer
The spectrum seems to match up fairly well with HD Radio even though the image from that article is not all that great. It might be worth checking if that particular station is broadcasting HD Radio.