“Subcarriers are, essentially, hangers on, areas of frequency that weren’t being used for the primary signal, but could find secondary uses in more specialized contexts.” Some FM subcarriers were used to provide a second channel for stereo, but “with secondary signals that may not be directly accessible at all by the primary receiver, completely unrelated, niche services were offered.” Those services have ranged from the original Muzak to specialized radio for doctors’ offices to services that read newspapers and books aloud for the blind to foreign-language broadcasts to GPS. (And there was one ill-fated Microsoft endeavor). – Tedium