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A subcarrier is separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth. This is a method of multiplexing.

Stereo is a subcarrier of an FM radio station, which takes the left channel and "subtracts" the right channel from it -- essentially by hooking up the right-channel wires backward and then joining left and right. The result is modulated with AM onto a 38kHz radio frequency, which is joined with the mono left+right audio (which ranges 50Hz~15kHz). A 19kHz pilot "tone" is also added to trigger radios to decode the stereo subcarrier, and voilá, you have FM stereo, fully compatible with mono. Once the receiver has the L+R and L-R signals demodulated, it adds the two ( L+R + L-R = 2L ) to get the left channel and subtracts ( L+R - L-R = 2R) to get the right.

Likewise, TV signals are transmitted with the black & white luminance part as the main signal, and the color chrominance as the subcarriers. A black & white TV simply ignores the extra information, as it has no decoder for it. Subcarriers on the video can also carry three audio channels, including one for stereo (same left-minus-right method as for FM), another for SAP second audio programs (such as descriptive video service for the vision-impaired, and bilingual programs), and yet a third hidden one for the studio to communicate with reporters or technicians in the field (or for a technician at a remote transmitter site to talk back to the studio), or any other use a TV station might see fit.

Other uses:

Note that higher frequencies drop off much more rapidly, which explains why FM stereo gets noisy at a distance, while switching to mono is still perfectly clear and easy to listen to.