How FM & Sidebands Work

The FM band that each station is authorized to use spans 200Khtz . In WBAI's case the space between 99.4 and 99.6 FM. This is called the station's bandwidth. But only a portion of that band is actually used for the FM stereo. The rest is either empty space or where sidebands can be broadcast. The picture below is a oscillascope representation of what a typical FM radio signal looks like, including sidebands and the new High Definition (HD) radio signal. The main signal is the peak in the middle and is for a Los Angeles radio station broadcasting at 91.5 FM. You can see the SCA (sidebands at 67 kHz and 92kHz-known as analog sidebands) to the right and left. Beyond that is the HD1&2 signals on either side. Further to the right is another station with its sidebands.

WBAI is broadcasting its main signal and leasing the 67kHz sideband. The other potential sideband where new programming could be broadcast on is empty. (Broadcast of audio on analog sideband channels is not regulated by the FCC.) Similarly space where WBAI would broadcast HD bands are also empty. (Broadcast on digital HD-radio requires permission from the FCC and special broadcast equipment.) It is possible to broadcast up to 3 channels on the HD sidebands. Stations like WNYC here in NY broadcast an HD-1 (same as their main signal), HD-2(new classical music station) and HD-3 (rebroadcast of AM talk radio) signal. With the HD-1 & 2 signals being stereo, FM quality and HD-3 being AM quality.