Subcarriers are additional carrier frequencies that are used within a frequency band for modulation with further information. Subcarriers are arranged at a fixed frequency distance from the carrier frequency and can each be modulated separately with their own modulation method. The number of subcarriers is unlimited, at least in theory. Some DSL methods work with 256 subcarriers.
Modulation methods with subcarriers are known from radio and television technology. FM broadcasting, for example, uses a subcarrier of 38 kHz for stereophony. Another subcarrier for the station identification of the Automobile Radio Information(ARI) is at 57 kHz. Subcarriers are also used in television. There is the sound carrier at a frequency spacing of 5.5 MHz from the picture carrier and the color carrier, which has a frequency spacing of 4.43361875 MHz from the picture carrier in the PAL television standard.
For satellite television programs, additional sound subcarriers are inserted within the television band of 8 MHz for the various languages, for stereo, and for other analog radio programs. For satellite communications, the audio subcarriers for analog television audio are at 7.02 MHz and 7.20 MHz. Analog radio programs are transmitted on the other subcarriers at 7.38 MHz, 7.56 MHz, 7.74 MHz, 7.92 MHz, 8.10 MHz and 8.28 MHz.
Subcarriers are also used in multicarrier processes for efficient use of the available frequency band. This is the case with DSL methods or Orthogonal Frequency Division Multiplex( OFDM).